Photons

1
Photons

• Particle Physics Lesson 3

2
Learning Objectives

• To know what a photon is.
• To know how to calculate the energy of a photon.
• To be able to calculate how many photons are
  emitted by a light source every second.

3
Electromagnetic Spectrum
Part of E-M Spectrum Wavelength Range



Visible 400-700 nm



4
Electromagnetic Spectrum
Part of E-M Spectrum Wavelength Range
Radio gt 0.1m
Microwave 0.1m to 1mm
Infrared 1 mm to 700 nm
Visible 400-700 nm
Ultraviolet 400 nm 1 nm
X-rays lt 1 nm
Gamma-rays lt 1 nm
5
Mnemonic?

• Rabbits ? Radio
• Mate ? Microwaves
• In ? Infrared
• Very ? Visible
• Unusual ? Ultraviolet
• X-rated ? X-rays
• Gardens ? Gamma Rays
• Highest energy? Highest frequency?

6
Video

• In Search of Giants (11 of 15) The Weird Quantum
  World.wmv

7
Wave Equation (GCSE)

• Recall from GCSE-
• Electromagnetic radiation travels at the speed of
  light which is 3.00 108 m s-1 in a vacuum.

8
Wave Equation (AS Physics)

• In AS we write this equation using symbols-
• where c the speed of light in a vacuum (m s-1)
• f the frequency of e-m radiation (Hz)
• ? the wavelength of e-m radiation (m)

9
Photon Energy

• We can work out the energy of an incoming photon
  using this equation-
• Where E Energy of Photon in Joules (J)
• f Frequency of the Radiation in Hertz (Hz)
• h Plancks constant 6.63 x 10-34 Js
• Or in words-

10
Photon Energy

• Recall from GCSE that f c/? so we can
  substitute this into the photon energy equation
  Ehf to get
• Or in words-

11
Worked Example

• Q What is the photon energy for UV radiation
  with a wavelength 400 nm?
• ? 400 nm 400 10-9 m
• E ?
• h 6.63 10-34 J s
• c 3 108 ms-1

12
Light from a Laser (GCSE)

• Intense
• In Phase
• Same Frequency
• ? the last one also means same wavelength
• ? which means monochromatic (one colour)

13
Laser Beam

• Light from a laser is monochromatic.
• So each photon has the same freq and energy.
• If we call the no. of photons leaving the laser
  per second, n, then-
• Because this gives you the energy emitted per
  second which is power.

14
Worked Example

• A 5 mW laser pointer emits a beam of red light of
  wavelength 670 nm, how many photons are emitted
  per second?
• P 5 mW 5 10-3 W
• ? 670 nm 670 10-9 m
• n ?