Instances of light refraction

In our last The world of Science page, we told you what refraction is. Today, we bring you more facts about refraction.

Did you know that due to atmospheric refraction, there would be an apparent shift in the position of the Sun during sunrise and sunset? During sunset, the Sun can be seen for several minutes after it has sunk below the horizon. Do you know how this happens?

The Earth's atmosphere is dense (thick) near the surface and becomes rarer higher up. Light travels faster in thin air than it does in dense air. The sunrays are coming from rarer to denser medium. Therefore, the rays bend towards the normal (as shown last week).

Light from the Sun, which is travelling for longer in thin air than in dense air, can get to us more quickly, even though the path of these rays is longer. These rays go through the atmosphere at a steeper slope. Therefore, for the observer, the Sun appears to be higher in the sky.

The time difference between an actual sunset and the visible sunset is about two minutes. The Earth's atmosphere is denser near the surface and rarer above it, and hence, the light path bends gradually to produce a curved path. This path of least time provides us with a slightly longer period of daylight each day.

A traveller in the desert thinks there is an oasis or water due to the apparent shimmering(glistening) reflection he/she sees. The refraction of light is responsible for many illusions; one of them is the apparent bending of a spoon partly immersed in water. The sunken part seems closer to the surface than it really is.

The optical(visual) characteristic of a material is given by what is called its refractive index (‘). It is defined as ‘ = Speed of light in air or vacuum (c)/Speed of light in the medium (v)

When you see a fish in the water, the fish appears nearer to the surface and closer than it really is. This is due to the refraction of light, which takes place when light rays pass from the water into the air.

The rays of light coming from the fish are travelling in water before coming out into the air. While coming out into the air, the rays refract and bend away from the normal and refracted rays, when produced backwards, meet at a position, which is slightly above the position of the fish.

In fact, the refracted rays, which enter the eye, appear to be coming from this slightly higher position. Hence, the fish appears to be nearer the surface than it actually is. It is also due to refraction of light that a thick glass slab appears to be less thick (when seen from above).

Similarly, an ink mark or a line drawn on a piece of paper appears to be raised and much nearer than it actually is, when viewed through a glass slab placed over it. A coin placed at the bottom of a container appears to rise as the container is slowly filled with water, due to the refraction of light.

The incident of refraction, or bending of light beams, therefore depends on the optical density of the media and the wavelength of light. The speed of light is c in a vacuum, but in optical media, it is seen that the speed of the violet-coloured wavelength is less than that for the red-coloured wavelength.

Summary

* Light bends towards the normal when the ray goes from a rarer medium to a denser medium, such as when light goes from air to glass.

* Light bends away from the normal when the ray goes from a denser medium to a rarer medium, such as when light goes from glass to air.

* Total internal reflection occurs beyond a certain critical angle of incidence when light goes from a denser to rarer medium.

* If light is along the normal, the refracted wave travels into the medium along the normal itself.

* The optical characteristic of a medium is defined by its refractive index.

Compiled by Janani Amarasekara