Understanding Physics Topics is essential for solving complex problems in many fields, including engineering and medicine.
What is Diffraction of Light? What Phenomenon Can Diffraction of Light be Observed?
We know from our daily experience that sound wave bends while passing round the edges of an obstacle or spreads in all directions while passing through a slit or aperture. The same thing happens with light waves also. A thin tin sheet placed in sunlight, casts its shadow on a wall. Sun rays can be treated as parallel rays and according to geometrical optics, they travel in a straight line. So, a sharp shadow of the thin tin sheet should be observed in the wall. But if the shadow is examined carefully, it will be seen that the edges are not very distinct. The direction of the light wave changes while passing through the edges of an obstacle or through an aperture. This is called ‘diffraction’ of light.
Definition: Light rays, while passing round the edges of an obstacle or aperture, instead of travelling in a straight line, bend to some extent. This phenomenon is known as diffraction of light.
In Fig.(a), shadow of a blade, formed by a monochromatic point source, is shown. In Fig.(b), the diffraction pattern, formed just outside the shadow of the blade, is shown in an enlarged form. The shadow of the side of the blade is not very distinct. In diffraction due to slit or aperture, the deviation of the propagation of the wave depends on its wavelength and on the size of the slit or aperture [Fig.].
Comparison of diffraction of light with diffraction of sound: The wavelength of an audible sound is sufficiently long (from 1.6 cm to 16 m). Even if there is a big hole in the line of propagation of the wave, the wave deviates considerably while passing through it.
On the other hand, wavelength of visible light is very small, 4000Å – 8000A. Even a very fine slit, like the eye of a needle, is large enough in comparison to the wavelength of light. For a light wave, while passing through a slit large enough in comparison to its wavelength, there is no noticeable change in the direction of light, i.e., diffraction of light is in distinguishable [Fig.(a)].
Now for the same wavelength of light, as the aperture is gradually made finer, the diffraction of light becomes more distinct [Fig.(b)]. On the other hand, a distinct diffraction can also be made to occur by increasing the wavelength of light used, so that the slit can now be comparable in size with the wavelength of light [Fig.].
Some Special Conclusions: Observing the phenomenon of diffraction of light, the following conclusions can be drawn:
i) Like other waves, light also spreads like a wave.
ii) If the size of the apertures are much larger than the wave length of light, diffraction of light is not easily detectable. In that case, it can be said that light travels in a straight line. In case of very fine aperture, when light bends from its straight path, we come to know of the limitations of geometrical optics. That is why, the rectilinear behaviour of light according to geometrical optics is actually an approximate behaviour.
iii) When the edges of the obstacle or aperture are sharp, diffraction is more distinctly detectable.
iv) Diffraction validates the wave theory of light, but it does not give any information about the nature of light waves (whether it is longitudinal or transverse).
v) As the wavelengths are long, sound waves and radio waves are diffracted more prominently than other kind of waves.