phas1223-1

Question 1

What is the definition of electromagnetic radiation?

Answer 1

it is a time varying electric field which produces a perpendicular time varying magnetic field both of which propagate.|(time varying refers to how field changes over time)

Question 2

Discuss the ways in which electromagnetic radiation can penetrate through earths atmosphere

Answer 2

most radiation is blocked out by eaths atmosphere and only a small fraction actually reaches the earths surface.|We position telescopes according to where the EM radiation it want to observe can travel e.g. infrared on mountains in infrared “windows” and hubble in space.

Question 3

What evidence is there for the wave nature of light?

Answer 3

this can be shown by youngs slits experiment in which you can see an interference pattern resembling what wed expect of a wave.

Question 4

what evidence is there for the particle nature of light?

Answer 4

1. Photoelectric effect (shows the quantum nature of light)|2. atomic spectra can also be used to provide evidence for the particle nature of light

Question 5

Describe the photelectric effect and it’s findings

Answer 5

1. UV Shone at negatively charged Zn plate|- electrons to escape immediately|- increasing intensity of the light increased number of photelectrons but not their max kinetic energy.|- red light wont cause the ejection of any electrons, not matter what intensity |(longer wavelengths of light will eject fewer electrons than shorter wavelengths regardless of intesnity)

Question 6

What are the implications of the photoelectric effect ?

Answer 6

energies of the emitted electrons independent to the intensity of the incident radiation.|1- Einstein said light comes in discrete packets of energy (quanta) which we call photons.|2- each photon has an energy E=hf|3- increasing the number of photons increases the amount of electrons ejected as more photons per unit time but the max k.e. of photoelectrons ejected is the same|4- This assumes that the dominant process consists of individual photons being absorbed by and resulting in the ejection of a single electron.||ejection energy is independent of the total energy of illumination so the light must be acting as a particle. (if it were acting as light, we’d expect energy to build up in the electron)

Question 7

What is spectroscopy?

Answer 7

decoding physical information from the details of lines in the spectrum of light coming from planets, stars and galaxies.|(this is based on the particle nature of light)

Question 8

What patterns do we see in the spectrum of stars at different temperatures?

Answer 8

lower temperature stars peak at lower red wavelengths—>red appearance |higher temp stars peak at blue wavelength but have a greater flux at all wavelength–>blue appearance.

Question 9

What is kirchoff’s 1st law of spectroscopy?

Answer 9

This law refers to continuous spectra and states that a hot solid object or dense gas produces light with a continuous thermal spectrum.

Question 10

what is Kirchoff’s 2nd law of spectroscopy?

Answer 10

This law refers to emission spectra and states that a hot tenuous (insubstantial) gas produces light with spectral lines at a discrete wavelength which depend on the energy level of the atoms in the gas.

Question 11

What is Kirchoff’s 3rd law of spectroscopy?

Answer 11

This law refers to absorption spectra and states that a hot solid object surrounded by a cool tenuous gas (just cooler than the hot object) produces light with an almost continuous spectrum but has gaps at discrete wavelengths. The position of the gaps are dependant on the energy levels of the atoms in the gas.

Question 12

Where do absorption lines come form?

Answer 12

An absorption line will appear in a spectrum if an absorbing material is placed between a source and the observer. The electrons in the material transition up energy levels when they atom absorbs a photon. This then creates gaps in the spectrum of the radiating object|(the object re-emits photon but in a random direction)

Question 13

What is Bohrs model of the atom

Answer 13

1. bohr postulated that only a discrete number of orbits are allowed and that in them the electron can’t radiate.||2. The only permitted orbitals were where the electrons orbital momentum is an integer multiple of angular momentum=n×h/2π= n×h(bar) n=1,2,3…..| =mvr||3. A single discrete quantum of radiation is emitted or absorbed as the electron jumps from an orbit to another, and|- the energy of the radiation equals the orbits’ energy difference

Question 14

What is Bohrs model of hydrogen?

Answer 14

In a stable state electrons orbit in the ground state defined by n=1.|there are an infinite number of discrete energy levels converging to infinity - this is called the ionisation potential.

Question 15

Why are electron energy levels negative?|What are the first three energy levels for the hydrogen atom? |how do you work out photon energy for n=3 to n=2 transition?

Answer 15

1. negative as you have to put energy in to move it up energy levels.|(they can also be shown on a normalised energy scale where they are not negative though)|2. 10.2, 12.1, 12.8, 13.6|3. E=hf=E(3)-E(2)

Question 16

How can an electron escape the atom?

Answer 16

it has to gain more energy than the ionisation potential.

Question 17

What are the different features of the spectrum from a hydrogen atom?

Answer 17

we can see emission/absorption lines corresponding to there energy changes.||lyman series–>UV light |Balmer series –> optical light |Paschen series –> infrared light

Question 18

Explain how emission lines are produced?

Answer 18

1. star emits continuum of radiation which passes through an optically dense nebula|2. photons at energy equal to the atomic transition is absorbed.|3. this light is then re-emitted in a random direction- some going towards the observer|- the only light going to the observer is that which has energy equal to the atomic transition in nebula.

Question 19

How are absorption lines produced?

Answer 19

1. star emits a continuum of light |2. this passes through a cooler tenuous gas which absorbs photons at an energy level equal to one of its energy level transitions.|3. the photon is the remitted in a random direction |4.observer sees all the wavelengths except those at the atomic transition energies ||(notes absorption and emission lines will be in the same place)

Question 20

Name 3 sources of absorption lines and the types of absorption spectra each produces.

Answer 20

1. STELLAR ATMOSPHERES: |outer layers absorb blackbody emission from the star |2.INTERSTELLAR GAS: |light is absorbed by an interstellar medium. If it moves through multiple gas clouds relative motion due to the expansion of the universe causes red shift and so the position of the absorption line is shifted–>multiple absorption lines all signalling the same energy level transition.|3. INTERGALACTIC LYα SYSTEMS OF CLOUDS AT DIFFERENT REDSHIFT: |light moves through lots of clouds at different redshifts so position of the absorption lines corresponding to energy level transition changes a lot. This is called a LYα-forest.|(called this as LYα absorption most common)

Question 21

What is meant by the doppler shift?

Answer 21

The observed wavelength λ is different from the emitted wavelength λ0 due to the relative radial velocity of the emitter with that of the observer.||λ > λ0 implies a ‘redshift’ of the light, v > 0, the emitter is moving away from the observer|λ < λ0 implies a ‘blueshift’ of the light, v < 0, the emitter is moving towards the observer

Question 22

What is the doppler method of planet detection?

Answer 22

1. the gravitational effect the planet has on the star causes it wobble |2. we can use doppler shift to measure the wobbles magnitude as planets orbits|3. we can then calculate the mass of the planet from this ||(note: we dont see the planet, we just infer its presence)

Question 23

What is the name and classes in the classification scheme for stars? |what is it based on?

Answer 23

Havard classification scheme||Based on spectral characteristics|• This gives information about temperature in|an alternative way|• Absorption lines can be observed only for a|certain range of temperatures|• The range involved shows atomic energy|levels which have been populated||also a further subdivsion 0-9 with 0 being hottest

Question 24

What classification is the sun?

Answer 24

G2

Question 25

Why can absorption lines only be observed for a certain range of temperatures?

Answer 25

fully ionised atoms cant absorb or emit radiation as no electrons to move between energy levels.

Question 26

How do absorption lines vary with temperature?

Answer 26

at different temperatures, you get different absorption lines. |at some temperatures, some particles will be fully ionised so no absorption lines present or not enough energy to create the energy transitions.|there is optimum temperatures to excite electrons to higher energy levels. To the right of this less photons have enough energy to excite atoms into a higher excitation state and to the left of this more photons have too much energy and begin to ionise the atom.

Question 27

if both absorption and emission lines are produced, why do we just see the absorption lines

Answer 27

absorption lines dominate when we superimpose the two on a spectrum as emission is given off in all directions

Question 28

how does the size of the star effect its luminosity?

Answer 28

L=4πr²σT⁴|a larger star will have a greater luminosity than a smaller star even if they are the same temperature|–>they will thus then have different spectra

Question 29

what are the different luminosity classes?

Answer 29

Ia- most luminous supergiant |Ib- supergiant |II - luminous giant |III- giant |IV - sub giant |V- main sequence

Question 30

what luminosity class is the sun?

Answer 30

its a main sequence making the sun a G2V star

Question 31

What is the Hertzspring Russel diagram?

Answer 31

if we plot luminosity against decreasing temperature, we can see that stars populate preferentially in different regions corresponding to the point they are in within their life cycle.

Question 32

what is absolute magnitude and how is it calculated?

Answer 32

another measure of luminosity |Mab=2.5log(L/Luminosity of the sun)|more negative value–>more luminous