What is an Image?

Personal Lab 12

Background

We use lenses everyday to correct eyesight, in microscopes, in telescopes, in simple magnifiers. The most useful lenses are those that are part of our eyes. What is their function? How do lenses work, and why is the use of images indispensable in our daily activities?

Purpose

1) To investigate how light interacts with different kinds of lenses.

2) To construct two types of telescopes.

piece of plastic wrap, 4 x 4 inch	water, drop
2 convex lenses	1 concave lens
1 plane mirror	1 sheet of white paper, 8.5 x 11 inch
1 fresnel lens	1 ruler with cm scale
2 index cards

Procedure

1. Place a drop of water on the piece of plastic wrap. Examine the water drop with one of your lenses that magnifies. Hold the plastic wrap above a newspaper. Describe what you see.

2. Examine each of the four plastic lenses by looking at them and touching the surfaces. Note how they differ, and how they are the same. Compare each lens with the mirror.

3. Turn on a desk or floor lamp. Hold each of the lenses one or two feet from the lamp, and look through each of the lenses toward the lamp. Enter your observations in your Lab Book.

4. Take the four lenses and the mirror to a room with a window during daylight hours. In separate experiments, use each lens and the mirror to form an image of the view out the window onto the index card. Record the distance from the center of the lens or mirror to the index card when you can see the sharpest image on the card. This distance is called the focal length of the lens or mirror. Enter the following information in your Data Table about the images you see: lens/mirror type (convex, concave or Fresnel (pronounced frehnel), focal length, orientation of image, size of image. Enter additional notes to complete the description of the image formed by a distant object from outside the window with each of the four lenses and the mirror. For example, do the orientations of the lenses or the mirror affect the images formed?

5. Predict what would happen to the image on the card if you moved another index card in front of the lens (on the side facing the window), covering half the lens? Record your prediction in your Lab Book, together with a sketch of the light path from the objects outside the window through the lens and forming an image on the card. Check your prediction by performing the experiment. Record the results.

6. Examine a printed page with each of the four lenses. Which lens or lenses act(s) as a magnifier? Make notes in your Lab Book, and estimate the magnification of each lens. Describe how you made your estimates.

7. Hold each of the four lenses up to your eye, and look out the window. Record what you see through each lens.

8. Explore what happens with various combinations of two of the four lenses in front of your eye. Try to find a combination that makes a telescope when you view distant objects such as a tree out a window. Make sketches of the lens combinations that are successful.

Questions

1. Explain what each of the following different types of lenses does to the light path: a) drop of water, b) convex, c) concave, d) Fresnel. Sketch the path of two light rays from an object through each lens to form an image.

2. Which combination of two lenses made a telescope? Sketch your telescope and show the path of light from a distant object to your eye, using two separate light rays. What is the function of each of the lenses in a telescope?

3. What do you predict would be the effect on the image viewed through a telescope if the area of the lens closest to the object were reduced by half? Perform the experiment by covering half of one of the lenses in your telescope.(The one closest to the object you are viewing, not the lens closest to your eye).

4. Carefully examine the shining curved surface behind the bulb of you flashlight or the behind the bulb in the headlight of a car. What do you think is the purpose of this "reflector" behind the bulb?

5. Why do you think Fresnel lenses are used in light houses? Sketch the path of light through a Fresnel lens. Sketch what you think the surface of the Fresnel lens would look like at high magnification.

6. What is the primary function of a lens in forming images?

7. That ostriches bury their heads in sand to escape from predators is a myth that arises from an interesting optical illusion. In hot climates the air near the ground becomes heated more than the air just above the ground. Thus the air changes density rather abruptly, giving rise to two transparent media with different densities. This is the inverse of the air to swimming pool interface. Can you trace the light path from the ostrich’s head while grazing in the hot desert from the ostrich to you, located 0.5 km or so away?

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Last modified 9 Aug 1997
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