Light Waves and Color

Metro Community College04/21/23

Electromagnetic Waves• Maxwell predicted that

an electric dipole would transmit changing electric and magnetic fields if a changing current was sent back and forth along the dipole.

• His equations proposed that light traveled at the same speed in all reference frames.

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What is an electromagnetic wave? • Electromagnetic waves are transverse electric field

waves in phase with magnetic field waves that travel away from the source at the speed of light in the medium.

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What is the speed of an electromagnetic wave?

• The speed of light, c is 3 X 10-8 m/s. the speed of light in medium is c/n where n is the index of refraction of the material.

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What is the frequency of an electromagnetic wave?

• The speed of light for our calculations is 3 X 108 m/s.

• The formula is v = f• The formula for the

energy of an electro-magnetic quantum is E = hf where h = 6.626X10-34 J-s

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Wavelength and Color

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Transmission and Reception

Transmission• Long wave – dipole• AM Radio – AM transmitter

varying the amplitude by changing energy

• FM Radio – FM transmitter varying the frequency of each wave.

• TV – Medium is changing• Infrared-light-ultraviolet –

atom emission.

Reception• Radio wave – dipole• AM receiver – to speaker,

current varies with amplitude of wave.

• FM receiver – various frequency waves become speaker frequencies

• TV Medium is changing• Diodes in eye, cameras.

Chemical changes in film.

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Transmission and Reception

Transmission• Ultraviolet – Atomic and

accelerated electron emissions

• X-rays – emissions by accelerating electrons

• Gamma rays – emissions by nuclei

Reception• Molecules with proper

energy levels, skin cells, and etc.

• Radiographic Film, photo sensors

• Electron collisions in material

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Eye Perception of Light

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Reflection of Light

Types of Reflection• Diffuse• Specular• Diffraction• Thin Film

Images differ from each one type of reflection.

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Transmission of Light

Types of transmission• Diffuse

– Scattering– Frosted glass

• Direct – clear glass• Diffraction

• single slit

• Interference– – double slit– grating

• Thin film

Examples

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Transmission of Light

Types of transmission• Diffuse

– Scattering– Frosted glass

• Direct – clear glass• Diffraction

• single slit

• Interference– – double slit– grating

• Thin film

Examples

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Interference of Light Waves• The bright lines occur

where the light is in phase and the path difference is one wavelength.

• The dark lines occur where the light is in phase and the path difference is one-half wavelength.

• Path difference = d(y/x)• = d(y/x) => bright• /2 = d(y/x) => dark

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Diffraction and Gratings

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y/x = (w/2)//2

Polarized Light• Polarized Light – has the electric field set in a

particular orientation. A second polarized can block the electric field completely.

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Color MixingSubtraction – Pigment mixing subtracts. Total subtraction gives black.

Addition – light mixing results in the addition of light. Total addition results in white

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Scattering

Noon sun has less dust to come through, so it is yellow.

Sunset sun is red because it passes through the most dust and all long wavelengths are scattered

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The short wavelength light (near the blue end of the spectrum) is scattered from the dust in the air.

Birefringence• Certain materials polarize

light passing through them in proportion to the stress on the material.

• Stress test – by putting the materials between crossed polarizing lenses. Light that is polarized again by the material will pass through.

• Since the amount of polarization depends on the wavelength the light that passes through appears in different colors.

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Summary

• Electromagnetic waves• Wavelength and color• Interference of light waves• Diffraction and gratings• Polarized light

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