Light

Radiation and Spectra

Chapter 5

What is Light?

• Newton– Prism shows white light contains all colors– Light made of particles (photons)

• Maxwell– Theory of electricity and magnetism– Light is electromagnetic waves

• Produced by wiggling electrons• Radiation = production of light

• Quantum Mechanics– Light is both: particle and wave

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Waves• Wavelength ( )

– Distance between crests (or troughs)

• Frequency ( f )– How often it repeats (wiggles up and down)

• Measured in Hertz (Hz)– number of times per sec

Waves• Speed c = 3 x 108 m/s

c = f• Wavelength inversely related to frequency

= c / f– high frequency = short wavelength

– low frequency = long wavelength

Particles as Waves

• “Wave Packet”– particle/photon = localized wave

Properties of Light

• Color– Depends on frequency

• blue = high frequency = short wavelength

• red = low frequency = long wavelength

• Carries energy (heat)– Photon energy

E = h f

• high frequency = high energy = blue

• low frequency = low energy = red

h = Planck’s constant

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Red light has ____ than blue light.

A. larger frequency, energy, and wavelengthB. smaller frequency, energy, and wavelengthC. larger frequency and energy, but smaller wavelengthD. smaller frequency and energy, but larger wavelength

Which of the following travels fastest?

A. radio waves B. infrared (heat) wavesC. microwavesD. blue light wavesE. none of the above

} All are types oflight!

All types of light travel at the same speed - the “speed of light”, c

Propagation of Light• Photons travel in straight lines

– energy spread over larger area at larger distances– produces 1/r2 decrease in brightness

• Double distance - brightness decreases by 4

If a 100-watt light bulb is placed 10 feet away fromyou, and an identical 100-watt light bulb is placed 100 feet away from you, which will appear brighter?

A. The closer oneB. The farther oneC. They will appear the same brightness

How much fainter will the far one appear compared to the close one?

A. Twice as faintB. 10 times fainterC. 100 times fainterD. 1000 times fainter

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~ 1/r2

Electromagnetic Spectrum

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• Visible light:– red, orange, yellow, green, blue,

indigo, violet (ROYGBIV)

• Invisible Light:– Ultraviolet = bluer than blue

– Infrared = redder than red

– Other wavelengths:• Short: X-rays, gamma-rays

• Long: microwave, radio

Thermal Radiation• All objects radiate (thermal radiation)

– Objects made of atoms– Atoms (and their electrons) vibrate

• Wiggling electrons radiate, producing light

– Bigger objects produce more light– Higher temperature = stronger vibration

• Hotter objects emit more light

• Perfect absorber is black– Absorbed light (energy) heats object– Temperature increases until

emitted energy = absorbed energy

– Emitted radiation called Blackbody Radiation

• Thermal radiation emitted by most objects similar to blackbody

Blackbody Radiation Laws

• Luminosity, L L = energy emitted per second

• Luminosity for a spherical object (a star)L = 4R2 T4

R = radius (size) of star; T = temperature– double size, luminosity increases by 2x2 = 4– double temperature; luminosity increases by

2x2x2x2 = 16

Stefan-Boltzmann Law

WORKBOOK EXERCISE:Luminosity Temperature and Size

Part I(pages 33-35 in workbook)

Blackbody Radiation

Blackbody Radiation Laws

• Color– Wavelength where most light emitted

max = 3 x 106 / T

T in Kelvin; max in nanometers (1 nm=10-9m)

• Cool stars are red

• Hot stars are blue

– Color indicates temperature!

Wien’s Law

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As T , Wavelength , Color = redder As T , Wavelength , Color = bluer

Homework: WORKBOOK EXERCISEBlackbody Radiation

(pages 37-40 in workbook)

The graph above shows blackbody spectra for three different stars. Which of the stars is at the highest temperature?

A. Star AB. Star BC. Star C

Because peak energy emissionoccurs at shortest wavelength

Doppler Shift

• Originally discovered using sound waves• Moving object

– emits light with slightly different color

• Frequency (pitch) of approaching object is higher– Blueshift

• Wavelength shorter (shifted blueward)

• Frequency (pitch) of receeding object is lower– Redshift

• Wavelength longer (shifted redward)

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Doppler Shift

Blueshift

Redshift

Spectroscopy

• Prism separates light into different colors– Continuous spectrum

• contains all colors

• Example: blackbody spectrum

Spectroscopy

Absorption Line Spectrum– Some colors are

missing (discrete lines)

N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

SolarSpectrum

Spectroscopy– Emission Line spectrum

• Only certain colors are present (discrete lines)

• Spectrum for each element unique (like fingerprints)

Model Atom• Nucleus

– contains protons and neutrons– number of protons = element

(1 proton = hydrogen, 2 protons = helium, etc.)

– number of neutrons about same as protons

• Isotope = different number of neutrons

hydrogen

helium

Isotopes of hydrogen

Model Atom• Electrons orbit nucleus

– Number of electrons = number of protons• Ionization = removing electrons

– Only certain orbits are allowed

hydrogen helium

Atomic Absorption• Atom absorbs photon energy

– electron “jumps” to higher energy orbit– only certain discrete orbits are allowed

• Atom can absorb only discrete colors (energies)

Atomic Emission• Electron “jumps” to a lower energy orbit

– Atom emits photon– can emit only discrete colors

• same colors (wavelengths/energies) as absorption

Atomic Energy Levels

• Energy Levels– Different for each element

• each element has unique set of absorption/emission lines

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Kirchoff’s Laws

• Continuous spectrum– Produced by hot solid (or dense gas)

• Emission line spectrum– Produced by hot, low density gas

• Absorption line spectrum– Produced when continuous source is viewed

through cooler low density gas

Kirchoff’s Laws

• Absorption lines same wavelengths as emission lines– Gas can only absorb and emit at certain discrete

frequencies/wavelengths/energies

WORKBOOK EXERCISE:“Types of Spectra”

(pages 41-42 in workbook)

If you analyze the light from a low density object (such as a cloud of interstellar gas), which type of spectrum do you see?

A. dark line absorption spectrumB. bright line emission spectrumC. continuous spectrum

Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight?

A. dark line absorption spectrumB. bright line emission spectrumC. continuous spectrum