Lecture 3: Nature of Light


Textbook: Chapter 5


Properties of Waves

A wave

For any type of wave (eg. sound, water, light)

  • The wave travels at a constant speed = c.
  • The distance between wavecrests is called the wavelength.
    Wavelength is denoted with the Greek letter "lambda" ()
  • The frequency is the number of times that the wave oscillates in a second.
    Frequency is denoted with the letter " f ".
For any type of wave, the frequency, wavelength and wave speed are related by
lambda = c/f
Since c is a constant, a wave with long wavelength vibrates slowly (frequency is small) and a short wavelength wave vibrates quickly.

For sound waves, our ears interpret the different wavelengths as different pitches. Long wavelength sound waves have low pitch and short wavelength sound waves have high pitch.


Light is an electromagnetic (EM) wave


An Electromagnetic Wave

  • The EM wave travels at a constant speed = c = 3 x 108m/s
  • equivalent to say radiation instead of waves
  • If the EM wave has wavelenth in the range
    400 nm < wavelength < 700 nm
    our eyes can detect the EM wave. (nm = nanometer = 10-9m)
  • EM waves in this range are called visible light.
  • Our eyes interpret EM waves with different wavelengths in the visible range as different colours.
  • In order of long wavelength to short wavelength:
    red, orange, yellow, green, blue, indigo, violet (ROYGBIV)






But visible light is not the only type of EM wave!

Order of types of EM waves from long wavelength to short wavelength: (also order of small frequency to high frequency)
TypeWavelength Range
Radio wavelength > 10-4 m = 0.1 mm
Infrared 700 nm < wavelength < 0.1 mm
Visible 400 nm < wavelength < 700 nm
Ultraviolet 20 nm < wavelength < 400 nm
X-rays 0.1 nm < wavelength < 20 nm
Gamma rays wavelength < 0.1 nm
The EM spectrum








Particle Properties of Light

  • Light also behaves as though it comes in particles called photons.
  • A photon has a wavelength and frequency associated with it.
  • The Energy carried by the photon is
    E = hf
  • In this equation, h is Planck's constant: h = 6.625 x 10-34J s.
  • Since fblue > fred, blue photons carry more energy than red photons!
  • (Opposite from most people's intuition)




Next Lecture: Blackbody Radiation
Read Chapter 5