When light enters a more dense material, (e.g. perspex), it will slow down…  This is called Refraction. The speed of light in a vacuum is 3x10 8 ms -1 and because of this … it will change directiontowards the normal

As the light returns to the less dense air, it will change direction away from the normal  The change in speed (and hence direction) depends upon the index of refraction, n of the materials concerned.

Material Refractive Index (no units) Vacuum1.00 Air/gases1.00 Ice1.31 Water1.33 Perspex1.50 Glass(1.5 ~ 1.7) Diamond2.41 Typical Refractive Indices

For a light ray moving from material 1 into material 2, the change in direction is given by where n 1 is the refractive index of material 1, and is the angle between the incident ray and the normal in material 1. n1n1 n2n2 Material 1Material 2

Example 1 A light ray enters a perspex block at 30 o to the normal. Calculate the angle of refraction in the perspex. nana npnp AirPerspex n a = 1.00 n p = 1.50

Determining Refractive Index by Experiment Perspex D Block D-

Providing that the light ray is moving from air to another material (e.g. perspex), the following will be true: and therefore: y = m x A straight-line graph can be obtained, which should go through the origin, with a gradient equal to the refractive index of the perspex.  This special case is only true if air is the first material!

Effect on Frequency and Speed The frequency of a wave is entirely determined by the source of that wave. The frequency of a wave does not change after leaving its source. The speed of a light ray will change on entering a different material. The change in speed is described by a variation of Snell’s Law: n1n1 n2n2 Material 1Material 2 n 1 v 1 = n 2 v 2

Effect on Wavelength Because the speed changes, the wavelength of the light ray must also change. This is because the frequency, wavelength and speed are linked by the Wave Equation:  Speed = Frequency x Wavelength If speed decreases, and frequency is constant, the wavelength must also decrease. n1n1 n2n2 Material 1Material 2 n 1 1 = n 2 2

Example 2 A red light ray (633 nm) enters a glass block at 30 o in air. The angle of refraction in the glass is measured as 17 o. Calculate: a)The refractive index of the glass block. b)The speed and wavelength of the light ray in the glass block. a)

b)