Refraction at Plane Surfaces — Part 1: Laws, refractive index and the glass block: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Why does a straw look broken in a glass of nimbu pani?

Part 1 — light changes direction when it crosses from one transparent medium into another. This bending is refraction.

In this lesson you will

State the laws of refraction and the conditions for a ray to pass undeviated
Say which of speed, wavelength and frequency change when light enters a denser medium
Calculate refractive index from speeds of light, and use v = f λ
Trace a ray through a glass block, mark the lateral displacement and explain multiple images in a thick mirror

Table of four everyday observations and the behaviour of light behind each
What you notice	What is happening
A straw looks broken at the surface of a drink	Light bends as it leaves the water
You see a faint reflection in a shop window	Part of the light is reflected at the glass
Writing under a glass paperweight looks raised	Light bends as it leaves the glass
A bathroom mirror shows a faint double image	Light reflects inside thick glass

Diagram

Partial reflection and refraction

When light meets the boundary between two transparent media, part of it is reflected back and most of it is refracted into the second medium. Going from a rarer medium (air) into a denser one (glass), the ray bends towards the normal ( $r < i$ ); going from denser to rarer it bends away from the normal. The cause is the change in the speed of light from one medium to the other.

Concept

The laws of refraction

First law: the incident ray, the refracted ray and the normal at the point of incidence all lie in one plane.

Second law (Snell’s law): for a given pair of media and a given colour of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant: $\dfrac{\sin i}{\sin r} = {}_{1}\mu_{2}$ , the refractive index of medium 2 with respect to medium 1. In the graph, every reading for one glass lies on a straight line of slope 1.5.

glass: slope 1.5

Snell’s law: sin i is proportional to sin r.

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