1. Lasers and Optics LO: to be able to calculate the refractive index using the critical angle of a substance.

2. Starter Questions: What is total internal reflection? Draw a diagram to show what the path of a light ray looks like if it enters a glass block at the critical angle. What is the formula for working out the refractive index when a ray travels from the air into a transparent substance? Give 2 applications of total internal reflection “in action.”

3. Refractive index recap Question: The refractive index of water is 1.33. A light ray enters a flat water surface at an angle of incidence of 35.0°. Calculate the angle of refraction for the light ray. A light ray travels from water into air. The angle of incidence of the light ray in the water is 45.0°. Calculate the angle of refraction of the light ray in the air.

4. Can you find the critical angle for the given glass blocks? Use these diagrams to help you Total internal reflection occurs when the angle of incidence is greater than the critical angle of a substance.

5. Total internal reflection The refractive index of a material and the critical angle of a substance are related. The higher the refractive index the lower the critical angle to make the light totally internally reflect. n = 1 or sin c = 1 sin c n n= refractive index c = critical angle

6. Total internal reflection the sine of the angle in air = n x the sine of the angle in glass This equation can be applied to the critical angle of a substance. The angle of incidence in air for a critical angle is 90°. sin 90 = n x sin c (where n = refractive index and c = angle in glass) sine 90 = 1 Therefore: n = 1 or sin c = 1 sin c n

7. What gives you that sparkle. Glass vs. diamond. Diamond is much more dense than glass. This means diamond has a higher refractive index than glass (it bends light more) This means light is totally internally reflected at a lower critical angle than glass. This is why diamonds sparkle so much more than glass. (more light gets twinkled into your eyes)

8. Practice q: The refractive index of water is 1.33. Calculate the critical angle of water. The critical angle for a certain type of glass is 42°. Calculate the refractive index of the glass.

9. Uses of total internal reflection The Endoscope Consists of 2 bundles of optical fibres. One is used to send the light rays down the endoscope the other is used to see the internal surfaces. It has a tiny lens over the bundle to form an image on the ends of the fibres in the bundle. A laser can be used alongside an endoscope to carry out surgical procedures. The energy is concentrated into a very small area and can be used for burning away or destroying diseased tissue and cauterising blood vessels.

10. Endoscopy and Lasers Kerboodle sheet