Photons (Unit 2)

Question 1

What is light consisted as?

Answer 1

discrete packets of energy

Question 2

How can the photoelectric effect be demonstrated?

Answer 2

-Clean a zinc plate with fine emery paper
-Attach the plate to the top disc on a gold leaf electroscope, so there is
good electrical contact.
- Charge the zinc plate and inner assembly of the electroscope
negatively, by rubbing the zinc plate with a polythene rod which has been rubbed with a duster. The gold leaf should now be raised, because the leaf and the BACK PLATE are both charged negatively and repel each other.
- Place an ultraviolet lamp near the zinc plate. Switch it on. The leaf
should be seen to fall. Clearly the plate is losing charge.
- Repeat the procedure, but charging the zinc plate and inner assembly of the electroscope positively, e.g. by rubbing the plate with a charged
perspex rod. This time the ultraviolet does not affect the leaf. Charge is not lost

Question 3

Ev to J?

Answer 3

Ev and multiply by 1.6 x 10^-19

Question 4

J to Ev?

Answer 4

J and divide by 1.6 x 10^-19 to get Ev

Question 5

Key observations from experiment to show photoelectric effect? FREQUENCY is low

Answer 5

-When frequency of light is too low no electrons were released no matter how bright (intensity) of the light was

Question 6

Key observations from experiment to show photoelectric effect? When you increase frequency of light?

Answer 6

-results in a rise in the energy of the electrons released

Question 7

What is f 0

Answer 7

The minimum frequency that causes emission of a metal
-threshold frequency

Question 8

What do we conclude from the photocell experiment?

Answer 8

One packet of light interacts with one electron, meaning that the light wave is not a continuous stream of energy; instead, it is made up of photons
- validates Plank’s constant

Question 9

What is work function equal to?

Answer 9

H f0 - planks constant x threshold frequency

Question 10

the graph of Ek max against frequency of illuminating radiation? What can we work out?

Answer 10

• X intercept = F 0 (we can work out work function)
• gradient planks constant
• y intercept = work function
• y = mx +c → KE max = hf - work function

Question 11

Equation for energy of photon?

Answer 11

-E = hf
-c = fλ

Question 12

What is released from metal in photoelectric effect?

Answer 12

Electron not photon

Question 13

What is meant by photoelectric effect?

Answer 13

When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface.

Question 14

What is meant by work function?

Answer 14

minimum energy needed to remove an electron from the surface.

Question 15

Experiment to show how a vacuum photocell can be used to measure the maximum kinetic energy, Ek max, of emitted electrons in eV and hence in J? (Photocell experiment)

Answer 15

-Set up a circuit including a vacuum photocell ,microammeter and a variable d.c power supply
-Set the variable d.c power supply to 0v
-Shine a light of known frequency on the vacuum photocell
-Increase the P.D on the power supply until the current on the microammeter reads 0 μ A
-Note the p.d on power supply V and calculate Ek Max= eV stop
-Repeat with different frequencies

Question 16

Wavelength of visible light?

Answer 16

750 (nM) (red) to 400 (nM) (purple)

Question 17

EM SPECTRUM WAVELENGTHS?

Answer 17

Gamma ray - 10^ -12
X ray 10^-10
Ultraviolet 10^-7 or 10^-8
Visible light 7x10^-7 to 4 x10^-7
Infrared 10^-5
Microwave 10^-2
Radio 10^2

Question 18

How to work out photon energy?

Answer 18

E = hc/λ

Question 19

Determination of h using LEDs experiment?

Answer 19

-By slowly increasing the p.d., across an LED, the minimum p.d. required for light to be emitted can be measured. This is known as the , V min , and can be used to calculate the energy of the photons emitted by the LED, E.
E=eV min
​Where e=1.60×10 −19
-As photon energy can also be calculated using E= hc/ λ
​-these equations can be combined to give an expression linking V
min and λ.
V min = hc/ e λ
​ -Plotting a graph of V min against 1/ λ
​should give a straight line through the origin, with a gradient equal to hc/e
- rearrange to work out h
​

Question 20

Explanations from goldleaf experiment?

Answer 20

The current produced due to the electrons being released
was unaffected by changing the brightness of the light.
• Changing the frequency of the light, changed the current.
• If the frequency of the light was too low, no electrons were
released no matter how bright the light was

Question 21

How is emission spectra produced?

Answer 21

-when electrons move down an energy level, then it must emit a photon of energy , photon energy must be equal to energy gap (coloured lines on black background)

Question 22

How is absorption spectra produced?

Answer 22

-If an electron wants to move up an energy level then it must absorb a photon (black lines on coloured background)

Question 23

Relationship between absorption and emission spectra?

Answer 23

-dark and bright fringes will have same wavelengths and frequency

Question 24

What is ionisation energy?

Answer 24

energy required to remove an electron from the ground state

Question 25

How to show particles have a wave like aspect and results?

Answer 25

• Through electron diffraction
  -When electrons are accelerated towards graphite crystals (ACT AS DIFFRACTION GRATING), a diffraction pattern is produced. Shows electrons behave like waves, shows particles have wave like aspects and particle like aspects

Question 26

Momentum change equation?

Answer 26

p (momentum) = h/λ

Question 27

Radiation pressure equation?

Answer 27

Momentum change per second / area of beam (units in pascal)

Question 28

Power equation?

Answer 28

energy/ time

Question 29

Significance of photocell experiment?

Answer 29

-proves EM spectrum
-waves are quantisised into packets of energy

Question 30

Number of electrons emitted per second?

Answer 30

Power/ Energy gap of photon (J)

Current / Charge of an electron

Question 31

Photon momentum?

Answer 31

H / Wavelength

Question 32

Momentum of light per second?

Answer 32

Photon momentum x number of photons emitted per second

Question 33

Number of photons striking metal disc (from light)

Answer 33

• Energy / Total ENERGY (WORK FUNCTION + MAX KE)

Question 34

Photon energy and electron energy?

Answer 34

Photon - Energy hitting/ total energy
Electron - energy / 1.6 x 1^-19