modern phy 101 all exp

7/25/2019 Modern PHY 101 All Exp

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Determination of the Angle of the Prism

Aim:To determine the angle of the given prism.

Apparatus:Spectrometer, prism, magnifying glass, mercury vapor lamp.

Principle: When a beam of light strikes on the surface of transparent material such as glass,

a portion of the light is transmitted while the other portion is reflected. When a beam of light

strikes on a plane surface, the angle of reflection will be the same as angle of incidence. If the

angle between two reflected ray is measured as , then the angle of the prism is A= /2, as shown

in the figure below:

Procedure:

a) Preliminary adjustments of the spectrometer

1. Turn the telescope towards a distant object and looking through eye-piece, adjust its position

till the cross wires are clearly seen.

2. Turn the telescope towards distant object; focus the telescope to a long distant object.

3. Place the telescope parallel to collimator.


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4. Place the collimator directed towards mercury vapor lamb. Switch on the lamp.

5. Focus collimator slit using collimator focusing adjustment.

6. Adjust the collimator slit width.

7. Place the prism on the table, note that the surface of the table is just below the level of

telescope and collimator.

8. Place spirit level on prism table. Adjust the base leveling screw till the bubble come at the

centre of spirit level.

b) Determining the angle of the prism

1. Prism table is rotated so that the sharp edge of the prism is facing towards the collimator.

2. Rotate the telescope in one direction up to which the reflected ray is shown through the

telescope.

3. Note corresponding main scale and vernier scale reading in both vernier.

4. Rotate the telescope in opposite direction to view the reflected image of the collimator from

the second face of prism.

5. Note corresponding main scale and vernier scale reading in both vernier.


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Physics Lab. manual for particle size estimation:: VIT Chennai

DETERMINATION OF SIZE OF FINE PARTICLE USING LASER

DIFFRACTION

Aim:To find particle size from laser diffraction pattern

Principle: This method is based on diffraction phenomenon and is based on Fraunhofer

theory. When a particle is lightened by a monochromatic source (laser source), a

diffraction pattern, called Airy`s pattern is obtained at the infinity.

This diffraction pattern gives the light scattering intensity (I), in the function of

diffraction angle. It is composed of concentric rings. The distance between the different

rings depends on the particle size. The size (D) of the particle is

r

dnD 22.1=

where is wavelength of source, nis order of dark ring, dis distance between particle

and the screen, r is the radius of the dark ring. The factor 1.22 is derived from a

calculation (Bessel function) of the position of the first dark ring surrounding the central

Airy disc of the pattern.

Assumptions:

1. Particles are spherical in nature and they do not absorb light

2.

Particle diameter must be at least 3-5 times bigger than the value (normally

m size)

3. The distance between 2 particles must be 3-5 times bigger than their diameter

Procedure:


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Physics Lab. manual for particle size estimation:: VIT Chennai

1. Place the imprinted diameter screen at the edge of the measuring bench.

2. Keep the glass plate (having particle of uniform diameter dispersed on it) in

between the laser source and screen.

3. Adjust the relative positions of laser and glass plate to get clear concentric

circular rings of bright and dark fringes on the screen

4. For three different position of glass plate with respect to screen estimate the

diameters of first and second order fringes

Data Table: To find the size of particle

Sl. No.Order of

diffractiond (cm)

Diameter of

dark ring

(cm)

Radius of

dark ring, r

(cm)

Size of

particle, D

(m)

11

2

21

2

31

2

Mean D value =

Suggestion: It is better to adjust distance (d) to make one of the two dark rings match

with imprinted diameter and make approximation in the size of diameter of other ring.

Result:

Interpretations:

Error estimation: Analysis should be done for diameter estimation by standard

deviation process for one position of dand measuring diameter of first dark ring for at

least for 10 times.


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1

Assessment of purity of a given liquid by determining its refractive index

AimTo determine the purity of a liquid in terms of refractive index using travelling microscope

Apparatus required

Travelling microscope, reading lens, 50 ml beaker, water, saw dust, etc.,

Formula

Refractive Index of the liquid =)(

)(

BC

AC

(no unit)

where

A- Reading of the microscope when the ink dot is focused directly (cm)

B-

Reading of the microscope when the ink dot is focused through water (cm)C-

Reading of the microscope when the saw dust is focused (cm)

Procedure1.

Focus ink dot marked at the bottom of the beaker directly and take the readings (MSR

and VSC) in the vertical scale (A).

2. Pour 20 ml of water into beaker without disturbing it.

3.

Now, focus the ink dot through water by adjusting the vertical adjustment screw (Do

not use the focusing knob) and take the reading (MSR and VSC) in the vertical scale.

(B).

4.

Sprinkle some saw dust on the surface of water gently without disturbing the set-up.5. Focus the saw dust by adjusting the vertical adjustment screw (Do not use the

focusing knob) and take the readings (MSR and VSC) in the vertical scale (C).

6. Pour out the water.

7.

Repeat the experiment (step 1 and step 5) for different quantities/levels ( 40 ml, 60

ml, 80 ml) of water and tabulate the readings.

8. Calculate the refractive index using the formula.


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2

Tabulation

TR= MSR + (VSC x LC)

Calculations

=)()(

BC

AC

(no unit)

= ____________

Result

The purity of the given liquid evaluated in terms of Refractive Index is determined to be

= ____________

Viva voce questions:

1. Why should we focus the ink dot in the beaker in this experiment?

2. Why should we use sawdust in this experiment?

2. Differentiate between actual depth and apparent depth.

3. How do you calculate the percentage of error?

4. What is the significance of this experiment is day to day life?


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1

Determination of the track width (periodicity) in a written CD

Aim: Determination of the track width (periodicity) in a given CD by a Laser diffraction method

and then determine the amount of data stored on a given CD.

Apparatus Required:

Laser source, written CD-R, Planer screen and Scale.

Theory:

A standard CD is a fairly simple piece of plastic disk having 1.2 mm thick and 120 mmdiameter. It can hold up to 80 minutes of uncompressed audio or 700 MB of data.

As shown in the Fig. 1, a CD have the following components, from the center outward: the

center spindle hole (15 mm), the first-transition area (clamping ring), the clamping area (stackingring), the second-transition area (mirror band), the program (data) area, and the rim. The inner

program area occupies a radius from 25 to 58 mm.

Fig. 1: A cross-sectional view of a CD skelton along with different components and their typicaldimensions.

There are different types of CDs are available in the market and CD-ROM (called stamped CD),CD-R and CD-RW are the most widely used types. Here we try to determine the track width of a

standard CD-R, using laser reflective diffraction method.


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2

Just like all kinds of CDs, a CD-R disc is a sandwich of a number of layers. The polycarbonate

disc contains a spiral groove (tracks), called the "pregroove" (because it is molded in before dataare written to the disc), to guide the laser beam upon writing and reading information. The

pregroove is molded into the top side of the polycarbonate disc, where the pits and lands would

be molded if data were written; the bottom side, which faces the laser beam in the player or

drive, is flat and smooth. The distance between the spiral tracks, the pitch, is ____ m. Ouraim is to determine the pitch using light diffraction experiment.This polycarbonate disc is coated on the pregroove side with a very thin layer of organic dye

(cyanine, azo or phthalocyanine). Then, on top of the dye is coated a thin, reflecting layer ofsilver, a silver alloy, or gold. Finally, a protective coating of a photo-polymerizable lacquer is

applied on top of the metal reflector and cured with UV-light. Some discs are also topped, on

lacquer layer, with additional layers that improve scratch resistance, increase handling durabilityor provide surfaces suitable for labeling by inkjet or thermal transfer printers. A cross-

sectional view of a CD-R is shown in the Fig.2.

Fig.2: A cross-sectional view of a CD-R.

The laser of your CD-R drive heats the dye to a temperature of about 200oC, irreversibly melting

a pitted pattern into the recording layer. A plastic layer alongside the dye expands into the newly

available space creating a pit pattern similar to that of a conventional CD. Your CD player reads

this highly reflective pattern for playback. Because the plastic layer melts into the dye layer toset the patter.

Digital data is stored in CD as a series of these "pits". The areas between pits (i.e.,

unmelted area) are known as "lands". Each pit is approximately 100 nm deep by 500 nm wide,and varies from 0.85 m to 3.5m in length. Pits have the same light reflecting surface as the

land, but pits reflect the read-laser's light in a diffuse and interferening way and thus lookrelatively dark compaired to the land areas.


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3

Fig.3: Data tracts and lands and pits in the written CD.

It is not simply so that a land is a "1" data bit, and a pit is a "0" data bit. A data bit is a "1"or "0" from the original data, but on a CD there are no data-bits but channel-bits. A channel bit is

the smallest time unit used on a CD (=1/4,321,800 sec). A "1" channel bit = a time with change

from land to pit, or from pit to land, a "0" channel bit = a time when there is no change, as shownin Fig.4. Channel bit length is computed just by dividing the speed by the bit rate. For example:

1.2 m/sec / 4321800 channel bit/sec = 277.662 nm.

Pit & Land Length varies a little depending on how fast the disk turns while recording. The

scanning velocity during recording shall be between 1.20 m/s and 1.40 m/s with a channel

bit rate of 4321800 channel bit/sec. The velocity variation for a disk when recorded shall be

within 0.01 m/s. In other words, CDs are recorded at a constant velocity within 0.01/1.3 =0.8% tolerance. Since the channel bit rate is held constant (4321800 channel bit/sec = 75

blocks/sec * 98 frames/block * 588 channel bits/frame.), then the density of the bits must

vary with recording velocity. In other words, those 4321800 channel bits that encode 1

second of audio could be stored in as little as 1.2 linear meters or as much as 1.4 linear

meters.


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4

Fig.4: Mapping of bits with lands and pits.

The spiral of pits behaves in much the same way as a reflective diffraction grating. That is whyyou see beautiful rainbow colors when white light illuminates the CD. When a laser beam is

reflected off the disc, a diffraction pattern is formed. If the angle of incidence is close to the

normal, the condition for constructive interference is identical to that for a transmissiondiffraction grating.

In your previous cycle (Modern Physics lab), you may have determined the wavelength (W/L) ofthe laser using a grating (ruler). Now you can use the W/L of the laser to to measure the spacingbetween tracks on a compact disc (CD)! Thus, you may determine the maximum amount of

information that can be stored on a CD.

The diffraction pattern that you see when you allow the reflected laser light to fall on a white

wall can be used to infer the track width.


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5

Fig.5: The schematic diagram of the experimental arrangement.

Formula used:

n =d sin (For reflected diffraction pattern) ..(1)

Where,

is the wavelength of the laser, is the angle of diffraction, n is the order of diffraction and d is

the track width (to be determined). Hence, the track width can be determined by using the

following equation,

d = n / Sin m (2)

Experimental Procedure:

1. The CD is held normal to the laser beam at a distance ~40cm such that the laser source

lies between the screen and the CD.

2. The laser source is switched on and it is diffracted by the CD by the phenomenon of

reflective diffraction.


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6

3. A central spot with equidistant spots on either side will be noticed on the screen.

4. The distance 2L between the spots on either side of the central spot is measured

corresponding to various orders (1,2,3.n).

5.

The experiment is repeated for various values of D, the distance between the screen andthe CD.

Observation Table: Given of laser =..

n D ( cm) 2L (cm) L (cm) tan=L/D = tan-1

(L/D) Sin Mean

Sin

Track-

width (d)

m)

1

2

Calculations:

d = n /Sin (m)

Result & Conclusion:

The track width of the CD is d =. m

Applications:


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7

Precautions:

1. Do not see the laser light directly.

Questions:

1.

Given the inter-track spacing that you find, can you estimate the number of tracks on theCD and the total length of the spiral track?

2. From this total length, and an average bit-length of about 0.6 micron, estimate how many

bitswould fit on the CD?

3. Find the number of bytes (8 bits/byte).

Further study:

1. Do the same for DVD

2. Create Transmission Gratings from a CD and try the above experiment to determine the trackwidth on the written CD.

3. Estimate the data size exist on the CD, by assuming standard value of data channel.

*****************

THE END


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ELECTRON DIFFRACTION- DEBROGLIE WAVELENGTH

Aim:

To observe the diffraction of electrons on polycrystalline graphite and to confirm the wave

nature of electrons.

To calculate and compare both the deBroglies and Braggs wavelength of electrons.

Apparatus Required:

electron diffraction tube

Tube holder

High voltage power supply

Analogue multimeter

Formula Used:

1. Braggs wavelength

d- the separation between two adjacent planes

D- diameter of the rings

L- distance between graphite target and fluorescent screen= 135mm

2. deBroglies wavelength

=

Where m- mass of electron,e - Charge of electron,

h- Plancks constant,

V- Applied voltage, kV.

Theory:

In 1923, in his doctoral dissertation, Louis de Broglie proposed that all forms of matter have

wave as well as particle properties, just like light. The wavelength, of a particle, such as anelectron, is related to its momentum, p, by the same relationship as for a photon:


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=h/p ----- (1)

where h is Planck's constant. The first experimental evidence of the existence of matter waveswas obtained by Davisson and Germer in 1927. The wave properties of electrons are illustratedin this experiment by the interference when they are scattered from successive planes of atoms in

a target composed of graphite microcrystals. The spacing between successive planes can bededuced from the interference pattern. When the beam of electrons strikes a family of parallelcrystal atomic planes, each plane will reflect part of the waves. If the reflected waves from O and

Q, as indicated in Figure 1, are to be in phase (interfere constructively), the path difference

PQ + QR = 2d sin ----- (2)

must equal an integral number of wavelengths, or

2d sin = n(n = 1, 2, 3..) ----------- (3)where d is the separation between two adjacent planes. Equation (3) is known as Bragg's law for

constructive interference. For any incident angle other than those satisfying equation (3), there isno reflected beam because of destructive interference.


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In the Polycrystalline graphite target, there are very many perfect microcrystals randomly

oriented to oneanother. Therefore the strongly emerging beam will be of a conical shape of half

angle 2asshown in Figure 2. If this beam falls on a phosphor-coated screen, rings of light will

then be formed. Now, the condition for constructive interference becomes

= n ----------- (4)

which for small angles and first order diffraction (n = 1) becomes

-------------- (5)

D is the diameter of the diffraction ring andL is the distance from the graphite target to the luminescent screen.

This experiment uses Thomson's method for sending electrons through a thin film of graphite

target to investigate the resulting ring diffraction pattern with the aid of Bragg's law. A schematic

diagram for the apparatus is shown in Figure 3. Electrons emitted by thermionic emission from aheated filament (the cathode) are accelerated towards the graphite target (the anode) by a

potential difference, V. Their kinetic energy, K, on reaching the target is equal to their loss of

potential energy:


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----------- (6)where p, m and e are, respectively, the momentum, mass and charge of the electron. Combiningequations (1) and (6), the wavelength of the electrons is given by

---------- (7)

With equation (7), equation (5) becomes

---------- (8)

Fig.5 Distance between adjacent net planes of graphite d1= 123 pm und d2= 213 pm.


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Graphite possesses a hexagonal sheet structure as shown in Figure 4. Each layer has strong

internal bonds but weak bonds between layers which stack into crystals. The properties parallel

and perpendicular to the sheets are markedly different. The two sets of parallel atomic planesresponsible for our observation are all perpendicular to the sheets as shown in Figure 5. Theelectrons experience Bragg-reflection at the planes of the carbon atom, the outer and the inner

ring originate from planes with the distances d1=123 pm and d2=213pm.

Circuit Diagram of the Electron Diffraction tube:

Fig: 6 Circuit Diagram of the Electron Diffraction tube

Procedure:

1. Do the experimental set up as shown in figure 6.2. Apply the heater voltage and wait about one minute for the heater temperature to achieve

thermal stability.

3. Apply an anode voltage of 4kV

4. Two diffraction rings will be observed on the fluorescent screen centered on theundeflected beam in the middle.

5. Determine the outer diameter D of the two diffraction rings (bright) using a vernier

caliper.6. Now repeat the same by changing the voltage.

7. Reducing the voltage will make the ring wider. Why?

8.

The de Broglie wavelength is calculated by the formula=

m= mass of electron, e= charge of electron, h= Plancks constant, V= applied voltage.

Using the Braggs formula, wavelength can be calculated by the formula n=2dsin

If is small the Braggs wavelength becomes,


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d1= 123pm when the outer bright rings outer diameter is measured

d2= 213pm when the inner bright rings outer diameter is measured

L= distance between graphite target and fluorescent screen= 135mm

To calculate deBroglie and Braggswavelength of electrons.

S.NoVoltage(kV) Diameter of the

ring=

Braggs

(),nm

deBrogli

(),nm

MSR

mm

VSC TR

mm

1. Outer

Ring

Inner

Ring

d=123pm

d=213pm

2. Outer

Ring

Inner

Ring

d=123pm

d=213pm

3. OuterRing

InnerRing

d=123pm

d=213pm

Error Analysis:

The error can be calculated by taking at least 5-10 values of diameter (D) of the diffracted

rings for a particular voltage and then calculating the standard deviation.

Result:

S.No Accelerating

Voltage, kV

deBroglie wavelength

of electron,nm

Braggs wavelength of

electron,nm1.

2.

3.


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Determining

Numerical

Aperture,

Acceptance

angle

and

optical

power

lossesofOpticalfibersforfindingtheirsuitabilityin

telecommunications

applications

Aims:

1.

To determine the Numerical Aperture (NA) and acceptance angle

( of the given two different (1 meter and meter cables) optical fibersto find their suitability in telecommunications applications.

2.

Observing the optical power losses, when light are passing through two

different (1 meter and meter cables) optical fibers during, (a) when

they are not coupled each other and (b) when they are coupled each other

through an in-line adaptor.

Apparatus required:

Fiber optic LED light source, Fiber optic power meter, Fiber Optic (FO)

cable 1 meter, FO cable meter, In-line adaptor (to connect 2 cables), NA-Jig (L-

shape with scale on one side and connector on other side).

Formula:

NA = sin =

(No unit)

where W - Diameter of the spot (m)

L - Distance between the fiber end and the screen (m)

- Acceptance angle (deg)

Procedures:

To determine the Numerical Aperture (NA) and acceptance angle (

:

1. Connect one end of the 1 meter FO cable and the other end to the NA Jig as

shown in the figure.

2. Plug the AC main. Light should appear at the end of the fiber on the NA Jig.


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3. Notice the horizontally movable acrylic screen-printed plate attached with NA-

Jig. This screen is drawn with concentric circles of 10, 15, 20, 25 and 30 mm

diameters).

4.

Now move the acrylic screen-printed plate to a distance L (say 5 mm) from the

fiber end, view the spot and measure its diameter W.5. Repeat the experiment for different distances L (say 10 mm, 15 mm, 20 mm, 25

mm and 30 mm). Note down the diameter values W of the corresponding spots.

6. Then calculate the NA and using the above relations.

7. Now fix the meter cable and repeat the same procedures to calculate the NA

and.

To observe the optical power losses, when light are passing through two

different (1 meter and meter cables):

1.

Connect one end of the 1 meter FO cable to the FO LED and the other end to

the fiber optic power meter and observe the displayed value and estimate the

power loss.

2.

Connect one end of the 1 meter FO cable to the FO LED and the other end to

the fiber optic power meter and observe the displayed value and estimate the

power loss.

3.

Connect both the FO fiber cables through the given in-line adaptor and thenconnect one end of this coupled FO cables to the FO LED and the other end

to the fiber optic power meter and observe the displayed value and estimate

the power loss.

4.

Analyze the power losses estimated in the 3 above mentioned cases.

To determine


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NA and :

1) For Optical fiber cable with length of 1 meter:

Sl.No.L W NA

mm mm No unit deg

1

2

3

4

56

Mean =

2) For Optical fiber cable with length of meter:

Sl.No.

L W NA

mm mm No unit deg

1

2

3

4

5

6

Mean =


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Calculations:

NA = sin =

(No unit)

NA =

sin = NA

sinNA deg

Results

i) The Numerical Aperture of the given optical fiber (1 Meter) =

.

ii)

The acceptance angle for the given optical fiber (1 meter) = .....

deg.

iii) The Numerical Aperture of the given optical fiber (1/2 Meter) =

.

iv) The acceptance angle for the given optical fiber (1/2 meter) =

..... deg.

v)

The optical power loss, when light is passing through optical fiber cable

(1 meter)

vi) The optical power loss, when light is passing through optical fiber cable

(1/2 meter)

vii) The optical power loss, when light is passing through optical fiber cable

(1/2 meter)

viii)

The optical power loss, when light is passing through optical fiber cables

(1 meter and meter) when they are coupled each other through an in-

line adaptor


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Error Analysis:

Do the error analysis for observations related to NA and measurements

for both the FO cables.


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1

Determination of the number lines of a given grating using a laser source for

display applications

Aim

To determine the number of lines in a given grating using a laser source of light.

Apparatus required

He-Ne laser or semiconducting laser,grating, scale, grating stand

Formula

nN

)sin( lines per meter

where- Wavelength of the laser light

used in the experiment (nm),

- Angle of diffraction (degree)

n - Order of diffraction,

N-The density of lines in the grating = lines/meter.

Procedure

1. The grating is held normal to the laser beam at a distance D (~ 30 cm) from the screen.

2. The laser light is switched on and it is diffracted by the grating.

3. Symmetric weaker spots corresponding to different orders (n= 1,2,3,4,5,..) of diffract ion

will be observed around a central bright spot

4. The distances (2L) between the spots on either side of the central spot corresponding to

various orders is measured and tabulated.

5. Step 4 is repeated for different values of D (~ 35 cm, 40 cm,45 cm, 50 cm).

6. The wavelength is calculated using the formula.


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2

Diffraction

Order

n

D 2L Ltan (L/D)

tan-1

(L/D)Sin

Mean

sin N

centimeters degrees Lines/meter

1

3035

40

45

50

2

30

35

40

45

50

3

30

3540

45

50

4

30

35

40

45

50

5

30

35

4045

50

Mean N =

Tabulation


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3

Observations

For n = 1; Mean sin =

For n = 2; Mean sin =

Result

The density of the lines in the given grating was determined to be N = ___________________

Viva voce question

1. What is the type of the laser you used in the laboratory?

2. What is the reason for serial of light spots appearing on the measuring scale?

3. Distinguish between laser source and conventional light source.

4. Define grating element.5. What are the requisites of good grating?

6. What is meant by diffraction of light?

7. Comment on: Grating with larger number of rulings per cm is always preferable.

8. what is the significance of the experiment


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1

Aim: To study Black body radiation and verify Wiens Law

Apparatus Required: Black body radiation kit, Desk Top Computer

Theory of the Black Body Experiment

An incandescent light source that emits light through a small cavity is a perfect emitter. Bydefinition, a perfect light emitter is one that emits light rays throughout an infinite number of

frequencies in the visible and invisible electromagnetic spectrum. When light from the black

body is cast through a prism, the observed spectrum is continuous, and no overlapping of the

spectral lines occurs.

In this experiment, parallel light rays travel through the collimating lens, which allows the light

rays to remain parallel. Passing through the prism, the light rays refract and project in front of theaperture slit over the light sensor. The light sensor detects and records the light intensity as

voltage.

Unlike other light sources, changes in light intensity from an incandescent black body is solely

dependent on temperature. Increasing the temperature of the black body light source increases

the light intensity. For any given temperature, there appears to be an optimal wavelength for

reaching a maximum light intensity.

The angle of the emitted light depends upon the refraction index of the prism and the wavelength

of the rays. Shorter wavelengths show more bend than longer wavelengths and thereforeexhibit higher indices of refraction. Here is a two dimensional plot of a spectrum of a black body

with different temperatures as shown below.


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2

Important Laws :

1. Rayleigh- Jeans law:

2. Plancks Law :


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3

Notes:

1. See the Operational Notes section of this manual for important setup reminders.

2. Before beginning the experiment, the following instructions are to be followed:a) Set the collimating and aperture slits.

b) Check the position of the prism.

c) Ensure the cables from the rotary motion sensor are properly inserted into Science Workshop.

d) Check to see that the black body light is turned on and is emitting steady light.The bulb can be turned on from the Signal Generator box in Data Studio. If the bulb does not

turn on or emits intermittent bursts of light, see the Troubleshooting section of this manual.

Calibration

1.Calibrate the Rotary Motion Sensor

Determining the wavelength from the prism spectrophotometer requires and exact measurementof the angle. To calibrate the rotary motion sensor, determine the ratio of the disk radius to the

pin radius (approximately 60:1) as follows:

a) Remove the prism mount and the light sensor bracket from the degree plate by unscrewing the

two small thumbscrews. Start the Data Studio program and select a rotary motion sensor. Make a

digits display of the angular position, and turn the degree plate so that the zero degree mark is

exactly aligned with the index mark.

b) Start recording data. Slowly and continuously turn the degree plate clockwise for exactly one

complete rotation.

c) Stop recording data. Record the maximum value of the angle. Divide this number by 360 (or

2if it is in radians mode). This is the ratio of the radii. Record this number in the calculator inData Studio.


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4

2. Tare the Light Sensor

Note: For best results and to avoid measurement drift, tare the light sensor before scanning

the spectrum and/or before each experiment run. To turn the light source on, click the On

button in Data Studios Signal Generator box.

a) Rotate the light sensor arm until it hits the stop against the angle indicator on the

spectrophotometer table.

b) Block the light source by placing your hands between the collimating slits and the collimatinglens.

c) While the light is blocked, press the tare button on the light sensor to zero the sensor.

Procedures

A. Scanning a Spectrum

1. Tare the light sensor (as described in the section above).

2. Remove your hand to unblock the light and start recording data (Click the Startbutton in the

Data Studio setup window) on the computer. Slowly rotate the light sensor arm through the

spectrum.

3. To determine the initial angle from the light source, continue to rotate the arm until the light

sensor has passed through the white light that passes under the prism (i.e the degree plate has

rotated pass the zero degree mark).

4. Stop recording data. The initial angle (when the stop is against the angle indicator) is required

to calculate the wavelength.

5. In Data Studio, make a graph of intensity vs. angular position. Measure the angle to the white

light that passes directly through the spectrophotometer and under the prism. This angle issubtracted from all angles, so that all angles are referenced from the reference line (the parallel

beams that travel in a straight line through the spectrophotometer).

6. Enter the initial angle as Init into the wavelength calculation in the Data Studio calculator.


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5

Experiment

A. Black Body Spectrum: Scanning Nested Curves from Black Bodies of Different

Temperatures

Note: Before running the experiment, perform the rotary motion sensor calibration, as describedin the Calibration section on page 9. Enter this calibration as Ratio into the Data Studio

calculator, and click the Acceptbutton.

1. Rotate the light sensor arm until it hits the stop against the angle indicator on the circular table.

Note: Before proceeding, the black body light source must be turned on and emitting light.If not, click the Onbutton in Data Studios Signal Generator dialog box.

2. Block the light source by placing your hand between the collimating slits and the collimating

lens. While the light is blocked, press the tare button on the light sensor to zero the sensor.

3. Remove your hand to unblock the light and start recording data on the computer. Click the

Startbutton in the Data Studio setup window, and slowly rotate the light sensor arm through thespectrum.


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4. After you have scanned through the spectrum, stop rotating the arm, continue recording andpush the tare button again. Place your hand between the collimating slits and the collimating lens

to block the light and return the light sensor arm to its original position against the stop. Blocking

the light during this time causes the return scan to be below the axis of the graph, so it does

not trace back over the black body data.

5. a) Change the temperature of the bulb by changing the voltage applied to the bulb. You get

temperature from current and voltage, as shown by

b) For each temperature, determine the maximum peak ( max ) of the wavelength. Use the Wien

displacement law, where maxT =2898 m. K, to calculate the max.

6. Repeat steps 1 through 5 to get nested curves. On the last scan, continue the scan to the zero

degree mark so that you can obtain an exact determination of the initial angle. When you havefinished all 5 runs, click the Stopbutton in Data Studio.

Note: To avoid measurement drift, you must tare the light sensor (steps 1 and 2) before eachsubsequent scan through the spectrum.

7. Repeat the experiment 5 times to record the intensity vs wave length data at 5 different

temperatures. For that 5 different voltages are to be applied to the bulb. The maximum voltageshould not exceed 10V.

9. Save intensity vs wavelength data for five different temperatures in the computer. Later theycan be plotted by any plotting software and print out are to be taken.

10. Find the max for each applied voltage and at corresponding temperature. Complete the

following table-1 in order to verify Wiens displacement law.


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Table-1

Voltage (V) Current (Amp) Temperature (K) max (nm) max (nm) x T

Data Collection and Analysis

When you are ready to scan the spectrum, click the Start button in the Data Studio setup

window. Data Studio records the results and automatically performs the calculations for you.

When you have finished collecting your data, click the Stop button and view your results in

either a graph or a table. If necessary, you can view voltage, wavelength, angle and temperaturedata points all in the same run. To view individual data points in a table, double click to open a

table icon, click on a colored data run icon, and drag and drop the colored data run into the table.

(For more information about data analysis using Data Studio, refer to the Data Studio online helpguide.)

IMPORTANT OPERATIONAL NOTES

The following notes are critical to maintaining the accuracy of the calibrations and experiment:

Setting the Collimating Slits - For best results, match the slit width size on the collimating lens

to the same slit width size on the aperture. For example, if you select the number 4 width from

the collimating slits, select the number 4 width on the aperture. (For more information aboutadjusting the collimating slits, see the Instructional Manual for the Model OS-8539 Educational

Spectrophotometer.)


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Ensure the slit opening on the aperture slides directly over the hole on the back of the aperture

bracket; otherwise light will not reach the light sensor.

Adjusting the Voltage in the Black Body Light Source - In Data Studio, set the voltage in the

Signal Generator box. The recommended voltage setting is 7 volts. The voltage can be varied

from zero to 10V, but continuous operation of the bulb at 10V will result in a shorter bulb life.

In Data Studio, display the temperature graph and ensure the voltage increase corresponds to a

temperature increase. The sampling rate is set to 50 Hz. Also, in Data Studio, ensure that youhave selected DC voltage in the Signal Generator box; do not select sine wave or another

function. The black body experiment requires direct and continuous current, not alternating or

pulsating current.

To improve the voltage signal display on the graphs, increase the gain on the light sensor to 10 or

100. In the Signal Generator window, use the arrow keys to adjust the gain.

Checking the position of the prism - The prism mount and prism must remain fixed at alltimes. If not, your data will be in error. If the prism mount rotates at any time during the

scanning, discard the data, recalibrate, and take another reading.

Checking the angular display against the reading on the degree plate Data Studio allowsyou to adjust the angle units to either degree or radians. If you set the units to rads in Data

Studio, remember that the number on the degree plate will not correspond to the number on the

display. You will need to mathematically convert degrees to radians.

If you have negative angle readings, you may have reversed the colored cables for the rotarymotion sensor, rotated the degree plate in the wrong direction, or improperly mounted the light

sensor arm and lenses. While taking a reading, the degree plate must rotate clockwise. For more

information about the rotary motion sensor or the light sensor arm, see the instruction manual forthe Educational Spectrophotometer.

Questions/Exercise

1) How does changing the temperature of the bulb affect the wavelength or light intensity? Do

you notice a pattern with increasing temperature?

2) From what you remember from the lesson on the grating spectrophotometer, what differenceshave you observed between using a prism and a grating?

3) On a piece of paper, draw a diagram showing the position of the reference angle, measuredangle and spectral lines. Do the spectral lines converge or diverge? Do the light rays overlap?4) What would happen if you removed the collimating lens?

5) What is the relationship between the lights angle, wavelength and intensity?

6) What differences do you notice between the black body and other types of light sources youhave used?

7) Replace the infrared light sensor with the high sensitivity light sensor. What differences do

you notice in the graph displays?


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Determination of Plancks constant using electroluminescence process

Aim: To determine the value of Plancks universal constant using LEDs.

Apparatus: LEDs, digital voltmeter, micro-ammeter, and ten turn linear potentiometer.

Principle: LED is a p-n junction and it works on the principle of electro-luminescence; a

phenomenon in which materials emit light in response to the passage of electric current.

The primary carriers in p and n-type semiconductors are holes and electrons respectively.

When a junction is formed from these materials, free electrons near the junction diffuse

across the junction into the P region and combine with holes. Filling a hole makes a

negative ion and leaves behind a positive ion on the N side. These two layers of positive

and negative charges form the depletion region; the region depleted of charge carriers. As

electrons diffuse across the junction a point is reached where the negative charge repels

any further diffusion of electrons; thus forming a potential barrier. External energy must

be applied to get the electrons to move across the barrier of the electric field. The

potential difference required to move the electrons through the electric field is called the

barrier potential. Barrier potential, Vo of a PN junction depends on the type of

semiconductor material, amount of doping and temperature. In an LED, the PN junction

is used in forward bias condition which helps in injecting large number of electrons with

additional potential energy required to overcome the barrier potential thus leading to

large flow of current through the junction. The recombination of electrons in the

conduction band with the holes of the valence band results in release of photons and the

wavelength of the light emitted depends on the band gap of the semiconductor material

used in the LED.

Fig. 1 Formation of Depletion Layer in

a p-n junction

Fig. 2 The foward-bias of the p-n junction

leads to large current. The energy

diagram below shows the recombination

of electrons and holes roducin hotons


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From the conservation of energy,

E=eVo(electron) =h= h(c/) (photon)

h=(eVo)/c (1)

Where e and h are the charge of electron and Plancks constant respectively while and

, are the wavelength or the color and the frequency of the photons emitted from the

LED.

The Eq. (1) can also be expressed as Vo=e

hc

-1 (2)

Therefore, the value hc/e and hence the Planck constant, h can be obtained from the slope

of Vo--1

curve which can be obtained from the Vovalues obtained for each LED from its

V-I plots. The V-I plot for four different LED's is obtained and the Vo--1curve is

obtained from the barrier potential Voobtained from V-I plot of each LED. If the linear

portion of the V-I plot is extrapolated back to x-axis the intercept represents the barrier

potential (the potential above which, I becomes independent of V).

Circuit: The LED circuit is shown in Fig.

3 and it consists of 5V supply; a ten turn

potentiometer to vary voltage across the

LED from 0 to 5 V that is measured

using voltmeter and an ammeter to

measure current through the LED. A 33

k resistor is connected in series with

the LED (find out why).


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Procedure:

1. Connect an LED (you have been given four LEDs) to the jack provided on the

front panel and switch on the unit.

2. Vary the voltage V (decide the appropriate step size) across the LED and note the

corresponding current I and tabulate as shown in Table 1 for V-I characteristic of

LED.

3. Repeat step 2 for the remaining LEDs.

4. Plot the V (along x-axis)-I (along y-axis) characteristics of all the LEDs on a

single graph sheet and obtain the Vofor each LED

5. Enter the values in Table 2.

6. Plot a graph of voltage Vo versus 1/ for LEDs of different wavelength and

determine the slope (=hc/e) of the line. Calculate h using standard values of c ande.

7. Calculate the slope also using least square fit method:

=

22 )( ii

iiii

xxN

yxyxNslope where N is the number of points. From the slope

obtain the value of h.


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Table-1 : I-V Characteristics of 4 different LEDs

Colour Colour Colour Colour

Sl.

No.

V(volts)

I

(mA)V

(volts)I

(mA)V

(volts)I

(mA)V

(volts)I

(mA)


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Table 2 : Barrier Potential Vs. inverse of wavelength of the photons emitted.

Sl.No.

LEDcolor

Wavelength

(nm)1/

(nm -1)

Barrier Potential

Vo

Slope of the 1/- Vocurve (hc/e) : S =

C=

h=Coefficient ( c/e) :

Planck Constant S/C :

Result: The value of Plancks constant was found to be _____________________

Conclusions

Hint for Error Analysis: Estimate the error in V at which I become nearly independent

of V.

Questions

1.

Suggest a method to measure h without using the value of any other universal

constant.

2. Identify the sources of systematic errors and random errors

3. Speculate the consequences if h were zero in our universe.

4. How is h useful to engineers?

5. Can you make LED like device using metals?

6. Can we think of LED as a device which annihilates mass (electron) to give energy

(photon)?

7. Principle explained above is rather over-simplified. Please look to a textbook on

semiconductor devices (Streetman, S. M Ze) to obtain more information on

energy diagram of LED.

modern phy 101 all exp

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