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JIMS-TEC, Greater Noida Engineering Physics Lab Manual

Applied Physics Lab 1

JIMS – TEC

Greater Noida

Engineering Physics Lab

Manual

B.Tech. 1st year

Faculty Name: Mr. Nafees Udin &Ms. Jyoti Bala



EXPERIMENT 1

AIM: To determine numerical aperture of an optical fiber.

APPARATUS: A fiber optic, 20X microscopic objective, He-Ne laser, screen, graph paper and

measuring scale

THEORY: Numerical aperture accounts for the ligth gathering ability of the fiber and it amounts the light

accepted by the fiber. NA refers to the maximum angle at which the light incident on the fiber end is

totally internally reflected and is transmitted properly along the fiber. NA depends only on the refractive

indices of the material of the core and the cladding. Mathematically, NA is defined as a sine of angle of

acceptance. Thus if θo is the angle of acceptance then:

NA = sin (θ0)

or

NA = (n12 – n2

2)1/2

/n0

NA is also called the figure of merit of optical fiber. The cone formed by the rotation of angle of

acceptance along the axis of the fiber is the cone of acceptance of the fiber. The light should strike the

fiber end within its cone of acceptance else it is refracted out of the fiber.

Fig.1.

For short length of straight fiber, light ray incident at angle θ at the input end and must come out at same

angle from out put end. So, at the output end of the fiber a cone of the same angle appears. Hence NA can

be determined by making measurements on the cone at the output end of fiber.

NA = D / (4L2 + D

2)1/2

where D = diameter of circular spot formed on the screen

L = distance between output end of the fiber and screen

Acceptance

angle Fig.1



PROCEDURE:

Fig.2.

1. Connect the power to the source.

2. Connect one end of the fiber cable to the output socket of the source through microscopic objective and

other end to the numerical aperture measurement jig as shown in Fig.2.

3. Hold the white screen facing fiber such that its cut face is perpendicular to the axis of the fiber.

4. Record the distance (L) of the screen from the fiber end and the diameter (D) of the spot.

5. Compute NA from the formula given above.

6. Repeat step 4 and 5 with varying distance.

7. Calculate the NA for each value of L and take the mean.

OBSERVATIONS & CALCULATIONS:

S.No Distance between output end of

the fiber and screen (L) cm

Diameter of circular spot

formed on the screen (D) cm NA = D / (4L

2 + D

2)

1/2

1

….

7

RESULT: Mean Numerical aperture of Optical fiber = ………………



PRECAUTIONS:

1. Optical source should be properly aligned with the cable.

2. Distance of the launch point from cable should be properly selected to ensure that maximum amount of

optical power is transferred to the cable.

3. The optical fiber provided should be handled carefully so as to prevent cracks.



Fig.1

EXPERIMENT 2

AIM: To find the refractive index of a material using spectrometer.

APPARATUS: Spectrometer, spirit level, glass prism, sodium lamp, magnifying glass with light

THEORY:

a) The spectrometer: Spectrometer consists of collimator (c) telescope (T), prism table (p) and vernier

table as shown in fig 1. The collimator consists of a convex lens fixed at one end and a slit of adjustable

width on the other end. The telescope consists of an objective (O) at one end and an eye piece fixed with

the cross wires on the other end. The prism table consists of two circular disc connected by three leveling

screws. The vernier tables have two verniers VA and VB each having main scale and vernier scale.

Here one main scale division is equal to half a degree. Each vernier scale has 30 divisions, which is equal

to 29 main scale divisions.

b) Refractive Index: If A is the angle of the prism and δm is the angle of minimum deviation at a

particular wavelength of light then, for that wavelength, the refractive index, μ, is given by



2sin

2

)sin(

A

A m

PROCEDURE:

a) Adjustment of the spectrometer:

1. Switch on the sodium source and allow it to warm up.

2. Remove the prism from the turntable.

3. Turn the instrument until you can point the telescope at a distant object, e.g. the far end of the

room.

4. Adjust the eyepiece (it slides in and out) until the crosswires are in focus.

5. Focus the telescope on the distant object. Once this has been done the telescope focussing

adjustment must not be touched again.

6. Now put the spectrometer back into the position in which you are going to use it. Make sure that it

is positioned so that you can see through the telescope when it is at least 60o on either side of the

line of the collimator.

7. Move the sodium source close to the slit at the end of the collimator.

8. With the telescope in line with the collimator pick up the image of the slit and now adjust the

collimator only to bring the image of the slit into focus.

9. Finally adjust the slit width until its image is just a little wider than the cross wire.

b) The angle of the prism:

The initial adjustments of the spectrometer are made. The

silt is illuminated by yellow light from the sodium vapour

lamp. The given prism is mounted on the prism table such

that light emerging from the collimator should be made to

incident on both the refracting faces of the prism as shown

in fig 2. The telescope is rotated (left or right) to catch the

image of the slit reflected by one of the refracting face of

the prism. The telescope is fixed. By adjusting the

tangential screw, the image is made to coincide with the

vertical cross wire. The main scale and the vernier scale



readings are noted from both the vernier 1 (V1) and vernier 2 (V2). Similarly readings are taken for the

image reflected by other refracting face of the prism. The difference between the two readings gives 2A,

where ‘A’ is the angle of prism.

a) The refractive index of glass:

The prism is mounted such that light emerging from the

collimator is incident on one of the refracting face of the

prism. The telescope is slowly rotated to catch the refracted

image which emerges from other refracting face of the

prism. Now by viewing through the telescope the prism

table is slightly rotated in such a way that the slit image

moves towards the direct ray and at a particular position it retraces its original path. This position is called

MINIMUM DEVIATION POSITION (fig.3). The prism table is fixed and adjusted to coincide the image

with the vertical cross wire and the readings are noted. The prism is removed and the direct ray reading is

noted. The difference between the direct ray and the refracted ray readings gives the angle of minimum

deviation (δm). Then, by substituting the values of δm and ‘A’ in the given formula, the refractive indices

(μ) for prism can be calculated.


a) Least count

LC = 1 MSD – 1 VSD

1 MSD = ½ degree

30 VSD = 29 MSD

1 VSD = 29/30 x ½ degree

1 VSD = 29/60 degree

LC = (1/2-29/60) degree = (30-29)/60 degree= 1/60 degree

Since 60’ = 1° › 1’ = 1°/60

LC = 1’

Angle of prism, A Angle of minimum deviation, δm

Vernier 1 Vernier 2 Vernier 1 Vernier 2

θ1 = θ'1 = θD1 = θ'D1 =



θ2 = θ'2 = θD2 = θ'D2=

A = A= δm = δm=

Mean A = Mean δm =

Refractive index of the material of prism,

2sin

2

)sin(

A

A m

RESULT:

Angle of prism (A) =

Angle of minimum deviation, δm =

Refractive index of the material of prism, μ =

Note: Enter all the readings in degrees and minutes. For example, 18o 30’ and not 18.5

o.

PRECAUTIONS:

1) The slit should be very narrow.

2) The prism should be clean.

3) The center of the source of the light, the center of the slit, and the center of the collimator lens

should be in the same line.

4) The prism should be in exact position of minimum deviation.



Experiment 3

AIM: -

To determine the specific rotation of cane sugar solution with the help of Polarimeter.

Apparatus: -

Polari meter, a balance, measuring cylinder, beaker, source of light and Polarimeter tube.

Formula used: - The specific rotation of the plane of polarization of sugar dissolved in water can be

determined by the following formula.

S = (θ X V ) / (m X l )

Where, θ= rotation produced in degrees.

l = length of tube in decimetre.

m = mass of sugar in gms dissolved in water.

V = volume of sugar solution.

Procedure: -

1. If the polarimeter is employing a half shade device, a monochromatic source should be used and if bi

quartz device is used then white light can be used.

2. Take the polarimeter tube and clean well both the sides such that it is free from dust. Now fill the tube

with pure water and see that no air bubble is enclosed it. Place the tube in its position inside the

Polarimeter.

3. Switch on the source of light and look through the eyepiece. Two halves of unequal intensity is

observed. Rotate the analyzer until two halves of the field appears equal bright. Take the reading of main

scale as well as vernier scale and find out the total reading.

4. Prepare the sugar solution of unknown strength. The procedure for preparing it can be seen under the

heading observations.



5. Take the polarimeter tube and remove the pure water and fill it with the prepared sugar solution and

again place it in the polarimeter.

6. Rotate the analyzer to obtain the equal intensity position, first in clockwise direction(I reading) and

then in anti-clockwise direction(II Reading). [When the tube containing sugar solution is placed in the

path of the polarized light, the plane of polarization is rotated which disturbs the previous position.] Note

down the position of the analyzer on main and vernier scales in the two directions. Find the mean reading.

The difference between this and previous reading gives the specific rotation.

7. Repeat the experiment with the sugar solutions of different concentrations.

8. Measure the length of the tube in centimeters and change it in decimeters.

Observation:

1. Least count of polarimeter = 0.1 deg = 6 minute

2 . Length of polarimeter tube = …

Analyzer reading with pure water

I Reading II Reading

A= (X+Y)/2 deg

M.S. V.S. Total

X deg

M.S. V.S. Total

X deg

Concent

ra-tion of

sugar

solution

C= m/V

Analyzer reading with sugar solution

I Reading II Reading

B =

(X'+Y')/2

deg

θ=A~B M.S. V.S.

Total

X deg M.S. V.S.

Total

X deg

Calculations: -

S = (θ X V )/ (m X l ) = ---------------deg/dm/kg/m3



Result: -

The specific rotation for cane sugar at a room temperature using monochromotic light is ----------

Percentage error:

% ERROR = (Standard value~ Observed Value) X 100 / Standard Value

Precaution:-

1. The polarimeter tube should be well cleaned.

2. Whenever solution is changed, rinse the tube with the new solution under examination.

3. The position of analyzer should be set accurately.

4. The temperature and wavelength of light used should be stated.

5. Reading should be taken when halves of the field of view becomes equally illuminate.



Experiment 4

AIM: -

To determine the wavelength of Monochromatic light (sodium light) by Newton’s Ring.

Apparatus: -

A Plano-convex lens of large radius of curvature, optically arrangement for Newton’s rings, plane glass

plate, sodium lamp and traveling microscope.

Theory:

The optical arrangement for Newton’s Ring is shown in Fig 1. A wedge shape air film in formal between

Plano-convex lens and glass plate. Interference take place between light reflected from concave surface of

lens and upper surface the plate as shown in Fig 2.

In this Experiment the Fringes are of equal thickness i.e. why fringes are circular as shown in Fig 3. In

this experiment path difference between reflected rays from lens and plate is 2 µt . Locus of points

having the same thickness then full on a circle having. Its center at the point of contact. Thus, the

thickness of the film is the same at all points on any circle having 0 as the center of the fringes one

therefore circular as shown in Fig 3..



Figure 3

Formula used: -

The wavelength of Sodium light is given by,

λ = D 2

n+p - D 2

n / 4 .P. R

Where,

Dn+p= diameter of (n+p)th ring

Dn = diameter of nth ring,

p = an integer number,

R = radius of curvature of the curved face of the plano‐ convex lens.



Procedure: -

1. If a point source is used only then we require a convex lens otherwise using an extended source, the

convex lens is not required.

2. Before starting the experiment, the glass platesG1, G2 & the Plano convex should be thoroughly

cleaned.

3. The center of lens L2 is well illuminated by adjusting the inclination of glass plate G1 at 45 deg.

4. Focus the eyepiece on the crosswire and move the microscope in the vertical plane by means of rack &

pinion arrangement till the rings are quite distinct clamp the microscope in the vertical scale.

5. According to the theory, the center of the interference fringes should be dark but sometimes the center

appears white, this is due to the presence of dust particles between glass plate G2 and Plano convex lens

L2.in this case lens should be again cleaned.

6. More the microscope in a horizontal direction to one side of the fringes. Fix up the cross wire tangent

to the ring and note this reading. Again the microscope is moved in the horizontal plane and the crosswire

is fixed tangentially to the successive bright fringes noting the vernier reading till the other side is

reached.

Observations: -

Least count of the travelling microscope = 0.01mm

Radius of curvature=….

No

of

ring

Microscope

reading

Diameter

= (a-b)

Mm

D2

= (a-b) 2

D2

n+p–

D2

n

Mean

P

λ =

D 2

n+p - D 2

n

4 .P. R

Left

end

(a)mm

Right

end

(b) mm

2

4

6

8

10

12



Calculations:-

The wavelength of sodium light is given by-

λ = D 2

n+p - D 2

n / 4 .P. R

Result: - The mean wavelength of sodium light = --------------------Å

Percentage error

% ERROR = (Standard value~ Observed Value) X 100 / Standard Value



EXPERIMENT NO. 5

AIM:

To determine the wavelength of sodium light source using plane transmission grating

Apparatus required: A diffraction grating, spectrometer, sodium lamp, reading lens and sprit level.

Formula used:

The wavelength λ of any spectral lines can be calculated by the formula:

(a+b) sin θ = n λ

λ = (a+b) sin θ / n

Where,

(a+b) = grating element,

θ = angle of diffraction and

n = order of the spectrum

Procedure: The following initial adjustments of the spectrometer and the grating are made first.

1. The spectrometer and the prism table are arranged in horizontal position by using the leveling

screws.

2. The telescope is turned towards a distant object to receive a clear and sharp image.

3. The slit is illuminated by a mercury vapour lamp and the slit and the collimator are

suitably adjusted to receive a narrow, vertical image of the slit.

4. The telescope is turned to receive the direct ray, so that the vertical slit coincides with

the vertical crosswire. The readings of one vernier are noted. The vernier table is firmly clamped.

5. Now, the telescope is rotated through 90° and is fixed in this position. The grating is mounted

vertically on the prism table with its ruled surface facing the collimator. The vernier table

I released and is slowly rotated till the reflected image coincides with the vertical crosswire.

6. The leveling screws are adjusted so that the image is at the centre of the field of view of

the

telescope. The prism table is fixed and after making fine adjustments with the tangential crew,

the reading of the vernier are noted. Now, the angle of incidence is 45° (Figure 1).

7. The vernier table is then released and rotated exactly through 45° (or 135°) in the proper

direction so that the surface of the grating becomes normal to the incident light. The

vernier table is firmly clamped in this position.

8. The telescope is then released and is brought to observe the direct image.



(B) Measurement of angles of diffraction for yellow light source.

1. The spectrum obtained in a grating is shown in Figure 2.

2. Rotate the telescope to the left side of direct image and adjust the yellow spectral line

on the vertical cross wire for first order. Note down the reading of

both the verniers in each setting.

3. Now rotate the telescope to the right and repeat the above procedure for first order.

4. Find out the difference of the same kind of verniers for each spectral line in the first order.

The angle is twice the angle of diffraction for that particular colour.

Half of it will be angle of diffraction.

Figure 1: Setting diffraction grating normal to the incident light.

Figure 2: Orders and spectrum obtain visible through the diffraction grating.



Observations:

No. of rulings per inch on the grating, N = ………

Least count of spectrometer = ……….

Table for determination of angles of diffraction:

Order of

spectrum

n=

Colour Vernier

Telescope reading of spectrum on Angle of

diffraction θ

= (a – b)/2

Mean

θ

Left side Right side

MSR VSR TR

(a) MSR VSR

TR

(b)

Yellow V1

V2

MSR = Main Scale Reading, VSR = Vernier Scale Reading,

TR = MSR+VSR = Total Reading.

Calculations:

Grating element, (a+b)= 2.54/N……..cm-1

.

Where, N is number of ruling per inch on the grating.

The wavelength of various spectral lines in the first order (n= 1) can be calculated by

λ = (a+b) sin θ

λ yellow = ........ Ao ,

Result:

Colour of spectral

line

Calculated

wavelength

Standard

wavelength % Error

Yellow 5770 Å, 5791 Å



Experiment No 6

Object: To verify Inverse square law of light using a photo cell.

Apparatus Required: Optical bench, Photocell housed with Red and Black sockets, lamp house with

lamps, DC Microammeter.

Theory: The Photoelectric emission may be regarded as a phenomena of liberation of an electron at the

surface of a metal when a photon of light having energy above threshold energy (metal work function)

incident on a metallic surface and transfer the enough energy to the electron to escape through the

potential barrier layer. The photo cell can be considered as the generation of a voltage across a circuit

element under illumination.

Let P be the illuminating Power of a source so the intensity of illumination I due to it at a distance r would

be

I= P/r2

Since the photo electric current (θp) produced is directly proportional to the intensity of illumination ie.

I α θp

I=Kθp

Where K is constant, hence I= P/r2 = Kθp. Since P and K are constant hence the relationship between 1/r

2

and θp is straight line which verifies the inverse square law of radiation.

Procedure:

1. Arrange the optical bench in such a way that both the lamp and the photo cell are at the same level as

shown in the figure.

2. Make the connection of photo cell to (+)ve and (–)ve terminal of the microammeter.

3. Adjust the distance of the lamp such that we will get the microammeter reading

4. Then decreases the distance in step of 5cm and each time note the reading in microammeter and note

your observation in table-1.

5. Draw the curve between 1/r2

and d.



Observation table:

Sr. No. Distance of lamp from

cell in cm ‘r’

1/r2

Reading in

microammeter

(θp) uA

Result:

The graph between θp and 1/r2 is a straight line. It show that microammeter reading is inversely

proportional to the square of the distance from the source. but deflection is directly proportional to the

intensity of illumination of the surface. Hence we can say that intensity of illumination varies inversely

square of the distance from the source. Thus inverse square law is verified.

Precautions:

1. Light should fall on normally on the photocell

2. The photocell should not be exposed to light for a long time continuously.

3. A Cover should be placed on the photocell to protect it



Experiment 7

Objective:

To determine the value of Planck’s constant using photoelectric effect.

Apparatus

Vacuum photo tube, light source, color filters, regulated voltage power supply, voltmeter, ammeter etc.

Theory:

The phenomenon of emission of electrons from the surface of a material when light falls on it, is known

as the photo electric effect. The emitted electrons are called photoelectrons. A typical experimental setup

for observing the photoelectric effect is shown in Fig. 1.

Figure 1

Light falls on a target metal plate T enclosed in a vacuum tube and as a result electron are ejected from

the surface of the plate. When the ejected electrons reach the collector electrode C placed opposite to T,

an electric current, called photocurrent flows through the circuit.

This photocurrent can be measured by an ammeter connected to the circuit. The kinetic energies of the

emitted electrons can be estimated by applying a negative potential to the collector C and tuning the

potential such that it is just enough to prevent the electrons from reaching the collector.



This negative potential, V0 to C at which the photocurrent becomes zero, is called the stopping potential.

eV0 =Kmax (1)

Where e is the electronic charge and Kmax is the maximum kinetic energy of the emitted electrons.

The photoelectric effect was first explained by Einstein on the basis of quantum theory of light. According

to it, light i.e. electromagnetic radiation consists of discrete energy packets or energy quanta. Each energy

packet behaves as particle and is called photon. The energy of a photon is given by,

E =h f (2)

where h is called Planck’s constant and f is the frequency of radiation.

When the photons falls on metal surface, an electron can absorb the energy of a photon and acquire

enough energy to escape the surface potential barrier φ (also called work function). The maximum kinetic

energy with which the electron can eject out , according to the principle of energy conservation, is given

by

Kmax =hf −φ (3)

Since

Kmax =eV0,

the relationship can also be written as

eV0 =hf −φ (4)

Let for two different frequency f1 and f2 , stopping potential is V1 and V2. Such that

eV1 =h f1 −φ (5)

eV2 =h f2 −φ (6)

using (5) and (6) equation we get,

eV2 - eV1 = (h f2 −φ) - (h f1 −φ )

eV2 - eV1 = (h f2 −h f1 )

e (V2 - V1 ) = h ( f2 − f1 )

h = e (V2 - V1 ) /( f2 − f1 )



Procedure:

1. Take the Planck’s constant setup & fix the photo vacuum tube at particular position.

2. Complete the circuit connections as shown in the diagram in the panel of the unit.

3. Set the range of the DC voltmeter at 200 mV and the ammeter at 2 µA.

4. Connect the mains cord and switch on the power supply and light source. Now you can observe some

value of current on ammeter.

5. First insert the ‘red’ color filter in front of photo vacuum tube.

6. If the observed current is too low, then slide the photo vacuum tube towards light source till you get

some appreciable current. Fix the photo tube at this distance (position 1).

7. Switch on the DC voltage source.

8. Now vary the DC voltage slowly by variable resistance pot and see the value of current. It should

decrease as the voltage is increased.

9. When the current becomes zero, note the value of applied voltage by DC voltmeter. This is the stopping

potential, V0 for the given color.

10. Switch off the DC voltage source.

11. Repeat steps 7-10 for the other color filters, e.g. orange, yellow, green and blue respectively, keeping

the position of the photo vacuum tube fixed.

12. Tabulate all the readings as indicated in Table 1.

Observations :

Velocity of light= c =3×108 ms−1

.

Sr.No. Color of filter

Wavelength, λ nm

Frequency,

f =c/λ

Stopping

potential, V0

1 Red 620-750

2 Yellow 570-590

3 Orange 590-620

4 Green 495-570

5 Blue 450-495



Calculation:

Value of e =1.6×10-19

C.

h = e (V2 - V1 ) /( f2 − f1 )

=…..

Result:

Value of Planck’s constant from the experiment, h=···.



P1

P2

O

P׳1

P׳2

Experiment 8

AIM: To study the laser beam characteristics like; wave length using diffraction grating aperture &

divergence.

APPARATUS: A diffraction grating with known grating element, He-Ne laser, screen, graph paper and

measuring scale.

THEORY: (a) Wavelength using diffraction grating:

A diffraction grating is an extremely useful device and it consists of a very large number of narrow slits

side by side. The slits are separated by opaque spaces. When a wave front is incident on a grating surface,

light is transmitted through the slits and obstructed by the opaque portions. The secondary waves from the

positions of the slits interfere with one another.

For normal illumination the grating equation is given by,

dsin = n

where, is the angle of diffraction,

d is grating element

is wavelength of light incident on grating

Fig. 1 Fig. 2

PROCEDURE:

1. Arrange the apparatus as shown in Fig. 1

2. Fix the graph paper on the screen and place it at appreciable distance D from grating so that

distinct maxima are seen, as in Fig.2.

3. Mark the central brightest spot and first three maxima on both sides of central maxima.

Diffraction

grating Screen

O

P1

P2

1P

2P

D

O

2

1 He Ne Laser



4. Using a scale find the distance between first maxima on either side of central spot. Mean distance

of first maxima from centre of screen is half of this separation.

5. Find the sine of angle of diffraction 1 by ,

11

2 2

1

OPSin

OP D

6. Repeat the steps 4 and 5 for 2nd

and 3rd

maxima. Then use these values to calculate wavelength of

LASER (λ) from equation (1).


(a) Wavelength using diffraction grating:

1. Number of lines per inch on the grating, N=…………..

2. Grating element, d= 2.54/N= ……………cm

3. Distance between diffraction grating and screen, D = --------------- cm

4. Table for wavelength of LASER , λ :

RESULT: (a) Mean wavelength of He-Ne LASER = ………………cm

PRECAUTIONS:

1. Laser light should not fall on eyes of observer directly.

2. All lengths should be measured in same unit.

3. Distance between the spots should be measured accurately.

4. The diameter size to begin with should be about 5mm.

S.No. Order of

diffraction,

n

Separation between

respective maxima

on either side of

central maxima (cm)

Position

of nth

maxima,

OPn (cm)

11

2 2

1

OPSin

OP D

Wavelength,

λ = dsinθn/n

(cm)

1.

2.

3.

jims tec - passhojao · 4) the prism should be in exact position of minimum deviation. jims-tec,...

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