Embedded Files
objectives
objectives
Extension
- Describe Internal and Total Internal Reflection using ray diagrams
- Give the meaning of critical angle
- Describe and explain the action of optical fibres particularly in medicine and communications technology
Total Internal Reflection
Total Internal Reflection
- The inside surface of water, glass, or other transparent material can act like a perfect mirror, depending on the angle at which the light strikes it.
- The diagrams below show what happens to three rays leaving an underwater lamp at different angles. Angle c is called the critical angle. For angles of incidence greater than this, there is no refracted ray. All the light is reflected. The effect is called total internal reflection.
The ray splits into a refracted ray and a weaker reflected ray.
The rays splits, but the refracted ray only just leaves the surface.
There is no refracted ray. The surface of the water acts like a perfect mirror.
- The value of the critical angle depends on the material. For example:
practical activity - finding a critical angle
practical activity - finding a critical angle
Complete the practical activity on page 10 of the workbook
Reflecting Prisms
Reflecting Prisms
- In the diagrams to the right, inside faces of prisms are being used as mirrors. Total internal reflection occurs because the angle of incidence on the face (45°) is greater than the critical angle for glass or acrylic plastic.
- Periscope This is an instrument for looking over obstacles. Prisms reflect the light, although they can be replaced with mirrors.
Applications of Total Internal Reflection
Applications of Total Internal Reflection
Rear reflectors (on cars and cycles) The direction of the incoming light is reversed by two total internal reflections.
- Binoculars The lens system in each 'barrel' produces an upside-down image. Reflecting prisms are used to turn it the right way up.
Optical Fibres
Optical Fibres
Optical fibres are very thin, flexible rods made of special glass or transparent plastic. Light put in at one end is total internally reflected until it comes out of the other end. as shown below. Although some light is absorbed by the fibre, it comes out almost as bright as it goes in - even if the fibre is several kilometres long.
Single optical fibre In the type shown above, the inner glass core is coated with glass of a lower refractive index.
- Bundle of optical fibres Provided the fibres are in the same positions at both ends, a picture can be seen through them.
- Optical fibres can carry telephone calls. The signals are coded and sent along the fibre as pulses of laser light. Fewer booster stations are needed than with electrical cables.
- This photograph was taken through an endoscope, an instrument used by surgeons for looking inside the body. An endoscope contains a long, thin bundle of optical fibres.
activity - questions on total internal reflection
activity - questions on total internal reflection
- Glass has a critical angle of 41 °. Explain what this means.
- Copy and complete the diagrams below to show where each ray will go after it strikes the prism, If the prisms on the right were transparent triangular tanks filled with water, would total internal reflection still occur? If not, why not?
- Give two examples of the practical use of optical fibres
- Give two other examples of the practical use of total internal reflection.
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