From the graph you can picture the diffraction pattern of a bright fringe in the centre of the screen with alternate light and dark fringes either side which fade away.
Diffraction of light in this way can be increased by making the width of the slit smaller or by increasing the wave length.
Explaining the diffraction patterns
The diffraction patterns which are produced from light diffracted by a single slit are a result of superposition. In the forward direction all the waves are travelling in phase and all have the same distance to travel to the screen - when they reach the screen their displacements add up creating a high intensity bright fringe.
To the sides of this central bright region however and some waves have to travel further to reach the screen. These waves have a path difference of - that is they have to travel half a wave length further to reach the screen. As a result the waves are out of phase and so the displacements cancel out causing a bright fringe.
Interference (Young's 2 slit experiment)
Interference occurs when waves meet and superposition occurs. Consider the light coming from a laser passing through two slits:
The light from the laser and therefore the waves leaving the slits is coherent, their phase difference is constant.
The dark green area, where the two light sources cross, is where interference occurs. Once again superposition can be used to explain the diffraction pattern produced.
Interference can be constructive - the two waves at a point are in phase and so add up creating a superpeak or it can be destructive - the two waves are out of phase and cancel each other out.
Explaining the result
Consider the following diagram:
Here we've focus on just 3 pairs of waves from the slits. At point 1, both waves have the same distance to travel to the centre of the screen, their displacements add up (as they are coherent) and so a bright fringe is formed.
At point 2 the wave from the lower slit has further to travel, when it arrives it has travelled further and therefore the two waves interfere destructively, causing a dark fringe.
At point 3 the wave from the lower slit has had to travel further still, but this time it has travel a whole wavelength further meaning it is back in phase with the other wave, the two interfere constructively.
Calculating wave length of light
From the above diagram the wavelength of the light can be calculated using the following formula:
Wave properties of light
This result provides evidence for light behaving as a wave - diffraction and interference are properties of waves.
Diffraction gratings
A diffraction grating may consist of thousands of slits per cm and so diffract light very well:
A formula exists which relates the angle of the fringe with the wave length:
Where d is the slit separation and the other values are obtained from the above diagram.
Diffraction by circular hole
When light is diffracted by a small pinhole, such as the pupil of the eye, a diffraction pattern of a central bright spot surrounded by alternating bright and dark rings is formed. The relationship between the angle of the first dark ring and the wavelength of the light is:
Where D is the diameter of the whole and is the wavelength
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