PHO 101 Photonics Concepts
Three Rivers Community College ONLINE

Home Lab #5

The Thin Lens Equation: More Fractions!

Purpose: In this lab you will use the thin lens equation to predict the locations of images and use two lenses to make a telescope.

Materials: small flashlight, two converging lenses from the OSA kit (labeled A and B), small piece of Aluminum foil, clothespins or other supports for components, card or heavy paper for screen, ruler or tape measure with metric (cm) markings

Background: To measure the focal lengths of the lenses, you will use the thin lens equation:

(equation 1)

If an object is placed to one side of the lens (d_o) and the corresponding image distance is found by placing a screen at the position of the sharpest image (d_i), equation (2) can be used to find the focal length of the lens. Remember that in this equation, the object distance is a negative number.

Procedure:

I. USING THE THIN LENS EQUATION

The thin lens equation method allows you to control the object distance. The set up for this lab is shown below, photographed from above:

The relative positions of object, lens and image. The distance from the end of the flashlight to the lens is d_o. The distance from the lens to the focused image on the "screen" is d_i.

1. To prepare the object, cut a small narrow triangle of aluminum foil and tape it to the center of the face of the flashlight. The triangle serves as the object (its orientation is readily observed.) Be sure it is small enough so that light can shine around it on all sides.

The object, lens and screen are supported by clip-type clothespins. It is important that the object, lens and screen be at the same height from the table top, as well as in a straight line along the ruler. That is, the optical axis, the line from the center of the object through the center of the mirror to the center of the screen, should be level and straight. (CD jewel cases make good bases for components!)

2. Use lens A.. Set the object so that it is 40 cm from the lens. Measure this distance (from the triangle on face of flashlight to center of lens support) as accurately as possible. Using the focal length you measured in the last lab, predict where the image will form and what the magnification will be (use the thin lens equation!). Locate the position of the image on a screen placed on the other side of the lens. Measure the distance from the lens to the image location as accurately as possible. Record the image distance on the data table (below). Note whether the image is upright or inverted, and in the column labeled "size", whether the image is larger or smaller than the object. Compare to your prediction.

3. The telescope part of the lab is in the document "TelescopeLab"

REPORT

1. How close was your prediction to the measured value for image distance? For magnification? Calculate percent error.

2. Suppose you have an object and lens aligned so that a clear image is projected onto a screen. If you cover the top half of the lens with a piece of paper, what happens to the image? (This question was answered incorrectly by the majority of college physics students who were surveyed! You can try the experiment and find out for yourself!)