3.3(a) - Optoelectronics

Question 0

What is the photoelectric effect?

Answer 0

This is when metals are exposed to high frequencies of electromagnetic radiation (eg. UV) and electrons are ejected from the surface of the metal.

Question 1

What is ‘Irradiance’?

Answer 1

The irradiance at a surface on which radiation is incident is the power per unit area

Question 2

State 2 characteristics the metal and radiation must have for photoelectric emission to occur

Answer 2

• The metal must be negatively charged

* The radiation must be short wavelength (high frequency)

Question 3

What is ‘threshold frequency’?

Answer 3

This is the value for which the frequency incident to a surface must be greater than to produce photoelectric emission, and this value depends on the nature of the surface

Question 4

What happens with frequencies smaller than the threshold value?

Answer 4

An increase in the irradiance of the radiation at the surface will not cause photoelectric emission

Question 5

What happens with frequencies greater than the threshold value?

Answer 5

The photoelectric current produces by monochromatic radiation is directly proportional to the irradiance of the radiation at the surface

Question 6

What is the ‘work function’?

Answer 6

The amount of energy the electron needs to absorb in order to be released from an atom

Question 7

What is meant by the statement ‘electrons in a free atom occupy discrete energy levels’ (3)

Answer 7

• Electrons are placed in circular orbits around the nucleus.
• There are a limited number of allowed energy levels, so the orbitals are quantised.
• An electron, bound in an atom, occupies certain states, equal to the allowed levels.

Question 8

What is the ‘ground state’?

Answer 8

The energy level with the least energy, that is closest to the nucleus

Question 9

What is an ‘excited state’?

Answer 9

An energy level higher than the ground state, that is further away from the nucleus and has more energy

Question 10

What is the ‘ionisation level’?

Answer 10

When an electron gains sufficient energy it can escape from the atom completely, and this occurs in the ionisation level

Question 11

When will an emission line in a spectrum occur?

Answer 11

When an electron makes a transition between an excited energy level and a lower level

Question 12

When will an absorption line in a spectrum occur?

Answer 12

When an electron in a lower energy level absorbs radiation and is excited to a higher energy level.

Question 13

Why does white light produced from the sun have certain frequencies missing?

Answer 13

The white light passes through a gas cloud outside of the sun which absorbs these frequencies of light

Question 14

What conditions must be met in order for an electron to absorb a photon?

Answer 14

The photon’s energy must be an exact match for the difference in energy levels.

Question 15

What is ‘Spontaneous emission’?

Answer 15

This is a random process analogous to the radioactive decay of nucleus

Question 16

What is ‘Stimulated emission’?

Answer 16

This is when the incident radiation and the emitted radiation are in phase and travel in the same direction

Question 17

For what purpose did lasers receive their name?

Answer 17

The conditions in a laser are such that a light beam gains more energy by stimulated emission than it does by absorption - so hence the name Light Amplification by the Stimulated Emission of Radiation

Question 18

What is the function of the mirrors in lasers? (2)

Answer 18

• The photons are reflected back and forth by the mirrors at each end, creating an avalanche effect, and this amplification creates a powerful pulse of light.
• One of the mirrors is only partially reflecting, so the powerful beam of light escapes the laser through this mirrors to create the laser beam

Question 19

Why would a beam of laser light having a power of just 0.1mW still cause eye damage? (2)

Answer 19

• The diameter of a circular laser beam may stay at approximately 1mm over a long distance.
• This means that it has a much higher irradiance than conventional light sources