Unit 10 Light

1. Unit 10 Light Discover PHYSICS for GCE ‘O’ Level Science Total Internal Reflection

2. 10.5 Total Internal Reflection Learning Outcomes In this section, you’ll be able to: • Explain critical angle and total internal reflection

3. 10.5 Total Internal Reflection Total internal reflection can occur when light passes from an optically denser to a less dense medium.

4. 10.5 Total Internal Reflection What is the critical angle? 1. When a ray of light is directed perpendicularly through the semicircular glass block, the light ray will pass through without deviation. Figure 10.46

5. 10.5 Total Internal Reflection What is the critical angle? 2. When the light ray is directed at an angle i at point O, the light ray is refracted away from the normal when it travels from the glass into air. Figure 10.47

6. 10.5 Total Internal Reflection What is the critical angle? 3. As the incident angle is increased, the refracted ray is seen to bend further away from the normal. It comes a point when the refracted ray is 90°to the normal. Critical angle is the angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90°. Figure 10.48 Critical angle

7. 10.5 Total Internal Reflection What happens if the angle of incidence is greater than the critical angle? When the angle of incidence is increased beyond the critical angle, the light ray will be reflected back into the glass block. No light ray is refracted through. This is known as total internal reflection. Figure 10.49 Total internal reflection occurs when i > c

8. 10.5 Total Internal Reflection Total Internal Reflection takes place only when • a ray of light travels from an optical denser medium to a less dense medium • The angle of incidence in the optically denser medium is greater than the critical angle.

9. 10.5 Total Internal Reflection How to determine the critical angle of a medium? When the angle of incidence is at the critical angle c, then the angle of refraction r = 90o. Since the light ray is from the medium into air, we use the principle of reversibility. Hence Figure 10.48 Critical angle

10. 10.5 Total Internal Reflection Key Ideas • Total internal reflection takes place only when light travels from optical denser to a less dense medium. • The critical angle c is the angle of incidence in the optically denser medium at which the angle of refraction in the less dense medium is 90°. • The critical angle c is given by

11. 10.5 Total Internal Reflection Test Yourself 10.5 • Determine the critical angle of a glass prism with a refractive index of 1.9. Answer: Given refractive index, n = 1.9 Applying the equation Hence the critical angle c = 32o.