(or

1
The Photoelectric Effect

• (orhow Einstein really became famous)

2
Photo (light) electric

• A photoelectric effect is any effect in which
  light energy is converted to electricity.
• First explained by Albert Einstein in 1905

3
When light strikes certain light-sensitive
materials

• It may cause them to give up electrons.

4
Photoelectric Effect
Metal Foil
5
Photoelectric Effect
Metal Foil
6
Photoelectric Effect

• As blue light strikes the metal foil, the foil
  emits electrons.

7
Photoelectric Effect
8
Photoelectric Effect

• When red light hits the metal foil, the foil does
  not emit (give off) electrons.
• Blue light has more energy than red light.
• How could we get more energy into the red light?
• Try increasing the brightness (intensity).

9
Photoelectric Effect
10
Photoelectric Effect

• That didnt work
• Maybe its still not bright (intense) enough.

11
Photoelectric Effect
12
Photoelectric Effect

• Still not working.
• What happens with brighter (more intense) blue
  light?

13
Photoelectric Effect
14
Photoelectric Effect

• More blue light means more electrons emitted, but
  that doesnt work with red.

15
Photoelectric Effect
16
Photoelectric Effect

• High FREQUENCY light even from a dim source can
  eject electrons from a photosensitive metal

• Low Frequency light, even very bright (very
  intense) cannot dislodge electrons

17
Photoelectric Effect

• Light, like all other radiation, is made up of
  small particles called photons.
• The higher the frequency, the more energy the
  photons have.
• Einstein stated that light interacts with matter
  as a stream of particle-like photons.
• Einstein received the Nobel Prize for this
  discovery.

18
Einstein won the Nobel Prize for the
Photoelectric Effect, not E mc²
19
Photoelectric Effect

• If a photon hits an atom of a certain material,
  it may be absorbed by an electron of that
  material.
• However, if the photon has enough energy, the
  electron is ejected, or emitted, from the atom.
• In this way, light energy changes into electrical
  energy.

20
Photoelectric Effect

• If wires are attached to a photoemittive
  material, the electrons can flow along the wires,
  forming an electric current.

21
WHY IS THE PHOTOELECTRIC EFFECT SO IMPORTANT?

• It helped explain the particle nature of light.
• It is the basis of the quantum theory.
• It is used in photocells e.g. in solar
  calculators, alarms, automatic garage door
  openers, flash of a camera

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Burglar Alarms

• When the light source to the semiconductor is
  interrupted by a person walking through the beam
  of light, a switch is activated that triggers the
  alarm.

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Review- Photoelectric Effect

• What it is
• The ejection of electrons from certain metals
  when light falls upon them. The metals are said
  to be photosensitive (sensitive to light)
• Who and When
• Einstein explained the photoelectric effect in
  1905, but won the Nobel Prize in 1921