Hence, diffraction patterns usually have a series of maxima and minima.

The slit must satisfy two conditions in order the diffraction occur: first, the slit should has dimensions of infinitely length to width and second, the width of the slit is on the order of the wavelength of light being used.

The wavefront from a light source will form secondary waves. The one located at the top edge of the slit interferes destructively with other secondary wave located at the middle of the slit, when the path difference between them is equal to '/2. Similarly, the secondary wave just below the top of the slit will interfere destructively with the secondary wave located just below the middle of the slit. Thus we can conclude that the condition for destructive interference for the entire slit is the same as the condition for destructive interference between double slits with distance equal to half the width of the slit. The path difference is given by:

When monochromatic light illuminates a double slit aperture having dimensions of the order of the wavelength of light, diffraction of light occurs if the slits width much narrower than there lengths. The incident wavefront will divided into two point sources of light which can interfere with each other to produce an interference pattern

1. Constructive Interference - When the path difference between the two beams in an integral multiplication of the wavelength. The result is brighter illumination in these regions when a crest of a wave meets a crest from another wave

2. Destructive Interference - When the path difference between the two beams in an odd multiplication of half a wavelength. The result is dark bands in these regions when a crest of a wave meets a trough from another wave

Constructive interference occurs when:

(3.5)

Where:

' is the wavelength of the light,

d is the separation of the slits, the distance between (b) and (c) in (Fig.3.1)

n is the order of maximum observed (central maximum is n = 0),

x is the fringe distance, the distance between the bands of light and the central maximum.

L is the distance from the slits to the screen.

This is only an approximation and depends on certain conditions.

It is possible to work out the wavelength of the used light using this equation and the above apparatus. If (d) and (L) are known and (x) is observed, then ' can be easily calculated.

Objectives:

Examine the diffraction pattern formed by laser light passing through single and double slits.

Verify that the positions of the minima in the diffraction pattern match the positions predicted by the theory

To compare
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