Young's Experiment

Thomas Young is one of the scientists who aimed to prove that light is a wave. He claimed that like other types of waves, light waves can also interfere with each other. To prove his claim, Young experiment using a monochromatic light source that will pass through barriers with slit arranged consecutively.

In 1801, Thomas Young experimentally proved that light is a wave, contrary to what most other scientists then thought. He proved that light is a wave by demonstrating that light, just like the other waves, also undergoes interference.

Young also was able to measure the average wavelength of sunlight as 570 nm which is impressively close to the modern accepted value of 555 nm.

In his experiment, he used a monochromatic light source and screens with different number of slits on them. These materials are arranged as shown in the figure below (Figure 1).

Figure

As the light wave pass through screen A, it illuminates the slit 0. The light that emerges from the slit will then spread through diffraction which then illuminates slits 1 and 2 in screen B. Light will again be spread from the two slits through diffraction, in which overlapping circular waves are formed on the region beyond screen B. The waves from each of the two slits will then interfere each other, thus forming interference of light.

A third screen is placed at the end part of the experiment, screen C in the figure, which intercepts the light to see an evidence that interference really happens. A pattern of bright and dark fringes appears on the screen which is called as the interference pattern. The bright fringes, also called as the bright bands results from constructive interference. It is also called as maxima. The dark fringes, or dark bands, which is also referred to as minima, is the result from fully destructive interference which can be seen in between two adjacent pairs of bright bands.

The figure (Figure 2) below shows the interference pattern that would be seen by an observer to the left of screen C.

Figure

How can the location of a fringe be determined?

To specify the location of the bright and dark fringes on the screen, the angle from the central axis to the particular fringe must be determined. To find the value of this angle, $$\theta$$, it must be related to the path length difference, $$\Delta L$$. Letting $$d$$ be the slit separation, then

$$\sin \theta = {\Delta L \over d}$$,     (1)

and this follows that the path length difference is

$$\Delta L = d \sin \theta$$.     (2)

For a bright fringe, $$\Delta L$$ must be either zero or an integer number of wavelengths. Letting $$m$$ be the integer, equation (2) can be written as

$$\Delta L = d \sin \theta = m\lambda$$     (3)

where m = 0, 1, 2, …

While for a dark fringe, the path length difference must be an odd multiple of half a wavelength. Using equation (2), the equation for the path length difference for a dark fringe can be written as

$$\Delta L = d \sin \theta = (m + \frac 12) \lambda$$     (4)

where m = 0, 1, 2, …

Equations (3) and (4) helps us to solve for the angle to any fringe, and it follows that the location of the fringe can be determined.