Take a glass of water, and set it on the kitchen counter. The glass should be smooth and without ridges or any other design. Then take a pencil and place it inside the water, so that it leans on the glass’s sides. Make sure the pencil is big enough to poke out of the water. Then lean down and look at the glass from the side. What do you notice about the pencil? Take the pencil out, make sure it’s still in one piece, and then put it back inside the glass. Close your eyes and pinch yourself—but, nope, you’re not dreaming, the pencil appears sluiced in the glass of water!
This apparent miracle is not quite as miraculous as it may seem, and as most ‘miracles’ around us are merely proof of scientific phenomena, this too is evidence of a phenomenon called refraction. Refraction is a variation to the reflection standard. As a quick recap, we see opaque objects because they obstruct light and reflect it into our eyes. But as mentioned before, this is not the case for transparent objects, because they don’t obstruct light; they allow it to pass through.
So what happens then? If you can recollect the diffraction theory, you will remember that light travels at different speeds in different media. In air, this speed is 3*108 m/s. However, when light enters a transparent material like glass, its speed differs—in this case, it slows down to 2*108 m/s. This is because glass is optically denser than air, that is, it obstructs the path of light waves more than air does. This very important piece of information gives rise to the bending of light.
This bending is easiest to explain if you think about incident light waves on a water body as a series of wavefronts, that is, representatives of the motion or propagation of the light waves, as shown below in the diagram. When the wavefront reaches the water surface (water being optically denser than air), the portion of the wavefront travelling through the new medium, i.e. water, slows down while the rest of the wave continues at a higher speed. As a result, the wavefront bends. In this case, the incident ray forms the angle of incidence with the normal, but the ray formed in water is called the refracted ray, and the angle formed with the normal is the angle of refraction. As you can see, the angle of refraction is not equal to the angle of incidence.
In fact, the angle of refraction is always bigger than the angle of incidence when light moves from an optically denser medium to a rare medium, and always smaller than the angle of incidence when the ray moves from a rarer medium to a dense medium (as in the case of the wavefront incident on the water). This is one of the laws of refraction, which are as follows:
- When the wavefront moves from an optically denser medium to a rarer medium, it bends away from the normal,
- When the wavefront moves from a rarer medium to a denser medium, it bends towards the normal,
- When the incident ray coincides with the normal, it is not refracted,
- An incident ray entering a second medium, passing through it and consequently emerging is displaced sideways but parallel to the original wavefront, and
- The refractive index n is found by dividing the sine of the angle of incidence by the sine of the angle of refraction, or alternatively, by dividing the speed of light in air by the speed of light in the medium.
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