Unit 10 Light

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Unit 10 Light
Discover PHYSICS for GCE O Level Science

• Total Internal Reflection

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10.5 Total Internal Reflection

• Learning Outcomes
• In this section, youll be able to
• Explain critical angle and total internal
  reflection

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10.5 Total Internal Reflection

• Total internal reflection can occur when light
• passes from an optically denser to a less dense
• medium.

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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
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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
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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
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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 gt ?c
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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.

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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
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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

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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.