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Activity 2: Images** Overall management

Some of the optical devices studied at the start of this topic can produce images on a screen. Certain curved mirrors can produce images on a screen.

Curved mirrors
Most curved mirrors are called spherical mirrors because their shape follows the surface of a sphere.

They can be either concave if they curve inwards or convex if they bulge out at the centre.

What sort of images can curved mirrors form?
What to use
Various concave and convex mirrors, a flat mirror, white A4-sized cardboard screen.
What to do
A    Images on screens
  1. Hold a concave mirror in your hand so that it faces the window. (It works best if you are several metres from the window.)
    An image from a concave mirror on a screen
  2. Reflect the light coming through the window onto a sheet of white paper as shown in the following diagram.
  3. Focus an image of the scene outside onto the paper by moving the mirror closer or further from the paper screen. What do you see on the screen?
    Focusing with a concave mirror
  4. Try the above activity with a convex mirror.
  5. Try the above activity with a plane (flat) mirror.

Real and virtual images
An image that can be focused onto a screen is called a real image.

An image that can be seen behind the surface of a mirror, and cannot be focused onto a screen, is called a virtual image.

Discussion
  1. Which mirrors will make an image on a screen?
  2. Look at the image of the distant scene produced on the screen by the concave mirror.
    1. Describe its appearance.
    2. Decide if it is a real or a virtual image.
  3. Would the image of the distant scene be the right way up if the mirror was turned upside down? Try it. Explain what happens.
  4. What type or types of mirror can produce a real image on a screen?
  5. What happens if different concave mirrors with different curvatures (concave mirrors that are curved by different amounts) were used to form images of the distant scene? What is the relationship between the size of the image on the screen and the amount of curvature of the mirror? Try it.
B    Images seen in mirrors
Look at the image of your face in a plane mirror.

  1. Does the image of your face seem to be in front of, or on, or behind the mirror? Is it the right way up (upright) or upside down (inverted)? Does it look larger, smaller or the same size as your face?
    Image on a mirror
  2. Move the mirror closer to and further away from your face. Does it make a difference?
  3. Look at the image of your face in a convex mirror. Does the image of your face seem to be in front of, or on, or behind the mirror? Is it the right way up (upright) or upside down (inverted)? Does it look larger, smaller or the same size as your face?
  4. Move the mirror closer to and further away from your face. Does it make a difference?
  5. Examine your nose by holding the concave mirror close to your face. Does the image of your face seem to be in front of, or on, or behind the mirror? Is it the right way up (upright) or upside down (inverted)? Does it look larger, smaller or the same size as your face?
  6. Move the mirror away from your face. Does it make a difference?
  1. Record what you have discovered in a summary table, such as the one shown.

    Record of observations of various images
Discussion
  1. Compare the image of your face in a plane mirror and a convex mirror.
    1. In what way were these images different from each other?
    2. In what ways were these images similar?
  2. Where are convex mirrors used?
  3. Consider a soup spoon. Which side is convex and which side is concave?
  4. You have probably discovered that one type of mirror can only make virtual images, and only one type can make real images. Which ones? You may need to check the definition of real and virtual images.
  5. Look at the image in the concave mirror.
    1. Was the image of your nose seen in the concave mirror real or virtual?
    2. Where is this type of image very useful?

Making perfect images
The image formed is clearest for mirrors that have less curvature or for mirrors with only a small face area.

No spherical mirrors give a perfect image. A sphere is actually the wrong shape to give a perfect image. The best images are produced by a parabolic shaped mirror.

Challenge
  • Find the name of the shape of a curved mirror that can give perfectly focused images.
  • What type of mirror is used in the following, and is the image real or virtual: security mirror in a shop, make-up mirror, rear vision mirror, astronomical telescope and mirrors on corners of blind intersections?


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