Reflection and Refraction Understand the origin of

Reflection and Refraction • Understand the origin of the term refractive index and be able to do calculations using refractive index formulae. • Understand the terms total internal reflection, critical angle, dispersion, incident ray, refracted ray, normal. • Know that refractive index depends on wavelength and causes dispersion of light.

Refraction of Waves • This is the bending or change in direction of light when it passes from one medium to another. • The reason why light changes direction is due to fact that light travels at a different speed in different mediums. It always travels at the same speed in a vacuum. • This property of a medium is described by the refractive index of the medium

Consider a ray of light passing from vacuum/air into a glass block Angle of incidence Air Glass Angle of refraction

Absolute Refractive Index = Relative Refractive Index =

Example 1 What is the angle of refraction in the following situation? Water n = 1. 33 Glass n = 1. 64 300

Properties of light inside a medium • The frequency remains unchanged • The speed is reduced by n • The wavelength is reduced by n (Driving force due to alternating electric field of em wave acts on electrons in the solid)

Why does light bend at the interface?

Total internal reflection When rays pass from a medium of higher refractive index into one of lower refractive index, θr > θi However θr cannot be more than 90°. So, there is a limiting θi above which no refracted ray can be formed. This is called the critical angle θc. For angles more than θc rays are reflected back into the first medium. This is known as total internal reflection

Using : If the ray travels from the optically more dense material to the optically less dense and if θ 2 = 90 then:

Example 2 A beam of red light enters a rectangular block of transparent material shown. If its speed in air is 3 x 108 ms-1, at what speed must the light travel through the block so that it is just totally internally reflected at A?

Example 3 which of the three drawings here (if any) show physically possible refraction?

Light travelling in optical fibres

Refractive index depends on the wavelength (or frequency) of the light.

If white light is incident on a glass prism it is dispersed; i. e. , it is split into all the visible colours – a continuous spectrum

LECTURE CHECK LIST LECTURE 18 Reflection and Refraction READING Adams and Allday 6. 4, 6. 5, 6. 6. At the end of this lecture you should: • Understand the origin of the term refractive index and be able to do calculations using refractive index formulae. • Understand the terms total internal reflection, critical angle, dispersion, incident ray, refracted ray, normal • Know that refractive index depends on wavelength and causes dispersion of light