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Computer-based optics experiments with a single lens, using a Java applet - Succeed in Physical Science. Also refer to physics, focal point, telescope, microscope, magnifying glass, lens laws, Ron Kurtus, School for Champions. Copyright © Restrictions

Simulated Optics Experiments:
Single Lens

by Ron Kurtus (revised 26 October 1999)

Have you ever experimented with a magnifying glass or other type of convex lens? Most people have. A convex lens is one in which the center is wider than the ends. A concave lens has the center "caved in" and is wider at the edges. Most lenses you see are convex.

This set of experiments will allow you to study the characteristics of a single lens by using a Java applet to simulate optical experiments on your computer.

(Note that your browser must be capable of using Java 1.1 for this simulation to work. It is a relatively large applet (115K), so it may take up to a minute until it is completely downloaded. You will see the illustration below in black with a line through the middle, when it is ready to use.)

In this computer-based experiment, you will:

  1. Place the lens on the optical axis.
  2. See how parallel light focuses at a point.
  3. See what happens when a source is put at the focal point of the lens.
  4. Experiment with creating images with a lens.
  5. Experiment creating virtual images with a lens.
  6. Experiment with a concave lens.

So, let's do some experiments!

Instructions

Follow these instructions to guide you through the experiments.

1. Place the lens on the optical bench

  1. Click your mouse on the Lens button. The text becomes colored, showing it is active.
  2. Click near the center of the yellow line to insert the lens. The two points are its focal points.

Jump to optical bench to start.

NOTE: If you make a mistake, you can click on the Clear Active button to erase the active element or the Clear All button to erase everything.

2. Show the effect of a parallel beam of light

A parallel beam of light will go through the lens and focus to a point. This principle is used with a magnifying glass to focus the almost-parallel light from the Sun to a point that will become very hot.

  1. Click the Beam button.
  2. Click about 1/4 of the way from the left side, near the yellow line.
    • A beam of light appears. The beam shines on the mirror, reflects, and focuses through a point on the yellow axis.
  3. Click on the focal point and drag it left and right.
    • This shows what happens when the curvature of the lens changes.
  4. Click on the source of the beam and then click the Clear Active button to erase the beam, or simply click the Clear All button to start all over.

Jump to the optical bench to proceed.

3. Show the effect of a source of light at the focal point

A source of light at the focal point of a convex lens will transmit a beam of parallel light. Some spotlights have a light bulb at the focal point of the lens.

  1. Start with a lens near the center of the optical axis.
    • If the lens focal points are not visible, click on the lens to show them.
  2. Click the Source button and click on the yellow axis near the left focal point to insert the source of light.
    • Light rays should come from the source and go out parallel from the lens along the axis.
  3. If the light is not parallel, drag the source until it is exactly on the focal point. If the light is not aligned with the axis, drag the source until it is exactly on the yellow optical axis.
  4. Click on the source and then click the Clear Active button to erase the source, or simply click the Clear All button to start all over.

Jump to the optical bench to proceed.

4. Creating images with a convex lens

A convex lens can create a real image that will project on a screen or on the film of a camera. It can also create a virtual image like seen in a magnifying glass. This experiment is in two parts: creating real images and then creating virtual images.

4a. Real images

A real image is inverted and smaller than the original object, just as the object on the camera film is certainly smaller than the real object.

  1. Start with a lens near the center of the optical axis.
    • If the lens focal points are not visible, click on the lens to show them.
  2. Click the Object button and click about 1/2 above the yellow axis and 1/2 the way the left of the lens.
    • An arrow should display with light rays going from its head, through the lens, and focusing to a smaller, inverted arrow on the other side of the lens. This is the real image.
  3. Drag the object arrow to see the effect its position has on the size and position of the image.
    • Note that when the object is at the focal point of the lens, the rays become parallel and there is no image. (The image is really at infinity, because we are transitioning from a real image to a virtual image.)
  4. Change the size of the image by dragging its point up or down.
  5. Change the focal length of the lens by dragging one in or out.

Jump to the optical bench to proceed.

4b Virtual images

  1. Continue from creating real images.
  2. Drag the focal length of the lens to make it longer.
  3. Drag the object inside the focal point, between the focal point and the lens.
    • The image is now on the same side of the lens, it is bigger, and it is upright. It is a virtual image and will not appear on a screen. This is the type of image you see when you use a magnifying glass close to an object.
    • If the object is at the focal point, the image is a virtual image at infinity.

Jump to the optical bench to proceed.

5. Experiments with a concave lens

A concave lens is wider at the edges than at the middle. Concave lenses are primarily used in eyeglasses for people who are nearsighted.

  1. Start with a convex lens near the center of the optical axis.
    • If the lens focal points are not visible, click on the lens to show them.
  2. Drag a focal point to the other side of the lens.
    • The shape of the lens changes to become a concave lens.

Jump to the optical bench to proceed.

Effect of beam of light

  1. Click the Beam button and place it to the left of the concave lens.
    • The beam is spread as if it came from the focal point.
  2. Drag the beam around to see what influence its position has.
  3. Click the Clear Active button to erase the beam.

Effect of light source

  1. Click the Source button and place it to the left of the concave lens.
  2. Drag the source around to see what influence its position has.
  3. Click the Clear Active button to erase the beam.

Creating images

  1. Click the Object button and place it to the left of the concave lens.
    • A smaller, upright virtual image is seen inside the focal point. Adjust the focal point, if necessary.
  2. Drag the point of the object to change the image size.
  3. Drag the object around to see what influence its position has on the virtual image.

Draw some conclusions about the images made by a concave lens.

The optical bench

The simulated optical bench is seen below.

Credit for applet

Special thanks go to Mike Lee, Wolfgang Christian and the WebPhysics staff at Davidson College in Davidson, South Carolina for developing this Java applet and making it available.

In conclusion

Concave mirrors can be used to focus light to a point as in a solar heater and send out parallel beam of light as in a flashlight. They can also be used to create images, but that is better illustrated with lenses.

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Experiments

Simulated Optics Experiments - Lenses

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Simulated Optics Experiments - Curved Mirror
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