3.1 The Photo Electric Effect

Question 1

What conduction eLectrons in a metal collide with

Answer 1

With each other and with the positive ions of the metal

Question 2

What did Heinrich hertz discover about the found that sparks produced by radio waves by his spark gap detector

Answer 2

The sparks produced by radio waves were greater than those of ultraviolet

Question 3

What is the photo electric effect

Answer 3

When electromagnetic radiation above a certain frequency was fired at a metal, electrons are emitted from the metals surface

Question 4

First problem with the idea that light is a wave

Answer 4

Photoelectric emission doesn’t take place if the frequency of the incident electromagnetic radiation is below the threshold frequency (dependent on type of metal)
This means that the wavelength of the incident light must be less than a maximum value equal to the speed of light because speed of light/threshold frequency = wavelength

Question 5

2nd problem with the idea that light is a wave

Answer 5

Number of electrons emitted per second is proportional to the intensity of the incident radiation (provided its greater than the threshold frequency)

If the frequency of the incident radiation is less than the threshold frequency no photoelectric emission will take place from that metal surface (no matter how intense the incident radiation is)

Question 6

3rd problem with light being a wave

Answer 6

Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface (provided the radiation exceeds the threshold) regardless of intensity

Question 7

What can the wave theory of light not explain

Answer 7

The existence of threshold frequency
Why photoelectric emission occurs without delay - according to wave theory, each conduction electron at the surface of the metal should gain some energy from the incoming waves regardless of how many waves arrive each second

Question 8

Who was the photon theory of light put forward by

Answer 8

Einstein

Question 9

What did Einstein assume about light

Answer 9

Light is composed of wave packets or photons, each of energy equal to hf
Where f is frequency of the light and h is the Planck constant

Question 10

For electromagnetic waves of a wave length energy of each photon is

Answer 10

E=hf=hc/lambda

Question 11

For electromagnetic waves of a wave length energy of each photon is

Answer 11

E=hf=hc/lambda
Where c = the speed of the electromagnetic waves

Question 12

Explanation of the photo electric effect

Answer 12

When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf (energy of a light photon)

Question 13

When can an electron leave the metal surface

Answer 13

If the energy gained from a single photon exceeds the work function of the metal
This is the minimum energy required by an electron to escape from the metal surface.
Excess energy gained by the photoelectron becomes kinetic energy

Question 14

Maximum energy of an emitted electron

Answer 14

Ekmax = hf - work function

Rearrangement = hf=Ekmax+work function

Question 15

How to work out threshold frequency of a metal

Answer 15

Emission can take place when Ekmax >0 or hf > work function

Therefore Fmin= work function / h

Question 16

What is stopping potential (Vs)

Answer 16

Electrons that escape from metal plate can be attracted back to it if it is given a sufficient positive charge.
The minimum potential needed to stop the photoelectric emission is called stopping potential

Question 17

What happens at this stoping potential

Answer 17

The maximum kinetic energy of the emitted electron is reduced to 0 because each emitted electrons must do extra work equal to e x Vs to leave the metal surface
Hence the maximum kinetic energy is equal to e x Vs 

Question 18

What experiment was evidence for Einstein’s theory

Answer 18

Robert Milllikan measured the stopping potentials for a range of different metals using different light frequencies.
His results fitted Einstein’s photoelectric equation closely.
After being peer reviewed and checked scientists excepted light as photons.

Question 19

Threshold frequency

Answer 19

Minimum frequency of light that can cause photoelectric emission

Question 20

Work function

Answer 20

Minimum amount of energy needed by an electron to escape from a metal surface

Question 21

Stopping potential

Answer 21

The minimum potential needed to stop the photoelectric emission