Light Is A Wave

1
Light Is A Wave
2
What IS Light?

• Christiaan Huygens (1690) Light is a wave
  traveling through ether (invisible medium)
• Issac Newton (1704) Light is a stream of
  corpuscles
• Thomas Young (1803) Interference of light ?
  light is a wave

3
What IS Light? (Still Looking)

• Planck (1900) Radiation theory included
  equipartition or quantization of energy
• Einstein (1905) Photoelectric Effect ? light is
  a stream of particles (photons)
• Bohr (1913) Model of the Hydrogen atom included
  absorption and emission of quantized units of
  energy

4
Whats Up With Light?

• Light behaves like both a wave and a particle,
  depending on the circumstances
• Physicists have come to accept this dual nature
• Here we will treat light as a wave to do our
  experiments

5
The Electromagnetic Spectrum
6
Diffraction Circular Aperture

• When light passes through a pinhole, it creates a
  bright central disk and a series of fringes
• This happens in telescopes, microscopes, anything
  round!
• This limits the instruments ability to see fine
  detail

7
What the Pattern Looks Like

• The bright disk in the middle is called the Airy
  Disk
• A 2.44l/D
• A is the angular diameter of the Airy Disk, l is
  the wavelength of the light, D is the diameter of
  the pinhole
• The wavelength of the light were using is 675 nm
  (6.75 x 10-5cm)

8
Linear Size of the Airy Disk

• We know A 2.44l/D
• If s is the diameter of the disk and l is the
  distance from the pinhole to the image
• A s/l
• s 2.44ll/D

9
Diffraction by a Slit

• When light passes through a slit, it looks like a
  1D version of the pinhole pattern
• Now you should pay attention to the light and
  dark fringes

10
Distance to the Fringes

• For a slit
• s nll/2D
• l is the distance from the slit to the image
• D is now the width of the slit
• n is an integer If n is odd, it is the distance
  to a bright fringe
• If n is even, it is the distance to a dark fringe
• NOTE n1 corresponds to the central fringe
• Note how the fringes change with different slit
  sizes

11
How to Measure the Fringes
12
Diffraction Two Pinholes

• The most convincing proof of the wave nature of
  light came from the interference of light beams
• Note the effect it has on the laser spot
• The apparatus here was originally built to prove
  the ether theory but ended up disproving it!

13
Whats Happening Here?

• The light passes through both pinholes and
  creates two waves
• These waves interfere with each other and create
  fringes on the screen
• There are two kinds of interference fringes
  constructive (bright) and destructive (dark)

14
Distance Between the Pinholes

• For the two pinholes
• s ll/D
• D is the distance between the two pinholes
• s is the spacing between two adjacent fringes
• l is the distance from the pinholes to the image

15
Basic Lab Procedure

• Pick a laser station and make sure you have a
  fresh piece of paper tacked up
• Using the different slides (pinholes, slits,
  double pinholes) measure the required quantities
  stated in the lab manual
• Dont forget to look through the diffraction
  grating at the end!