Photons and Lasers

Question 1

Define Photoelectric Effect

Answer 1

When light or ultraviolet radiation of short enough wavelength falls on a surface, electrons are emitted from the surface.

Question 2

Define Work Function

Answer 2

The work function of a surface is the minimum energy needed to remove an electron from the surface. Unit: J or eV

Question 3

Define Electron Volt

Answer 3

This is the energy transferred when an electron moves between two points with a potential difference of 1 V between them.

Question 4

State one experiment which provides evidence that electromagnetic radiation behaves like waves.

Answer 4

Young’s double slit experiment.

Question 5

State one experiment which provides evidence that electromagnetic radiation behaves like a stream of particles (photons)

Answer 5

The photoelectric effect experiment.

Question 6

Define the term threshold frequency.

Answer 6

The threshold frequency for a particular metal is the minimum frequency of the incident electromagnetic radiation required to eject electrons from a metal surface.

Question 7

Describe and explain the photoelectric effect

Answer 7

• a photon is absorbed by an electron (in a metal surface); causing the electron to be emitted
  from the surface.
• Energy is conserved in the interaction.
• Only photons with energy above the work function energy (or frequency above the threshold
  frequency) will cause emission
• Einstein’s photoelectric energy equation:
  (energy of photon) = (work function of metal) + (maximum possible kinetic energy of
  emitted electron)
• The work function energy is the minimum energy required to release an electron from the
  surface of the metal.
• The number of electrons emitted also depends on light intensity of the incident radiation.
• Photoelectric emission is instantaneous.

Question 8

The wave theory predicts a time delay between light shining on the surface of a metal and electrons being emitted. In practice, there is no delay. How can this be explained?

Answer 8

• Light travels in the form of discrete packets of energy called photons.
• The photon energy is transferred to the electron in a ‘one to one’ instantaneous interaction.

Question 9

Define Ionisation.

Answer 9

The removal of one or more electrons from an atom.

Question 10

Define Ionisation Energy.

Answer 10

The minimum energy needed to remove an electron from the atom in its ground state.

Question 11

Explain what is an atom at an excited state.

Answer 11

It is an atom in a state of raised energy above the ground state.

Question 12

Explain what is an atom at ionised state.

Answer 12

It is an atom with an electron removed.

Question 13

Describe briefly one piece of evidence which shows that electrons sometimes behave like waves.

Answer 13

electrons passing through a thin sheet of graphite are diffracted and interfere to produce diffraction rings on a fluorescent screen.

Question 14

Explain the term diffraction

Answer 14

Diffraction is spreading out of a wave after passing through a gap or around an obstacle.

Question 15

Describe what makes laser light different

Answer 15

• Laser light is special because it is plane polarised, coherent, monochromatic and forms a parallel beam
• All photons are in phase, have the same frequency, wavelength, energy, direction and plane polarisation.

Question 16

Explain what is meant by the term coherent when describing laser light

Answer 16

Stimulated photon and incident photon are in phase and have the same frequency.

Question 17

State three possible interaction between light and matter

Answer 17

• Absorption of photon by electron (electron transition up energy levels)
• Emission of photon by electron (electron transition down energy levels)
• Stimulated emission of photon by electron (incident photon causes it, electron transitions down energy levels and emits second photon, with same phase and frequency as the
  incident).

Question 18

Explain the term spontaneous transition

Answer 18

Electrons transition down (drop) from higher to a lower energy level randomly, without stimulation

Question 19

Explain the term stimulated transition

Answer 19

A passing photon which causes electron to transition down emitting a second photon together with original photon.

Question 20

Explain how stimulated emission produce light amplification

Answer 20

• There are two photons (one passing/incident and one by stimulated emission) where there was one (passing/incident) and the process repeats.

Question 21

Explain what is meant by population inversion

Answer 21

There are more electrons in the upper level than the lower level.

Question 22

Explain why a population inversion is needed for the laser to work

Answer 22

If more electrons are at the lower energy level than the higher (i.e no population inversion), absorption of photons is more likely than stimulated emission.

Question 23

Explain what is meant the term pumping

Answer 23

Pumping is feeding energy into the amplifying medium of a laser to produce population
inversion by raising electrons from lower level to pumped level

Question 24

Explain why pumping is required for a laser to operate

Answer 24

• Electrons spontaneously transition down from pumped to upper level
• With pumping a population inversion can arise between upper and lower (ground) level
• And stimulation emission becomes more likely than absorption

Question 25

Explain what a two level system cannot produce amplification by stimulated emission

Answer 25

- No population inversion is possible, at best upper and lower level have equal populations - As such it is equally likely that (pumped) incident photons cause absorption and stimulated emission

Question 26

Explain how in a four level system the introduction of a level above the ground state makes the population inversion easier to establish than in a three level system

Answer 26

- The new level above ground state is almost empty because electrons transition quickly to the ground state. - As such even a low number of electrons at the higher level are enough to cause population inversion between higher and lower levels

Question 27

What are the advantages of semiconductor (diode) lasers compared to other types of lasers.

Answer 27

- They are much cheaper to produce - They are much smaller and easier to produce in numbers - They require much less input energy for a given energy output

Question 28

Explain how a cavity design with one fully reflective mirror on one end and a partially reflective mirror on the other end promote laser operation

Answer 28

- The mirrors cause the light (photons) to travel back and forth in the cavity - This increases the probability of stimulated emissions - This increases the amplification (intensity)