Electromagnetic Radiation and Quantum Phenomena

Question 1

What were the three main conclusions of the photoelectric effect?

Answer 1

1) no photoelectrons are emitted if radiation is of a frequency lower than the threshold frequency
2) the value of maximum kinetic energy of photoelectrons increases with the frequency of the radiation. ONLY DEPENDS ON FREQUENCY
3) intensity only affects the number of photoelectrons emitted per second.

Question 2

What is the (incorrect) wave theory explanation of the photoelectric effect?

Answer 2

1. Energy carried is proportional to the intensity of the beam
2. Energy carried by light would be spread evenly over the wavefront
3. Therefore each free electron on the surface of the metal would gain some energy
4. Gradually each electron would gain enough energy to leave the metal.

Question 3

What is the work function?

Answer 3

The minimum amount of energy required to break the bonds holding an electron in the metal surface.

Work function = h x threshold frequency

Question 4

What is the stopping potential?

Answer 4

The pd needed to stop the fastest moving electrons with maximum kinetic energy.

Question 5

Why do the kinetic energies of emitted photoelectrons range to a maximum value?

Answer 5

Energy is lost as an electron leaves a metal.

Electrons deeper down in the metal lose more energy than the electrons on the surface.

Question 6

How can electrons move up (or down) energy levels?

Answer 6

By absorbing (or emitting) a photon of energy equal to the difference in energy levels

Question 7

Define the threshold frequency.

Answer 7

The minimum frequency that a photon must have to supply enough energy for a photoelectron to be emitted.

Threshold min = work function / plancks constant

Question 8

What is the 
1. Gradient
2. X intercept
3. Y intercept
on a graph of E Max against frequency

Answer 8

1. Gradient = plancks constant, h
2. X intercept = threshold frequency
3. Y intercept = negative work function

Question 9

State one characteristic of an ion?

Answer 9

The number of electrons in an ion is not equal to the number of protons.

Question 10

What is the electron volt?

Answer 10

Unit of energy equal to the work done when an electron is moved through a pd of 1V.

Question 11

What is a metal?

Answer 11

Lattices of ions surrounded by delocalised electrons.