3.1- THE PHOTOELECTRIC EFFECT

Question 1

What do metals contain?

Answer 1

conduction electrons

Question 2

What do conduction electrons do in metals?

Answer 2

move about freely inside the metal

Question 3

What do the conduction electrons in metals collide with?

Answer 3

each other and with the positive ions of the metal

Question 4

What did Heinrich Hertz discover?

Answer 4

how to produce and detect radio waves

Question 5

What did Heinrich Hertz find when he discovered how to produce and detect radio waves?

Answer 5

sparks produced in his spark gap detector when radio waves were being transmitted were stronger when ultraviolet radiation was directed at the spark gap

Question 6

What did further investigations on the effect of EM radiation show?

Answer 6

electrons are emitted from the surface of a metal when EM radiation above a certain frequency was directed at the metals

Question 7

What is the term for when electrons are emitted from the surface of a metal when EM radiation above a certain frequency was directed at the metals

Answer 7

photoelectric effect

Question 8

What observation were made following the observations from the photoelectric effect that were a major problem as they could not be explained using the idea that light is a wave? (3)

Answer 8

photoelectric emission of electrons from a metal surface does not take place if the frequency of the incident EM radiation is below a certain frequency

no. of electrons emitted per second proportional to intensity of incident radiation

photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface

Question 9

What is the threshold frequency?

Answer 9

minimum frequency of the incident electromagnetic radiation for photoelectric emission of electrons from a metal surface

Question 10

What does the minimum frequency depend on?

Answer 10

type of metal

Question 11

What must the wavelength of the incident light be like for photoelectric emission?

Answer 11

less than the maximum value equal to the speed of light divided by the threshold frequency

Question 12

Provided, what is the number of electrons emitted per second proportional to the intensity of the incident radiation?

Answer 12

frequency is greater than the threshold frequency

Question 13

Provided what, allows the photoelectric emission without delay as soon as the incident radiation is directed at the surface?

Answer 13

frequency of the radiation exceeds the threshold frequency

Question 14

What can the wave theory not explain about the photoelectric effect?(2)

Answer 14

existence of threshold frequency

why photoelectric emission occurs without delay

Question 15

What does the wave theory explain about the conduction electrons in photoelectric emission?

Answer 15

each conduction electron at the surface of a metal should gain some energy from the incoming waves, regardless of how many waves arrive each second

Question 16

What did Einstein put forward in 1905?

Answer 16

photon theory of light

Question 17

Why did Einstein put forward the photon theory of light in 1905?

Answer 17

to explain the photoelectric effect

Question 18

What did Einstein assume about light when putting forward the photon theory of light?

Answer 18

assumed light is composed of wavepackets or photons

Question 19

What is the energy of each photon?

Answer 19

hf

Question 20

What did Einstein say to explain the photoelectric effect? (2)

Answer 20

when light incident on metal surface, an electron absorbs a single photon from the incident light so gains energy equal to hf

electron can leave metal surface if energy gained from single photon exceeds work function of metal. excess energy gained by photoelectron becomes its K.E

Question 21

What is the definition of work function?

Answer 21

minimum energy needed by electron to escape from metal surface

Question 22

How can electrons that escape from the metal plate be attracted back to it?

Answer 22

by giving the plate a sufficient positive charge

Question 23

What is the definition of stopping potential?

Answer 23

minimum potential needed to stop photoelectric emission

Question 24

What happens to the maximum kinetic energy of the emitted electron at stopping potential?

Answer 24

maximum K.E. of the emitted electron is reduced to zero

Question 25

Why is the maximum K.E. of the emitted electron reduced to zero at stopping potential?

Answer 25

each emitted electron must do extra work equal to e * Vs

Question 26

Who obtained the conclusive experimental evidence for Einstein’s photon theory?

Answer 26

Robert Millikan

Question 27

What did Robert Millikan do?

Answer 27

measured the stopping potential for a range of metals using light of different frequencies