Chapter 19.4 Energy Levels

Question 1

How do electrons exist in atoms
What does discrete mean

Answer 1

They exist in shells which have discrete ENERGY LEVELS
- this means that electrons can only have certain amount of energy, can’t be IN BETWEEN

Question 2

Why are the levels negative, what’s the lowest state

Answer 2

Levels are negative because external energy is needed to out in to ‘liberate’ the electron. Also negative energy makes it seem like it’s stuck in the atom

Lowest energy level is called ground state

Question 3

What happens when electron gains energy and how

What happens when it releases

How to find frequency of photon

Answer 3

If it gains energy normally by ABSORBING A Photon, it excites to a higher energy level

2) when it de excites back to a level, it releases same energy in form of photon .
Use e=hf to find frequency of these (take care for ev)

Question 4

What special about electrons sittting in energy levels that makes analysis of thing possible

Answer 4

Different elements have DIFFERENT energy levels, and so will absorb and emit different frequencies of photons

• this gives a fingerprint like thing which allows us to identify what elements are in something based on spectra

Question 5

What is an absorption spectrum shows

Answer 5

Shows all the wavelengths that pass through, and where black lines the wavelengths of light ABSORBED BY ATOM due to excitation

(Context, imagine white light shining at atom and then whatever light goes through picked up. All the wavelengths absorbed Wont snow through, and are shown as BLACK LINES).

Question 6

What are EMISSION Spectra, what do they show and how

Answer 6

This is where you have an atom and as it de excites it releases photons

• all these photons detected produce an emission spectra

Question 7

In absorption spectra, even tho an electron will absorb a photon , surely it EMITS it again, so why does this NOT APPEAR ON THE ABSORBTION SPECTRA?

Answer 7

it does re emit, but in ALL directions, so the intensity in required direction is much less, and so rather not picked up

Question 8

Do you think the emission spectra will always be the ANTI of the absorbtion?

Answer 8

NO, not necessarily
- emission spectra , atom could release the electron in DIFFERENT STAGES, resulting in different photons being released, so different wavelengths show up
- BUT an absorption spectrum all start from the ground state, so they absorb the specific exact photons , AND NOT THE IN BETWEEN ENERGY LEVEL ONES

Thus these likely to be anti but very slightly different , emission has few more lines!

Question 9

There are 2 ways an electron can become excited

List them both and for each way, tell if the energy provided must be EXACTLY the difference. Eternal energy shells to excite and WHY?

Answer 9

• through absorbing a photon calm
• also interacting with accelerated electron

1) through photon energy must be exact
2) through electron the energy must be ATLEAST, it could be more, as it will transfer the required energy and KEEP THE REST

photons are DISCRETE however, either all transferred, or none!!

Question 10

What happens when exactly enough energy provided for it to escape?

Answer 10

The electron escapes and the atom is now IONISED

Question 11

1) So how can does light coming from stars appear with Missing lines so like an absorbtion spectra
(Misconception)

2) thus how can we use this light to determine elemental composition and why

3) why don’t we detect the re emitted photons

IMPORTANT

Answer 11

1) to clear up, the light produced in any case is always light of a CONTINOUS SPECTRUM
- what happens after produced is in the LAYERS SURROUNDING STARS are gases of elements which are cooler
- they absorb light if they can use it to excite, and ALL OTHER WAVELENGTHS OF LIGHT ARE ABLE TO PASS THROUGH
- we see them on earth as a continous spectrum with some missing lines.
- we compare these lines to known elements absorbtion spectra on earth as the spectra for any element SHOULD BE THE SAME as it has the same electron shell energy differences
- = this is thus an ABSORBTION SPECTRUM!

2) as each element has the same absorbtion spectra each time, analysding the spectra we can match it to elements absorbtion spectra and deduce what ELEMENTS IN THERE

3) we don’t detect photons that have been re emitted because these won’t necessary be in our direction, so intensity less and we ignore

Question 12

Off topic but this is how Helios discovered

Answer 12

Light from sun missing lines that didn’t correspond to any event so far, then named Helios and found later

Question 13

What is a diffraction grating, why do we use this over just a double slit to analyse !

Answer 13

Diffraction grating is equipment with 1000s of slit separations in a small space.

We use this because fringes produced by double slit are not VERY SHARP, so can be difficult to determine the position of the centre if each maximum (and thus use equation).

• instead as so much superposition happens in diffraction grating, the interference pattern is much clearer and BRIGHTER TOO

Question 14

What is the equation for diffraction grating then

Answer 14

N lambda = d sin theta

Where N is the order of MAXIMUM (same in both sides of normal)
Lambda is wavelength
D is the SLIT SEPARATION
Theta is the angle formed between nth order and 0 order!

Question 15

How to determine the MAXIMUM order the light can produce with the diffraction grating

What to do if decimal achieved?

Answer 15

The maximum diffraction possible that can occur coming is at 90° realistically

• so sub in 90 and rearrange

N lambda = d sin 90
N = d / lambda

2) IF YOU GET ANSWER LIKE 2.3, the maximum order is always a WHOLE NUMBER = therefore 2!

Question 16

How ti experimentally find the wavelength of a LASER in class
Why use laser

• how to find theta
  -how to find slit separation

Answer 16

1) lasers are a source of monochromatic coherent light, saves using a single slit first

2) - clamp your diffraction grating and laser hopefully at a point that will project decent orders far a lug h such that yiu can SEE THEM .
- in our case form top table to floor was good
- to find the ANGLE, measure distance direct to floor and then distance from 0 order to 1st or 2nd . Use TAN to find
-to find slit separation check the information and how many slits, do 1/ that

Make sure use a SET SQURE for accurate measurements

Now rearrange

Question 17

What is a black body ?

Answer 17

1)An IDEAL object that absorbs all the EM radiation that shines on to it
- as nothing is reflected, it appears BLACK

2) (when in thermal Equilibirum ) will emit a CHARACTERISTIC distribution of wavelengths at different intensities , when at a SPECIFIC temperature

Question 18

Explain the black body thing again, what does it emit?

What is peak wavelength intensity

Answer 18

At specific temperatures if the object is modelled as a black body, based on its temperature, it will emit a series of wavelengths

However these will be at different intensities, and often you get a wavelength produced at a PEAK intensity so this is the one you’ll see the most !

Question 19

What is WIENS law relating TEMPERTAURE and peak wavelength?

• so what’s the constant

Answer 19

For any black body:

Is that TEMPERTAURE is inversely proportional to the PEAK wavelength intensity produced

Therefore, for any black body, the temp and the peak wavelength multiplied gives a CONSTANT

Question 20

So how can we use wiens law for black bodies for generally most objects!

Answer 20

We model a lot of things as black bodies , including metal, filament lamps and stars…

As a result we explain why a higher TEMPERTAURE produces different wavelength of light

Question 21

So what type of light is produced when temp of object is Hotter
- thus how does the intensity wavelength graph shift in x direction

Answer 21

Higher temp = lower wavelength = higher frequency = closer to blue

Thus the wavelength decreases, graph shifts to the LEFT

Question 22

What happens to intensity of all waves emitted when temp lowers and Why

Answer 22

The intensity generally decreases for peak and every wavelength

2) I don’t know

Question 23

Remember, how many wavelengths are produced when a black body emits?

Basicslly is it discrete or continous

Answer 23

They are considered to be the perfect emitters of LIGHT , they emit AT ALL WAVELENGTHS AT A CONTINUOUS SCALE,

and can sometimes creep to UV too

Question 24

What is definition for luminosity again

Answer 24

The total power output for a star
- this because it absorbed some power too and emits , so not technically the same as power but basically can be thought of it like that

Question 25

What’s STEFANS equation and law

How L related to temp
Radius
Surface area

Answer 25

That L = (4pi r2 sigma ) T ^4

Essentially the luminosity is proprtiosnl to the absolute SURFACE TEMP of a black body to the power of 4

And to the radius sqaured

Or SURFACE AREA too (L prop to 4 pi r2)

Question 26

What can we measure coming in from stars and thus how can we work out radius and thus other stuff like mass and density

And how do we work out the peak wwvalenetgh!

Answer 26

We know the luminosity + peak wavelength coming in (using diffraction grating!)
- can work out temo
- thus the radius
- thus mass and density