Background

Light enters the corneas of our eyes directly from the air. Most of the focusing of light happens at the interface (boundary) between our corneas and the air. Our eye lenses serve merely to "fine tune" our focusing. Some of us wear "correcting lenses" in the form of glasses or contact lenses. Without correcting lenses some objects would appear blurred to us or not "in focus". How can an optometrist predict what shape lens to prescribe for our correcting lenses? What defect in our eyes causes distant objects to appear blurred for some people? What causes the difficulty some people have in reading newspaper print without corrective lenses? How do your eyes form images? What does it mean to "see" something?

Purpose

1.) To investigate how lenses form images.

2.) To investigate and understand various optical devices that use lenses, and how the properties of lenses can be used as an aid to sight.

Materials

candles, 2	glass plate, clear
large paper, 11x14"	clay
matches	lenses, convex & concave, assortment of 6
ground glass, 3x4" & index card	ruler
lens cleaner	tissue

Procedure

1. Parallel Light Rays from Distant Objects:

WARNING: This lab uses candles and matches. Please use caution when placing flammable objects near a lit candle or match.

a) When coming from great distances, light rays travel in parallel paths which you will simulate in this part of the experiment. Place a lit candle at one end of your group's table. Make sure the candle is situated on a piece of paper so wax won't drip onto the tabletop. In the middle of the table, place one of the lenses upright in the clay and use the index card or ground glass to locate an image of the flame.

Why do you want to place the light source (the candle flame) as far from the lens as possible?

What happens to the image on the screen if you bring the lens closer to the flame but keep the distance between the lens and the screen the same?

b) Explore the images formed when each of the assorted lenses provided is placed in front of the light rays originating from the candle flame. Use the clay to support each lens upright in front of the candle. Record a description of each lens, its focal length, and how far from the lens you observed an image, the size of the image, whether the image is upright or inverted, which side of the lens the image was formed, etc.

2. Exploring Images further:

a) Using the materials available, can you find ways to make the image of the flame on the screen fainter?

If you keep the lens and screen stationary, can you change the brightness of the image? Record in your lab book any experiments you can think of to change the brightness of the flame image. Also record any ideas you might have to explain your observations. Test those ideas with the appropriate experiments.

b) i) Place a lens in the clay at the middle of the table and look through the lens at the candle flame. What happens to the image of the flame you see if you bring your eyes closer or farther from the lens?

ii) Explore the image of the flame viewed through each of the assorted lenses seperately. Record your observations and generate hypotheses for the phenomena you observe. What is the image orientation (inverted or upright)? Is the image real or virtual? What is the image size compared to the size of the original candle flame (the object)?

iii) If you stand far from the object (the flame), can you use the lens (or lenses) to make the flame appear closer and in focus? Record your observations and any orientations of the lens/lenses that allow you to view the distant candle. What type of optical instruments have you made?

iv) Chances are, one of your group members wears eyeglasses. If not, ask one of your colleagues from another group to borrow their glasses for a few minutes. Repeat some of the experiments from earlier in the lab session. Compare the results you find for the lenses in a pair of eyeglasses to the lenses provided for you. What type of lenses does a nearsighted person use? a farsighted person?

c) Prepare a five minute White Board presentation on two or three of the phenomena observed in this lab. (15 min)

White Board Report

Your White Board should include: 1) a title in the form of a question, 2) a summary or demonstration of the phenomenon, 3) your proposed explanation or hypothesis for the phenomenon. White Board "grading criteria" are summarized on a separate sheet.

Suggestions to include:

Summarize in a data table your results for the assorted lenses. Include careful and clearly labeled sketches of each lens and the properties of each lens, including its focal length, type of image, size of image, etc. Describe any experiments you proposed and performed with the assorted lenses. Discuss predictions, outcomes, hypotheses, and conclusions of each experiment.