Chapter 13 - Light Waves

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Reflection

When a ray of light hits a plane (flat) mirror, it is reflected. The angle of incidence is equal to the angle of reflection. Mirrors are often used to change the direction of a ray of light. for example, a perescope uses two mirrors to see what they wouldn't originally be able to see. 

Images created by a plane mirror:

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Properties of an image in a plane mirror:

  • The image is as far behind the mirror as the object is in front
  • The image is the same sze as the object appears to be
  • The image is virtual - connot be produced on a screen
  • The image is laterally inverted - the left side and the right side of the image appear to be interchanged.
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Refraction

Similar to the refraction of all waves, light waves change direction slightly as they pass through a different medium. As a ray enters a glass block, it is bent towards the normal. As the ray keaves the block, it speeds up and is refracted away from the normal (at the same angle as when it entered the block). The change in speed causes the ray to change direction.

If the block id perpendicular to the ray of light, the ray is not refracted it just goes straight through on a straight line. 

Here is a diagram of refraction:

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Total Internal Reflection

1) When light passes from a dense medium to a less dense medium (e.g. glass to air) most is refracted but some can be reflected from the boundary. 

2) As the angle of incidence increases, the angle of refraction also increases until it reaches the critical angle (where the refracted ray runs perpendicular to the normal). 

3) If you further increase the angle of incidence, all of the light is reflected and none is refracted. This is total internal reflection.

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Using Total Internal Reflection (TIR)

Prismatic Perescope:

  • Instead of mirrors, prisms can be used in perescopes.
  • They use the principle of TIR to reflect the light rays.
  • It is effective because no multiple images are created.

Optical Fibres:

  • An optical fibre is made of a core of dense glass with an outer layer of less dense glass.
  • As the fibres are so narrow, when light enters it is always greater than the critical angle, meaning that the light is totally internally reflected. 
  • It does this several times so the the image bounces many times, sending itself to the other end of the fibre.
  • These are used in endoscopes so doctors can see inside their patients.

Dispersion:

  • When white light is passed through a prism, the light separates into the 7 colours.
  • This allows us to see ROYGBIV.
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