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Introduction
Questions to
think about
How we see
Reflection
Refraction
What is refraction?
Exploring prisms
Lenses and images
Refraction
Gems
Summary
Colour
Conclusion
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Activity 3: Lenses and refraction**

Gadgets that use lenses can enable us to see images.

When you hold up a lens and look through it, you can usually see an image through the lens (eg what you see when you look through a magnifying glass or telescope).

Sometime we want to see an image on a screen. Projectors use lenses to produce images on screens.
How do lenses change the direction of light rays and form images?

What to use
Light box and various lens shapes, white paper.
What to do
  1. Set up the light box so it throws four narrow beams of light.
  2. Use this to find out what happens when these rays strike a convex lens shape and a concave lens shape.
  1. Trace the rays to record your observations.
    Light box with four rays travelling through a convex lens
  2. Suggest a reason why the convex lens could focus an image on the screen but the concave lens could not.
  3. What happens to the rays when a thicker convex lens is used? How would this affect the position of the image?

Rules for refraction
See Activity 1

  1. Light rays may change direction when they travel from one material into another material (eg air and glass or plastic or water).
  2. When the light rays travel into a denser or thicker material they change direction away from the surface.
  3. When the light rays travel into a less dense or thinner material they change direction towards the surface.

Discussion

  1. Using the words 'converge' (move closer together) or 'diverge' (move further apart), describe the effect refraction had on the rays of light from the light box as they passed through the different lens shapes.
  2. What type of lens could not form an image on the screen?
  3. Why was the convex lens able to focus an image on the screen, but the concave lens could not?
  4. Which lens was the only type that could produce a real image?
  5. What was the only type of image a concave lens could produce?

Real and virtual images
A real image is an image that can be focused onto a screen.

An image that can be seen through a lens, but which cannot be focused onto a screen is called a virtual image.



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