Material

A large lens such as a "Page Magnifier" fresnel lens at least 7" x 9", 20 cm x 25 cm.

2 Binder clips, large to mount the lens.

A distant object placed far outside the focal length of the lens. I often use a bare lightbulb.

2 people.

Assembly

Mount the lens so that it is in a vertical plane using the binder clips.

Place the object as far away as possible.

To Do and Notice

Stand far from the lens, outside the focal length of the lens.

Have one person look through the lens at the distant object.

They see an inverted image of the object. Can they tell where the image is located?

Have a second person look through the lens at the object.

Have both people point at the object.

Notice where the directions they are pointing cross, this is the location of the image.

If you are using a glowing lightbulb as an object place a piece of white paper at this location and notice that the image of the bulb appears on the paper.

What's Going On?

The light spreads out from the object and then hits the lens where it is bent back together. All the light from one point on the object is assembled into one point on the image.

The image is called a real image because the light actually comes back together at the image location. The image can be seen on a piece of paper placed at the location of the real image.

Two rays of light are traced from the large arrow object on the left, through the giant lens which assembles the rays into a real arrow image on the right of the lens.
Two people standing in these rays will point along the rays. Where the rays cross is the location of the image.

Going Further

If one person walks up to the lens he or she will see the image flip over and become a virtual image. This creation of a virtual image is not predicted by the above ray tracing image. You can explore more about this image by going here, real or virtual?

Scientific Explorations with Paul Doherty

© 2005

16 November 2005