3.3(a) - Optoelectronics Flashcards

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0
Q

What is the photoelectric effect?

A

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

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1
Q

What is ‘Irradiance’?

A

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

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2
Q

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

A
  • The metal must be negatively charged

* The radiation must be short wavelength (high frequency)

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3
Q

What is ‘threshold frequency’?

A

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

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4
Q

What happens with frequencies smaller than the threshold value?

A

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

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5
Q

What happens with frequencies greater than the threshold value?

A

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

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6
Q

What is the ‘work function’?

A

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

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7
Q

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

A
  • 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.
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8
Q

What is the ‘ground state’?

A

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

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9
Q

What is an ‘excited state’?

A

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

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10
Q

What is the ‘ionisation level’?

A

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

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11
Q

When will an emission line in a spectrum occur?

A

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

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12
Q

When will an absorption line in a spectrum occur?

A

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

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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

What is ‘Spontaneous emission’?

A

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

16
Q

What is ‘Stimulated emission’?

A

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

17
Q

For what purpose did lasers receive their name?

A

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

18
Q

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

A
  • 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
19
Q

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

A
  • 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