3. sifat cahaya.pdf

7/24/2019 3. Sifat Cahaya.pdf

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Sifat Cahaya


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What is Light ?

Light is a form of electromagnetic energy detectedthrough its effects, e.g. heating of illuminated objects,conversion of light to current, mechanical pressure(Maxwell force) etc.

Light energy is conveyed through particles: photons

ballistic behavior, e.g. shadows

Light energy is conveyed through waves

wave behavior, e.g. interference, diffraction

Quantum mechanics reconciles the two points of view,through the wave/particle duality assertion


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Particle properties of light

Photon = elementary light particle

Mass = 0

Speed (c) = 3 108m/sec

According to Special Relativity, a mass-less particle

travelling at light speed can still carry momentum!

Energy E = h relates the dual particle

& wave nature of light;

h : Plancks constant = 6.6262 10-34 J sec

: the temporal oscillation frequency of the light waves


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The sinusoidal (in space) wave

One wavelength takes 2 radians

Described by

A=Amaxcos(2 x/ )

A

x

A

A=Amaxsin(2

x/ )

A

A=Amaxcos(2 x/ )

A

A=Amaxsin(2

x/ )


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The sinusoidal (in time) wave

A cycle completes one period in 2 radians

Described by

A=Amaxcos(2 t/T+ )A=Amaxsin(2 t/T+ )

A

t

T


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Wave vocabulary

wavelength = distance per cycle

wave number k= radians per distance

2 / = rad/cycle (cycle/m) = rad/m = k

period T= time per cycle angular frequency = radians per second

2 /T = rad/cycle (cycle/s) = rad/s =

frequencyf= number of cycles per second

f= cycles/second = 1/(second/cycles) = 1/T

Speed v= distance per time; wave travels in T

v= /T


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The traveling wave A wave varies in both space and time:

At one location, the amplitude varies in time

At one time, the amplitude varies in space

A sinusoidal wave moving toward positive x isdescribed by

A =Amaxcos(kx t+ )

A sinusoidal wave moving toward negative x isdescribed by

A =Amaxcos(kx+ t+ )


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8

Wave properties of light

: wavelength

(spatial period)

k=2/

wavenumber

: temporal

frequency

=2

angular

frequency

E: electric

field


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Light In Matter

E.g. vacuum n=1, air n 1;

glass n1.5; glass fiber has 0.25dB/km=0.0288/km


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Overview of light sources

non-Laser

Thermal: polychromatic,

spatially incoherent

(e.g. light bulb)

Gas discharge: monochromatic,

spatially incoherent

(e.g. Na lamp)

Light emitting diodes (LEDs):

monochromatic, spatially

incoherent

Laser

Continuous wave (or cw):

strictly monochromatic,

spatially coherent

(e.g. HeNe, Ar+, laser diodes)

Pulsed: quasi-

monochromatic,spatially coherent

(e.g. Q-switched, mode-locked)


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Monochromatic, spatially coherent

light nice, regular sinusoid , well defined

stabilized HeNe laser

good approximation

most other cw lasers

rough approximation

pulsed lasers & nonlaser

sources need

more complicateddescription

Incoherent: random, irregular waveform


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The concept of a monochromatic

rayt=0

(frozen)

direction of

energy propagation:light ray

wavefronts

In homogeneous media,

light propagates in rectilinear paths


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The concept of a monochromatic

rayt=t

(advanced)

direction of

energy propagation:light ray

wavefronts




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Spring 2008 14

Colors Light is characterized by frequency, or more commonly, by

wavelength

Visible light spans from 400 nm to 700 nm or 0.4 m to 0.7 m; 0.0004 mm to 0.0007 mm, etc.


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Additive Colors

Red, Green, and Blue lightsources can be used tosynthesize almost anyperceivable color

Red + Green = Yellow

Red + Blue = Magenta Green + Blue = Cyan

These three dual-sourcecolors become theprimary colors for

subtraction why? because absence of

green is magenta

absence of red is cyan, etc.


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Spring 2008 16

Introduction to Spectra

We can make a spectrum out of light,dissecting its constituent colors

A prism is one way to do this

A diffraction grating also does the job

The spectrum represents the wavelength-by-wavelength content of light

can represent this in a color graphic like that above

or can plot intensity vs. wavelength


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The concept of a polychromatic

rayt=0(frozen)

wavefronts

energy from

pretty much

all wavelengthspropagates along

the ray




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Spring 2008 18

White light White light is the combination of all wavelengths,

with equal representation red hot poker has much more red than blue light

experiment: red, green, and blue light bulbs makewhite

RGB monitor combines these colors to display white

blue light green light red lightwavelength

combined, white light

called additive color

combinationworks

with light sources


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Fermat principle

is chosen to minimize this

path integral, compared to

alternative paths

(aka minimum path principle)

Consequences: law of reflection, law of refraction


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The law of reflection

a) Consider virtual source P

instead of P

b) Alternative path POP is

longer than POP

c) Therefore, light follows

the

symmetric path POP.

mirror


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The law of refraction

reflected

incident

reflected

Snells Law of Refraction


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Total Internal Reflection (TIR)

becomes imaginary when

refracted beam disappears, all energy is reflected


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23

Prisms

airair

air air

air air

glass

glass


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Dispersion

Refractive index n is function of the wavelength

white light

(all visible

wavelengths)

Newtons prism

air glass

red

gree

nblue


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Frustrated Total Internal Reflection

(FTIR)

Reflected rays are missing

where index-matched surfaces

Touch shadow is formed

Angle of incidence

exceeds critical

angle

glass other

material

air gap


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Fingerprint sensor

air

finger

glass | air glass | finger

TIR occurs TIR does not occurs

(FTIR)


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Optical waveguide

Planar version: integrated optics

Cylindrically symmetric version: fiber optics

Permit the creation of light chips and light cables, respectively,

wherelight is guided around with few restrictions

Materials research has yielded glasses with very low losses

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