P8.2 – Refraction of Light

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1. Describe an experimental demonstration of the refraction of light.

When you put a pencil in a full glass of water, the submerged pencil, when viewed from the side, seems to be bent in the water. This is because the light rays bend when they enter a different medium to the one they were originally travelling in – in this example, they enter the air from the water. This phenomenon is called refraction.

 

2. Describe, using ray diagrams, the passage of light through parallel-sided transparent material, indicating the angle of incidence i and angle of refraction r.

When light travels from a less dense material to a more dense material (e.g. from air to glass), the light ray bends towards the normal. That is, the angle of incidence > angle of refraction.
When light travels from a more dense material to a less dense material (e.g. from glass to water), the light ray bends away from the normal. That is, the angle of refraction > angle of incidence.

 

3. State the meaning of critical angle

The critical angle is the angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally internally reflected.

In other words, it is the angle between the incident ray and normal, when the refracted ray is parallel to the medium’s surface.

 

This will make more sense in the next point.

 

4. Identify and describe internal and total internal reflection using ray diagrams. 

When the Angle of Incidence is equal to or more than the Critical Angle, total internal reflection is achieved and the ray of light is completely internally reflected.

 

5. Describe the action of optical fibres and their use in medicine and communications technology.

Optical fibres are cables made from high-quality glass or plastic. When light enters one end of an optical fibre cable, it undergoes total internal reflection until it reaches the end of the cable. Digital signals are emitted as pulses of light (either Visible Light or Infrared Light) and reflected along the cable until they reach their destination. This method allows the transmission of information at high speeds.

When Infrared is used, the cables are lined with glass. When Visible Light is used, the cables are lined with plastic.

The Visible Light and plastic combo is cheaper but lower quality – the different colours in visible light will have slightly different critical angles, so the signal becomes more distorted the longer it travels. Also, at a molecular level, plastic will have more irregularities than glass, which affects the direction that the rays will be reflected.

This is why the glass and IR combo is always used when the information has to be transmitted over long distances.

 

 

 

Notes submitted by Lintha and edited by Sarah.

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