Last lesson Refraction of light

Refraction When a wave changes speed (normally when entering another medium) it may refract (change direction)

Water waves Water waves travel slower in shallow water

Light waves Light slows down as it goes from air to glass/water

Refraction of light Glass block Beam of light Glass block Beam of light

Refraction Glass block Beam of light Light going from air to glass bends towards the normal Light going from glass to air bends away the normal

Snell’s law There is a relationship between the speed of the wave in the two media and the angles of incidence and refraction i r Ray, NOT wavefronts

Snell’s law i r Ray, NOT wavefronts speed in substance 1 = sin(i) speed in substance 2 sin(r)

Snell’s law i r Ray, NOT wavefronts In the case of light only, this quantity is called the refractive index (η) η = sin(i) sin(r)

Refraction – a few notes The wavelength changes, the speed changes, but the frequency stays the same Ripple Tank Simulation

Today’s lesson Critical angle Total internal reflection

Critical Angle?

Critical Angle Critical angle (when angle of refraction is 90º)

Critical Angle Critical angle (when angle of refraction is 90º) Copy! 90º

Total Internal Reflection Critical angle sin c = 1/η sin(critical angle) = 1/(refractive index)

Read pages 148 to 151

Examples of Total Internal Reflection

Questions! Page 149 Questions 1,2 and 3.