Lesson 58: Young's Double Slit Experiment
Thomas Young

In 1801 Thomas Young was able to offer some very strong evidence to support the wave model of light.

If light is a particle…

We set up our screen and shine a bunch of monochromatic light onto it.

Figure 1

If light is a wave…

If light is a wave, everything starts the same way, but results we get are very different.

Figure 2
Figure 3

We must conclude that light is made up of waves, since particles can not diffract.


When you set up this sort of an apparatus, there is actually a way for you to calculate where the bright lines (called fringes) will appear.

The formula that we will use to figure out problems involving double slit experiments is easy to mix up, so make sure you study it carefully.

λ = wavelength of light used (m)
x = distance from central fringe (m)
d = distance between the slits (m)
n = the order of the fringe
L = length from the screen with slits to the viewing screen (m)

Figure 5

It is very easy to mix up the measurements of x, d, and L.


Example 1: A pair of screens are placed 13.7m apart. A third order fringe is seen on the screen 2.50cm from the central fringe. If the slits were cut 0.0960 cm apart, determine the wavelength of this light. Roughly what colour is it?

Just to make sure you’ve got all the numbers from the question matched with the correct variables…

L = 13.7 m
n = 3
x = 2.50cm = 0.0250 m
d = 0.0960cm = 9.60e-4 m

It’s probably a yellow light being used given the wavelength we've measured.

If a white light is used in the double slit experiment, the different colours will be split up on the viewing screen according to their wavelengths.

There is also a version of the formula where you measure the angle between the central fringe and whatever fringe you are measuring.

Figure 6

Example 2: If a yellow light with a wavelength of 540 nm shines on a double slit with the slits cut 0.0100 mm apart, determine what angle you should look away from the central fringe to see the second order fringe?

Do not forget to:

  1. Change the wavelength into metres.
  2. Change the slit separation into metres.
  3. "Second order" is a perfect number and has an infinite number of sig digs.

The Single Slit

Figure 7

A surprising experiment is that you can get the same effect from using a single slit instead of a double slit.

Example 3: For a single slit experiment apparatus like the one described above, determine how far from the central fringe the first order violet (λ = 350nm) and red (λ = 700nm) colours will appear if the screen is 10 m away and the slit is 0.050 cm wide.

We need to solve the formula for “x”, the distance from the central fringe.

For the violet light…

For the red light…

You can actually do the single slit experiment wherever you are right now! Hold two of your fingers very close together; there should be only the tiniest little gap between them that you can barely see through. Look towards a light source, light a light bulb, through the gap in your fingers. In the gap between your fingers you shold see very faint gray lines that run parallel to your fingers... these are the destructive interference "dark" fringes!