AP Class 10 Physical Science Chapter 5 Refraction of Light at Plane Surfaces

You might have noticed a coin kept at the bottom of a vessel filled with water appears to be raised. This is an illusion created by the refraction of light. In AP Board Class 10 Physical Science Chapter 5 Refraction of Light at Plane Surfaces why such illusions occur.

What is Refraction?

The phenomenon of the change in direction of the light at the interface of two media is known as refraction. It is a result of the change in the speed of light at the interface.

Refractive Index

The ratio of the speed of light in vacuum to the speed of light in the medium is defined as refractive index ‘n’. It is called the absolute refractive index. Mathematically, it is represented as follows:

\(Absolute\,Refractive Index=\frac{Speed\, of\, Light\, in\, Vacuum}{Speed\, of\, Light\, in\, Medium}\) \(n=\frac{c}{v}\)

It is a dimensionless quantity and gives an idea of how fast or slow the light travels in a medium.

The refractive index of various materials is given below in a table.

Material Refractive Index
Water 1.33
Diamond 2.42
Ice 1.31
Air 1.0003
Benzene 1.50

In the next section, let us look at a few answered chapter questions, to better understand the concepts discussed in the chapter.

Chapter 5 Refraction of Light at Plane Surfaces Questions

  1. What is a total internal reflection?


The phenomenon in which the angle of incidence is greater than the critical angle resulting in the light being reflected into the denser medium at the interface is known as the total internal reflection.


  1. Why do stars appear twinkling?


The light from the stars while reaching the earth gets refracted due to the movement of air molecules in the atmosphere. This refraction results in the changes in the position and brightness of the star’s image, causing the twinkling effect of stars.

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