Lecture 4 - Light Spectra and Matter Flashcards
what is the distance to the moon?
1 light second
what is the nearest solar system? how far away is it?
Alpha centauri
4.4 light years away
each day you see into universe, you see how many light days further away?
each day you see into universe, you see 1 light day further away
how is light related to matter?
light originates from matter and interacts with matter
matter can emit/absorb/transmit/reflect light
what is matter made of?
atoms
describe the energy levels of electrons
electrons orbit the nucleus in different energy levels –> only have specific energy and nothing in btwn
what happens when atoms emit light?
electrons are stepping DOWN btwn energy levels and emit light that is equivalent to the difference btwn levels
what happens when atoms absorb light?
atoms absorb light if it has energy exactly equal to the difference btwn energy levels –> atoms absorbs the light so its electron will jump up
what does the energy of emitted/absorbed photons represent?
the energy of emitted/absorbed photons represent the fixed energy levels
what is the equation for light energy and what does each variable represent?
E = hf
E = photon energy
h = Planck’s constant (6.62x10^-34 J/s)
f = light frequency
when do energy level transitions occur? why?
energy level transitions only occur when an atom gains or loses an electron bc energy must be conserved
what is light intensity?
amount of light
what are the 3 types of spectra?
- CONTINUOUS spectrum
- EMISSION line spectrum
- ABSORPTION line spectrum
what is continuous spectrum?
spectrum of traditional/incandescent light bulb that is a rainbow of colour
what is emission spectrum? what does it depend on? describe what is happening with electrons?
thin cloud of gas emits light at specific wavelengths depending on its composition and temp
has bright emission lines against a black background
collisions within gas clouds allow electrons to jump to higher energy levels but then they eventually fall down, releasing energy as emitted light
- the emitted photon has the same amount of energy that the electron loses so it has a specific wavelength and frequency
- the bright emission lines occur due to the electrons that undergo their energy change
what is absorption spectrum? when does absorption spectrum occur?
what produces the absorption lines?
if gas cloud lies btwn us and a light bulb AND the cloud is cooler than the light bulb then we will still see most of the continuous spectrum
but the cloud absorbs light of specific wavelengths so has dark absorption lines against the rainbow background
the light bulb emits light of all wavelengths but the hydrogen atoms absorb photons with enough energy to raise electron to higher energy level
what is the relationship btwn the change in electron’s energy and wavelength of light emitted and absorbed?
what is the change in electron’s energy equal to?
deltaE = hf = (hc/wavelength)
change in electron’s energy = photon energy
example: what is the energy difference btwn Hydrogen’s n=3 and n=2 energy levels? the resulting emission line from this transition has wavelength 656nm
wavelength = 656nm = 656x10^-9m
deltaE = hc/wavelength = (6.62x10^-34 Js * 3x10^8 m/s) / 656x10^-9m = 3x10^-19 J
does every element have the same spectrum?
no, it is unique for each element
when do things glow? how is this visible?
everything glows at any temp –> visible with infrared goggles
what are chemical fingerprints? why are they helpful?
each element produces a specific set of emission/absorption lines so looking at a spectrum you can determine what elements make up the cloud
do different ions of the same element produce the same lines? why is this significant?
no, they all have diff spectra
this is significant bc it helps us determine the temp of a hot gas or plasma –> more highly charged ions are present at higher temperatures
what type of spectrum is the solar spectrum? why?
ABSORPTION line spectrum
the Sun’s surface acts as a cloud of gas btwn us and the light source (sun interior)
why is it helpful to look at light before and after it reflects off a planet?
if you take light from sun, reflect it off a planet, then look at it on Earth, the spectrum will look diff as it goes thru diff parts of the atmosphere
this lets you see the elements present in diff atmosphere
how is spectroscopy measured? what is it used for?
spectroscopy measures intensity (Y) as a function of wavelength or frequency (X)
used to measure gas composition of atmosphere, interstellar gas, etc.
all objects emit _________ because of……
all objects emit electromagnetic energy because of the motion of their aotms/molecules –> heat
what type of spectrum is thermal radiation?
continuous
what is the only thing that thermal radiation depends on?
temperature of the object
when would there be no thermal radiation?
if temp = 0 but this is impossible bc it means atoms would stop moving
what are the 2 laws of the thermal radiation spectrum?
if a curve has a higher peak what 2 things does this mean?
higher peak = higher T at any wavelength = brighter per unit square area
what does the colour of a star indicate?
indicates how much energy the star has
what is Wien’s law?
peak wavelength = (2,900,000K / T)nm
if the temp of the sun is 5777 K, what is the peak of its continuous emission spectrum? what colour is this?
peak wavelength = (2,900,000K / 5777K) nm = 502 nm –> GREEN
if the temp of a neutron star is 1,000,000K what is the peak of its continuous emission spectrum?
peak wavelength = (2,900,000K / 1,000,000K)nm = 2.9nm
if we see orange/red stars, white stars, blue stars, why no green stars?
green is in the middle of the spectra and other colours are quite strong so the stars don’t appear green
