3.1 The photoelectric effect

Question 1

What is the photoelectric effect?

Answer 1

» Is where where photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it

Question 2

What is the certain frequency?

Answer 2

» It is different for every type of metal and is called threshold frequency

Question 3

What is the number of electrons emitted per second proportional to?

Answer 3

» The intensity of the incident radiation, provided the frequency is greater than the threshold frequency

Question 4

What will happen if the frequency of the incident radiation is less than the threshold frequency?

Answer 4

» No photo electric emission from the metal surface can take place, no matter how intense the incident radiation is

Question 5

When does photoelectric emission occur?

Answer 5

» As soon as the incident radiation is directed at the surface, provided the frequency of the radiation exceeds the threshold frequency
» And occurs without delay

Question 6

Why couldn’t the threshold frequency be explained by wave theory?

Answer 6

» it suggests that any frequency of light should be able to cause electron emission as the energy absorbed by each electron will gradually increase with each incoming wave

Question 7

What did Einstein assume?

Answer 7

» Light is composed of wavepackets or photons

Question 8

What is the energy of a photon equal to?

Answer 8

» hf

Question 9

What is h?

Answer 9

» Planck’s constant which is roughly about 6.63 x 10^-34 Joules

Question 10

What did Einstein say happens when light is incident on a metal surface?

Answer 10

» An electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon

Question 11

When can an electron leave the metal surface?

Answer 11

» If the energy gained from a single photon exceeds the work function of the metal

Question 12

What is the work function?

Answer 12

» It is the minimum energy required for electrons to be emitted from the surface of a metal, and is denoted by ϕ

Question 13

What happens to the excess energy gained by the photoelectron?

Answer 13

» Becomes its kinetic energy

Question 14

What is the maximum kinetic energy equal to then?

Answer 14

» hf - ϕ

Question 15

What is the photoelectric effect equation?

Answer 15

» hf = E(kmax) + ϕ

Question 16

What equations can reflect when emission can take place from a metal surface?

Answer 16

» E(kmax) > 0
» hf > ϕ

Question 17

What is the equation for the threshold frequency?

Answer 17

» f(min) = ϕ/h

Question 18

What is the stopping potential?

Answer 18

» The minimum potential needed to stop photoelectric emission

Question 19

What happens at the stopping potential?

Answer 19

» The maximum kinetic energy of the emitted electron is reduced to zero because because each emitted electron must to extra work equal to e x Vs

Question 20

What do metal contains?

Answer 20

» Conduction electrons, which move around the positive ions and collide with one another

Question 21

What does the photoelectric equation show?

Answer 21

» The photoelectric equation is E = hf = Φ + E and it shows the relationship between the work function, maximum kinetic energy, and the frequency of light.

Question 22

Explain why, when ultraviolet light is shone on a positively charged plate, no charge is lost by the plate?

Answer 22

» Photoelectric effect involves the emission of electrons which are negative
» Any electrons emitted will only make the positive charge greater

Question 23

Explain why photoelectric emission from a metal surface only takes place if the frequency of the incident ray is greater than a certain value?

Answer 23

» hf = work function + Ekmax
» Ekmax > 0 for electrons to be emitted
» Therefore hf has to be > work function
» So f > Work function / planks constant
» For electrons to be emitted

Question 24

Why is it important to test any new theory by testing its prediction experimentally?

Answer 24

» If the test does not provide supporting evidence, the prediction is incorrect
» So the theory is incorrect and must be changed

Question 25

In the photoelectric equation, what is the name of characteristic quantity of the metal that emits photoelectrons?

Answer 25

» Work function

Question 26

What is meant by an excited atom?

Answer 26

» When an electron promotes to a higher energy level from a lower energy state

Question 27

Explain the meaning of the threshold frequency?

Answer 27

» Minimum amount of energy required for a photon for the photo electric effect to happen