Lecture 4 - Light Spectra and Matter

Question 1

what is the distance to the moon?

Answer 1

1 light second

Question 2

what is the nearest solar system? how far away is it?

Answer 2

Alpha centauri

4.4 light years away

Question 3

each day you see into universe, you see how many light days further away?

Answer 3

each day you see into universe, you see 1 light day further away

Question 4

how is light related to matter?

Answer 4

light originates from matter and interacts with matter

matter can emit/absorb/transmit/reflect light

Question 5

what is matter made of?

Answer 5

atoms

Question 6

describe the energy levels of electrons

Answer 6

electrons orbit the nucleus in different energy levels –> only have specific energy and nothing in btwn

Question 7

what happens when atoms emit light?

Answer 7

electrons are stepping DOWN btwn energy levels and emit light that is equivalent to the difference btwn levels

Question 8

what happens when atoms absorb light?

Answer 8

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

Question 9

what does the energy of emitted/absorbed photons represent?

Answer 9

the energy of emitted/absorbed photons represent the fixed energy levels

Question 10

what is the equation for light energy and what does each variable represent?

Answer 10

E = hf

E = photon energy
h = Planck’s constant (6.62x10^-34 J/s)
f = light frequency

Question 11

when do energy level transitions occur? why?

Answer 11

energy level transitions only occur when an atom gains or loses an electron bc energy must be conserved

Question 12

what is light intensity?

Answer 12

amount of light

Question 13

what are the 3 types of spectra?

Answer 13

1. CONTINUOUS spectrum
2. EMISSION line spectrum
3. ABSORPTION line spectrum

Question 14

what is continuous spectrum?

Answer 14

spectrum of traditional/incandescent light bulb that is a rainbow of colour

Question 15

what is emission spectrum? what does it depend on? describe what is happening with electrons?

Answer 15

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

Question 16

what is absorption spectrum? when does absorption spectrum occur?

what produces the absorption lines?

Answer 16

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

Question 17

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?

Answer 17

deltaE = hf = (hc/wavelength)

change in electron’s energy = photon energy

Question 18

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

Answer 18

wavelength = 656nm = 656x10^-9m

deltaE = hc/wavelength = (6.62x10^-34 Js * 3x10^8 m/s) / 656x10^-9m = 3x10^-19 J

Question 19

does every element have the same spectrum?

Answer 19

no, it is unique for each element

Question 20

when do things glow? how is this visible?

Answer 20

everything glows at any temp –> visible with infrared goggles

Question 21

what are chemical fingerprints? why are they helpful?

Answer 21

each element produces a specific set of emission/absorption lines so looking at a spectrum you can determine what elements make up the cloud

Question 22

do different ions of the same element produce the same lines? why is this significant?

Answer 22

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

Question 23

what type of spectrum is the solar spectrum? why?

Answer 23

ABSORPTION line spectrum

the Sun’s surface acts as a cloud of gas btwn us and the light source (sun interior)

Question 24

why is it helpful to look at light before and after it reflects off a planet?

Answer 24

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

Question 25

how is spectroscopy measured? what is it used for?

Answer 25

spectroscopy measures intensity (Y) as a function of wavelength or frequency (X)

used to measure gas composition of atmosphere, interstellar gas, etc.

Question 26

all objects emit _________ because of……

Answer 26

all objects emit electromagnetic energy because of the motion of their aotms/molecules –> heat

Question 27

what type of spectrum is thermal radiation?

Answer 27

continuous

Question 28

what is the only thing that thermal radiation depends on?

Answer 28

temperature of the object

Question 29

when would there be no thermal radiation?

Answer 29

if temp = 0 but this is impossible bc it means atoms would stop moving

Question 30

what are the 2 laws of the thermal radiation spectrum?

Answer 30

Question 31

if a curve has a higher peak what 2 things does this mean?

Answer 31

higher peak = higher T at any wavelength = brighter per unit square area

Question 32

what does the colour of a star indicate?

Answer 32

indicates how much energy the star has

Question 33

what is Wien’s law?

Answer 33

peak wavelength = (2,900,000K / T)nm

Question 34

if the temp of the sun is 5777 K, what is the peak of its continuous emission spectrum? what colour is this?

Answer 34

peak wavelength = (2,900,000K / 5777K) nm = 502 nm –> GREEN

Question 35

if the temp of a neutron star is 1,000,000K what is the peak of its continuous emission spectrum?

Answer 35

peak wavelength = (2,900,000K / 1,000,000K)nm = 2.9nm

Question 36

if we see orange/red stars, white stars, blue stars, why no green stars?

Answer 36

green is in the middle of the spectra and other colours are quite strong so the stars don’t appear green