Lecture 3: Nature of Light

Textbook: Chapter 5

Properties of Waves

Wave is periodic disturbance in space that oscillates

  • The distance between wavecrests is called the wavelength. Wavelength is denoted with the Greek letter "lambda" &lambda
  • The frequency "f" is the number of times that the wave oscillates in a second.
  • Time it takes for one complete oscillation is called period "T"
    T = 2 &pi / f
  • Most fundamental case: The wave travels. Position of crests move at velocity V . Velocity depends on properties of the medium.
  • For any type of wave, the frequency, wavelength and wave speed are related by
    f &lambda = V
    A wave with long wavelength vibrates slowly (frequency is small) and a short wavelength wave vibrates quickly.

Frequency of the wave determines its energy. Travelling wave transports energy.

  • When velocity of a wave changes, its frequency remain unchanged, but wavelength changes — f=V/&lambda =const. Why ? Energy conservation !
(animations courtesy of David Harrison collection at UofToronto)


Light is an electromagnetic (EM) wave

An Electromagnetic Wave is an alternating oscillations of the Electric (E) and Magnetic (B) fields

  • The EM wave travels in vacuum at a constant speed
    V = c = 3 x 108m/s
  • f &lambda = c
  • If the EM wave has wavelenth in the visibile range
    400 nm < &lambda < 700 nm
    our eyes can detect the EM wave. nm = nanometer = 10-9m)
  • 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)

EM Waves are produced my moving electric charges

Animation is here or here (courtesy of David Harrison at UofToronto)

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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 > 1 cm
Microwave 1 mm < wavelength < 1 cm
Infrared 700 nm < wavelength < 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

Richard Feyman on the EM waves and light

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Light Rays versus Light Waves

Far from the source the wave is planar

the crests of the wave form planes (in 3D) or straight lines (in 2D)
  • Planar wave has well-defined direction of propagation, thus we can think about light rays
    Physicists say "we can use geometrical optics approximation here"

Point source emits spherical waves.

  • Far from the source, segment of spherical surface looks like plane and we speak about rays . EM waves from distant stars is very planar !

Wave properties become important when wavelength is similar to the size of obstacles on the wave path.


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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