Unit 10 LightTotal Internal Reflection

Discover PHYSICS for GCE ‘O’ Level Science

10.5 Total Internal Reflection

Learning OutcomesIn this section, you’ll be able to:• Explain critical angle and total internal

reflection

10.5 Total Internal Reflection

Total internal reflection can occur when lightpasses from an optically denser to a less densemedium.

10.5 Total Internal ReflectionWhat 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

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

10.5 Total Internal ReflectionWhat 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

10.5 Total Internal ReflectionWhat happens if the angle of incidence isgreater 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

10.5 Total Internal ReflectionTotal Internal Reflection takes place only when1. a ray of light travels from an optical denser

medium to a less dense medium2. The angle of incidence in the optically denser

medium is greater than the critical angle.

10.5 Total Internal ReflectionHow 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

osin 90 1n = =

sin c sin c

1sin c =

n

Figure 10.48 Critical angle

10.5 Total Internal ReflectionKey Ideas1. Total internal reflection takes place only

when light travels from optical denser to a less dense medium.

2. 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°.

3. The critical angle c is given by

1sin c =

n

10.5 Total Internal ReflectionTest Yourself 10.51. Determine the critical angle of a glass prism

with a refractive index of 1.9.

Answer:Given refractive index, n = 1.9Applying the equation

o

1sin c =

n1

sin c = = 0.531.9

c = 32Hence the critical angle c = 32o.