Light and Spectroscopy AST 111 Lecture 11

Light Charges interact via electric and magnetic forces Light is a repetitive disturbance in these forces! Electromagnetic wave A form of energy Depending on conditions, light can also act like a particle A photon

The Electromagnetic Spectrum Higher Energy Lower Energy Blue light: 400 nm Red light: 700 nm

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

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?

Spectrum of Mars Page 170:

Spectrum of Mars Is there evidence of continuous spectra? Are there emission lines? Absorption lines?

Spectrum of Mars The boxed region shows reflected sunlight What color is Mars? The Sun is a source of a continuous spectrum

Spectrum of Mars What causes the lower-energy bump? Does Mars glow? Do we see its glow?

Spectrum of Mars This is the signature of carbon dioxide.

Spectrum of Mars Emission lines These come from hot gas in the atmosphere

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