Astronomy Picture of the Day

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

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

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

CPS Question

● The color of visible light is determined by its ____.

● A)brightness● B) amplitude● C)speed● D)wavelength

CPS Question

● If the wavelength of light increases, the frequency must ____.

● A)increase also● B) decrease● C)remain unchanged

CPS Question

● The bending of light that occurs when moving between media of different densities is called ___.

● A)reflection● B) refraction● C)diffraction● D)distortion

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

~ 1/r2

fc =

1 nm = 10 -9 m , 1 Angstrom = 10 -10 m

The Electromagnetic Spectrum

Electromagnetic Spectrum

● 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

What does the spectrum of an astronomical object's radiation look like?

Many objects (e.g. stars) have roughly a "Black-body" spectrum:

Brightness

Frequency

also known as the Planck spectrum or Planck curve.

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

Spectroscopy and Atoms

How do you make a spectrum?

When you bend light, bending angle depends on wavelength, or color.

Refraction of light

Questions

● How is temperature related to the amount of energy radiated?

● How is temperature related to the color of the object?

(Blackbody Demo)

"cold" dust

"hot" stars

"cool" star

Sun

frequency increases, wavelength decreases

The wavelength of peak emission tells us the temperature of the object!

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

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

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) video

Video

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)

Pattern of lines is a fingerprint of the element

For a given element, emission and absorption lines occur at the same wavelengths.

Helium discovered in Sun’s spectrum before being found on Earth!

Sodium emission and absorption spectra

Spectrum of the Sun• Absorption spectrum

• What causes emission/absorption of light at specific wavelengths?

Interactive

Video 1, 2, 3

Types of Spectra

1. "Continuous" spectrum

2. "Emission" spectrum

3. "Absorption” Spectrum

video

The Particle Nature of Light

Light interacts with matter as individual packets of energy, called photons.

c

photon energy is proportional to frequency:

E f (or E 1

example: ultraviolet photons are more harmful than visible photons.

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

The Nature of Atoms

The Bohr model of the Hydrogen atom:

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+

proton

electron

"ground state"

_

+

an "excited state"

(Fair Analogy)

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)

When an atom absorbs a photon, it moves to a higher energy state briefly

When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system!

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

Other elements

Helium Carbon

neutron proton

Each element has its own allowed energy levels yielding a unique spectral fingerprint.

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/energiesvideo

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

CPS Question Which ONE of these is constant for all forms of

EM radiation in a vacuum?

A) amplitude B) wavelength C) frequency D) speed E) energy

CPS Question

Which ONE is NOT a property of a blackbody?

A) It appears black, regardless of its temperature. B) It emits radiation in a continuum of

wavelengths. C) Its spectrum peaks at a wavelength determined

by its temperature. D) The total energy that it radiates increases rapidly

with temperature.

CPS Question

The Sun's observed spectrum is _____.

A) A continuum with no lines, like the rainbow. B) A continuum with bright emission lines. C) Only absorption lines on a black background. D) Nearly a continuum with some absorption lines.

Ionization

+

Hydrogen

_

+

_

++

Helium

_

_

++

"Ion"

Absorbing a high energy photon and atomic collisions can both lead to ionization.

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_

Spectrum of the Sun• Complicated objects

=> many different elements

• Nearly continuous absorption spectrum

• What causes emission/absorption of light at certain wavelengths?

Why emission lines?

.

..

...

hot cloud of gas

- Photon absorption/atomic collisions excite atoms

- Electron drops back to lower level

- Photons at specific frequencies emitted

Why absorption lines?

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. .

.

.

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.

..

.

.

cloud of gas

(Shockwave Demo) (Web Link)

Stellar Spectra

Star

Fusion generates

continuous spectrum

Sun's 'atmosphere' absorbs specific wavelengths

Kirchhoff's Laws

1. Continuous spectrum

2. Emission spectrum

3. Absorption spectrum

Question How does the pitch or tone of a sound wave

change when the source of the sound is moving towards or away from you?

What about when you are moving towards or away from the source?

Does this effect occur for all types of waves or just for sound waves?

Doppler Shifted Atomic Spectra

• Why don’t we see the color of everyday objects change as they move?

We've used spectra to find planets around other stars! (Ch. 4)

• Star wobbling causes Doppler shift of its absorption lines.

• Only gives information about velocity along line of sight!