Light and Spectroscopy. Light Charges interact via electric and magnetic forces Light is a...
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Transcript of Light and Spectroscopy. Light Charges interact via electric and magnetic forces Light is a...
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