Properties of LightProperties of Light

Light as a WaveLight (or electromagnetic radiation), can be thought of as either a particle or a wave. As a wave, light has

• a wavelength, (distance between waves)

• a frequency, (number of waves passing you each second)

• a speed, c = (this is always the same: 300,000 km/s)

• an energy, E = h (where h is just a constant)

Note that because the speed of light is a constant, , , and E are linked: if you know one, you know the other two.

The Electromagnetic Spectrum

Atmospheric Windows

Not all light from space makes it through the earth’s atmosphere. In fact, only visible light, radio waves, and some infrared light makes it to the ground. The rest of the electromagnetic spectrum can only be observed from space.

The Doppler Shift

The wavelength emitted by an object is not always the wavelength you observe. If you are moving towards an object, you will see more waves per second (i.e., a higher frequency, like swimming upstream). The light will appear bluer and be blueshifted. Conversely, if you are moving away from an object, its light will be redshifted.

v

c

The faster the relative motion, the larger the red or blue shift.

Scattering of Light

Dust in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength (blue) light gets scattered more than red light. That’s why the sky is blue.

Scattering of Light

The long path through the atmosphere means all the blue photons are scattered away.

Dust in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength (blue) light gets scattered more than red light. That’s why the Sun is red at sunset.

In interstellar nebulae, stars behind large piles of dust will be reddened. Other parts will appear blue, due to the scattering by dust.

This is just like the daytime sky.

Scattering of Light

Anything that is hot (i.e., above absolute zero) produces light at all wavelengths – a continuous spectrum. But the amount of light given off at each wavelength is very sensitive to the object’s temperature.

For hotter objects:

• Peak intensity shifts to shorter (bluer) wavelengths

peak 1/T• more light is created

Total energy T4

1st Type of Radiation: Blackbody (or Thermal)

1st Type of Radiation: Blackbody (or Thermal)

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

Our bodies are a lot cooler than the Sun. So at which wavelength range do our bodies produce most of their light?

A) Gamma rayB) VisibleC) InfraredD) Ultra-violetE) X-ray

We are all shining!

2nd Type of Radiation: Emission Line

In the Bohr model of the atom, the nucleus contains protons and neutrons. Circling around the nucleus (in orbitals) are electrons. Since electrons are attracted to the protons, they normally orbit in their lowest energy state (i.e., closest to the nucleus), called the ground state.

Important: electrons can only orbit at very specific distances from the nucleus, and these distances are different for different elements.

+

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level, or an excited state.

+

2nd Type of Radiation: Emission Line

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level, or an excited state.

+

Eventually, when the electron falls back down, it has to give this energy back. It does so by giving off a photon of light.

Since each orbital has a very specific level, electron transitions between the orbitals emit very specific amounts of energy. The spectrum from this process would not be continuous.

Ephoton = E2-E1

E2

E1

E3

2nd Type of Radiation: Emission Line

2nd Type of Radiation: Emission Line

QuickTime™ and aAnimation decompressor

are needed to see this picture.

2nd Type of Radiation: Emission Line

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

2nd Type of Radiation: Emission Line

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

Emission Line SpectraSince every element has a different set of atomic orbital energies, the emission line spectrum of every element is different. They are as unique as fingerprints!

Blackbody Spectra

Emission Line Spectra

Absorption Line Spectra

An object (like a star) emits a hot blackbody spectrum. Somewhere between you and the star (like on the outside of the star) is some cooler gas. That gas can absorb the photons which correspond to the atom’s energy levels. The result is an absorption spectrum.

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E2-E1

E2

E1

E3

Absorption Line Spectra

Absorption Line Spectra

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Absorption Line Spectra from the Sun

Why does the Sun produce absorption lines?

(1 Million K)

?

CBA

?

CBA

?

CBA

Continuous, Emission, and Absorption Spectra

Imagine yourself as an astronaut orbiting the Earth. You look out the window of your vehicle and see a flare on the surface of the Sun. You know that solar flares emit light at all wavelengths. How long until your ship is hit by the dangerous x-rays and gamma-rays?

a) 8 days

b) 8 hours

c) 8 minutes

d) 8 seconds

e) No time at all. You are already getting hit by the x-rays and gamma-rays.

optical

X-rays

An electron in an atom absorbs a photon and jumps from level 1 to level 3. It then falls from level 3 to level 2 and emits a photon, and then falls from level 2 to level 1 and emits another photon. Which of the statements below is true?

a) Each of the 2 emitted photons has a higher frequency than the absorbed photon.

b) Each of the 2 emitted photons has a higher energy than the absorbed photon

c) Each of the 2 emitted photons has a longer wavelength than the absorbed photon.

d) The combined energy of the 2 emitted photons is less than that of the absorbed photon.

e) The speeds of the two emitted photons are different from that of the absorbed photon.

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level, or an excited state.

+

Eventually, when the electron falls back down, it has to give this energy back. It does so by giving off a photon of light.

Since each orbital has a very specific level, electron transitions between the orbitals emit very specific amounts of energy. The spectrum from this process would not be continuous.

Ephoton = E3-E1

E2

E1

E3

2nd Type of Radiation: Emission Line

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level, or an excited state.

+

Eventually, when the electron falls back down, it has to give this energy back. It does so by giving off a photon of light.

Since each orbital has a very specific level, electron transitions between the orbitals emit very specific amounts of energy. The spectrum from this process would not be continuous.

E2

E1

E3

2nd Type of Radiation: Emission Line

Ephoton = E3-E1

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level, or an excited state.

+

Eventually, when the electron falls back down, it has to give this energy back. It does so by giving off a photon of light.

Since each orbital has a very specific level, electron transitions between the orbitals emit very specific amounts of energy. The spectrum from this process would not be continuous.

Ephoton = E3-E1

E2

E1

E3

2nd Type of Radiation: Emission Line

Ephoton = E3-E2

Suppose something collided into an electron orbiting in the lowest energy level. Some of the energy of the collision could kick the electron up to a higher level, or an excited state.

+

Eventually, when the electron falls back down, it has to give this energy back. It does so by giving off a photon of light.

Emitted photons:Lower energiesLonger wavelengthsLower frequencies

Ephoton = E3-E1

E2

E1

E3

2nd Type of Radiation: Emission Line

Ephoton = E3-E2

Ephoton = E2-E1

While moving toward the Earth, a spaceship emits a photon of infrared radiation toward the Earth. Which of the following could happen at the surface of the Earth?

a) The photon is detected at radio wavelengths.

b) The photon is detected at optical wavelengths.

c) The photon is detected at X-ray wavelengths.

d) The photon is detected at gamma ray wavelengths.

While moving toward the Earth, a spaceship emits a photon of infrared radiation toward the Earth. Which of the following could happen at the surface of the Earth?

a) The photon is detected at radio wavelengths.

b) The photon is detected at optical wavelengths.

c) The photon is detected at X-ray wavelengths.

d) The photon is detected at gamma ray wavelengths.

Summary• Wave characteristics of light

• wavelength (), frequency (), speed (c), energy (E)

• The electromagnetic spectrum• light at different wavelengths

• Atmospheric absorption• Light at some wavelengths is absorbed by the atmosphere and

can’t reach the ground

• Doppler shift• Light shifts wavelength if the light source is moving toward or

away from you

• Why is the sky blue? • The atmosphere scatters blue light more than red light

Summary• 1st type of light: blackbody (thermal)

• Produces a continuous spectrum

• As an object becomes hotter:

• The peak of intensity shifts to shorter (bluer) wavelengths: peak 1/T

• Total energy of its emitted light increases T4

• 2nd type of light: line emission

• As an electron moves between discrete orbitals in an atom, light is emitted and absorbed at discrete wavelength

• The wavelengths of these lines are different for different elements, so they can be used as “fingerprints” in determining what stars are made of