The Human Eye and the Colourful World — Part 2: Prisms, rainbows and the sky: visual lesson, CBSE Class 10 Science

Lesson

The Colourful World

Part 2 — how a glass prism bends and splits light, how raindrops make a rainbow, and how the atmosphere bends and scatters sunlight.

In this lesson you will

Trace a ray through a triangular prism and identify the angle of deviation
Explain dispersion and recombination of white light
Explain how a rainbow forms
Explain twinkling of stars, advance sunrise and delayed sunset using atmospheric refraction
Explain the Tyndall effect and the blue colour of the clear sky

Concept

The triangular glass prism

A prism has two triangular bases and three rectangular lateral faces. Light enters through one lateral face and leaves through another.

Table of prism terms: refracting faces, angle of the prism between the two refracting faces, the base opposite it, and the angle of deviation between the incident ray produced and the emergent ray produced back
Term	Meaning
Refracting faces	The two lateral faces through which light enters and leaves
Angle of the prism (A)	The angle between the two refracting faces
Base	The face opposite the angle of the prism
Angle of deviation (δ)	Angle between the incident ray produced forward and the emergent ray produced backward

Diagram

Path of a ray through a prism

Here i = 48°, r = 29.7°, e = 49.2° and δ = 37.2° for glass of refractive index 1.5 and A = 60°.

At the first face light bends towards the normal (air to glass); at the second face it bends away from the normal (glass to air). The ray is bent towards the base.

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