AST 112Light and Spectroscopy

Light Charges interact via electric and magnetic

forces

Light is a repetitive disturbance in these forces! Electromagnetic waveA form of energy

Depending on conditions, light can also act like a particle A photon

Light Wavelength relates to COLOR

Shorter wavelengths have more energy

Higher-energy light interacts more strongly with matter

○ Infrared can pass through dust

○ X-Rays can pull electrons off of atoms

The Electromagnetic Spectrum

Blue light: 400 nmRed light: 700 nm

Higher Energy Lower Energy

Light and Atoms

Light comes from electrons moving to lower energy levels in an atom

Atoms can also absorb light, promoting an electron to a higher level

Emission and Absorption

Because the energy levels in atoms are specifically spaced apart:

A given type of atom can only emit and absorb specific colors of light

Energy Level Diagrams

Light and Atoms

Different atoms have different energy levels

A given type of atom emits and absorbs specific colors of light

Light and Atoms

Atomic spectra therefore provide a wealth of information about the physical properties,

especially chemical composition, of an object.

Spectra

A prism spreads light into its different colors

Spectra

Spectra

We will consider three types of spectra:

Emission

Absorption

Continuous

Spectra Emission and Absorption spectra:

An atom can both emit and absorb light

We consider a gas that is not dense

○ If we don’t, the atoms interact and alter the energy levels

○ This ruins the unique spectral fingerprint of the individual atoms

Emission Spectra

If a gas has enough microscopic energy (high enough temperature):

Collisions between atoms will transfer energy to electrons

Electrons then drop to a lower energy level, emitting a photon

Emission Spectra

An aside…

What does conservation of energy say will happen to the temperature of the gas?

Emission Spectrum

Absorption Spectra White light is shining through a cold gas cloud.

White light contains all colors of the spectrum.

The atoms in the gas cloud absorb photons with energies corresponding to differences in atomic energy levels

These colors are therefore removed from the spectrum

Absorption Spectra

Absorption Spectra

Continuous Spectra

Hot, dense objects emit essentially continuous spectra

Continuous Spectra The spectrum of a hot

dense object has a bump-shaped graph

The graph shows the brightness of each color (wavelength)

What determines the exact shape?

Continuous Spectra The amount of energy emitted by such an

object is given by the Stefan-Boltzmann law:

Brightness = sT4

The wavelength at which the peak occurs is given by Wien’s Law

Higher temperature object peaks at shorter wavelengths

Temperature Estimation So we can use a

glowing object’s color to estimate its temperature

Does this apply to the lava? How about an orange shirt?

Why or why not?

Doppler Effect

Because of the wave nature of light, its frequency or wavelength change if the source moves toward or away from an observer.

Doppler Effect An object’s

spectrum blueshifts if it moves toward us

An object’s spectrum redshifts if it moves away from us

Doppler Effect

Based on an object’s redshift or blueshift, we can ascertain how fast it is moving toward or away from us

We know where the lines should be when the object is at rest, so we can easily measure the shift