Photoelectric Effect

Question 1

Photon Model of EM radiation

Answer 1

1905 Einstein showed atoms absorb and emit light in packets of energy. De Broglie showed light can behave as a particle and as a wave. Quanta of light became known as photons.

Question 2

Energy of Photon Equation

Answer 2

E = hf E = hc/wavelength

h = Planck’s Constant(6.626 x 10-34)

Question 3

Electronvolt

Answer 3

The kinetic energy gained by an electron when its accelerated through a pd of 1 volt.

1eV = 1.602 x 10-19

Question 4

Observations of the photoelectric effect using a gold leaf electroscope

Answer 4

When visible light is incident on a positive metal plate, no movement. This is also true when UV is shone on positive plate.

When visible light is shone on negative plate, no movement.

However when UV is shone on negative plate, leaf falls. The metal loses its negative charge through the emission of electrons which are repelled by negative charge of electroscope. Cant be caused by ions in the air as its in a vacuum.

Question 5

Threshold Frequency

Answer 5

Lowest frequency of radiation that results in the emission of electrons

Question 6

Work function

Answer 6

Minimum energy required to release an electron from its surface, overcoming the electrostatic attraction.

Question 7

Frequency & Intensity affects on photoelectric affect

Answer 7

Incident radiation < threshold freq: photons dont have enough energy to overcome work function of metal

Incident = Threshold: Elections emitted with 0 KE

Incident > Threshold: Electrons ejected with some KE. As freq increases KE increases.

Increasing intensity ejects more electrons per second but doesnt affect KE.

Question 8

Einstein’s Photoelectric equation

Answer 8

Each photon can only eject one electron

hf = work function + KE_max

Question 9

Wave particle duality

Answer 9

Interference & Diffraction can only be explained by wave model of light. Einstein proposed light can behave as a stream of particles called photons.

Idea light can behave as both a particle and a wave

Question 10

Evidence supporting wave-particle duality

Answer 10

De Broglie suggested all matter can display wave like properties.
Electrons from electron gun accelerated through vacuum to polycrystalline graphite made of tiny crystals, each consiting of a large number of regularly arranged atoms. Electrons diffract as they emerge from layyers of graphite & Interfere constructively.

Gives circular pattern as graphite atoms aren’t all lined up in same direction.

Question 11

De Broglie Equation

Answer 11

wavelength = h/mv

wavelength = h/p

wavelength = h/√2mVe