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Activity 1: Exploring prisms* Overall management

A pair of binoculars is really two telescopes side by side. However, telescopes can be fairly long pieces of apparatus.

To make binoculars shorter the light is made to follow a zigzag path. This is done using prisms.
How do prisms affect the path of light rays?

What to use
Rectangular glass prism, triangular glass prisms (various angles), light box with prisms, plain paper, a page of text from a book or newspaper, 'Refraction' sheet.
What to do
Record your observations for each of the following activities.
A    Rectangular glass prisms
Try standing a rectangular glass prism on various faces on a page of a book or newspaper.

  1. How does the writing on the page appear when viewed through the top of the rectangular prism?
  2. Where have you seen this effect before?
    Prisms on different faces
  1. Record your observations.
  2. Can you see the text through all faces of the prism? Look at the writing through each side of the prism at several different angles (eg directly above and sideways). Can you see the text through all faces and at all angles?
    Viewing text at different angles through a prism
  1. Record your observations.

Transparent means that light can pass through and you can see clearly what is on the other side.

Is glass really colourless?

Examine several glass prisms to find your answer.

Adjacent sides
Sides that are alongside each other are called adjacent sides.

B    Triangular prisms
  1. What are the shapes of the triangles in the various triangular glass prisms? There could be at least two triangular shapes.
  1. Make a sketch of each and describe (or measure) the angles in each.
  2. How does a triangular prism affect the direction of your view when you look through it?
  3. Look through two sides of a triangular prism which are alongside each other.
    Looking through a prism
  1. Describe what you see.

C    What is happening to the rays of light?
The light box kit contains several types of prisms. They can be used to trace the path of a ray of light as it passes through a prism.

  1. Hints Start by using the diagrams on the Refraction sheet.
    Rays through various prisms
  1. To record your observations, trace around each prism on a sheet of white paper.
  2. Trace the path of the ray(s) of light as they enter and then leave the prism. Once you have drawn the ray going into the prism and the ray coming out of the prism, you can join them to show the path of the ray inside the prism.
    Path of a ray through a triangular prism
Sometimes the rays are reflected a little off the faces of the prism, producing extra rays. Ignore these at this time.

  1. After you have completed the sheet, try some more of your own. Predict where you think the rays will go before you turn on the light box.
Discussion Strategies

  1. Was there any consistent change of direction that occurred each time when the light ray entered the prism? (Hint: Look at the prism surface and the ray. Are they closer or further apart?)
    The change of direction of a light ray travelling through a prism
  2. Was there any consistent change of direction that occurred each time when the light ray left the prism?
  3. Hints  Summarise these observations into a 'Law of refraction' (ie describe what happens when light passes into or out of something like glass which is thicker or denser than air).

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