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Activity 2: Lenses and images* Overall management

Many of the optical devices you explored at the start of this topic used lenses. What can these lenses do?

Often a scientific word is invented to give a name to something to make it easier for scientists to be sure that they are all talking about the same thing. It will be easier to do this activity if you can all talk about lenses by name rather than have to describe them in detail each time.

Convex and concave
Concave and convex lenses
When the edges of the surfaces of the lens are curved away from each other it is said to have a convex shape, as shown in the side view of the lens.

When the edges of the surfaces are curved towards each other it is said to have a concave shape, as shown in the side view of the lens.

Can you identify the types of lenses?

What to use
The tray of mixed lenses (labelled A, B, etc).
What to do Strategies
  1. In your group, each student in turn picks a different lens from the tray of mixed lenses.
  2. Tell the rest of the group whether it is concave or convex.
  3. Others in the group check that the lens has been correctly named.
  4. Go through the tray several times until you are sure everyone in your group can name lenses correctly.
What do some images look like?
What to use
Range of lenses (at least four) including one fat and one thin lens, and one that is concave and one that is convex, a book with text and pictures.
What to do
Do each of the following steps individually and compare your observations with the others in your group.

  1. Stand a book vertically on the bench at least 1.5 m away from your eyes.
  2. Hold a convex lens at arms length and look at the image of the book seen through the lens.
    Looking through a convex lens at a book
  1. Record what happens if you move the lens and your eyes closer to the book? Look for:
    1. any changes in the size of the image of the text
    2. any changes in the nature of the image (upright or inverted).
  2. Hold a concave lens up at arms length and look at the image of the book seen through the lens.
  1. Record what happens if you slowly move the lens and your eyes closer to the book? Observe any changes that occur to the image of the book.
  2. What happens if you keep moving towards the book until the lens is touching the page? Look for:
    1. any changes in the size of the image of the text
    2. any changes in the nature of the image (upright or inverted).
Which lenses can form images on a screen?
What to use
Range of lenses (at least four) including one fat and one thin lens, and one that is concave and one that is convex, a piece of white cardboard to form a screen.
What to do
Do each of the following steps individually and compare your observations with the others in your group.

  1. Hold a convex lens up to the window and place a white cardboard screen behind the lens.
    Looking through an image through a convex lens
  1. Move the card towards or away from the lens, until an image of the scene outside the window is focused on the screen.
  2. Record the appearance of the image on the screen.
  3. Investigate what happens if lenses of different thickness are used to form the image on the screen.
  1. Record the appearance of the image on the screen.
  2. Investigate the images formed on screens by concave lenses.
  1. Record the appearance of the image on the screen.

Describing images
To describe an image you should observe:

  1. whether it is upright or upside down
  2. whether it is larger or smaller than the object
  3. whether it is coloured or black and white
  4. the distance between the lens and the screen
  5. any other difference.

Discussion

  1. What optical instrument do you think uses a single convex lens to view a small object?
  2. Name the optical instrument that uses several convex lenses to produce greatly enlarged images of small objects.
  3. What optical devices can you think of that use lenses to form an image on a screen?

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