Optics Experiment #4 NOTE: For submitting the report on this laboratory session you will need a report booklet of the type that can be purchased at the McGill Bookstore. The material of the course that is relevant to this laboratory exercise is that of lectures 11, 12, 13, 14 and 15. References to the Material in Hecht can be obtained from the Index of that text under "optics". Introduction Objects that change your view of things have fascinated mankind since they were first discovered. The earliest of these were mirrors that were formed naturally by the surface of quite pools of water. Very early people realized that the image seen by looking down onto such a pool was indeed of themselves, although they had never before seen themselves to know what they really looked like. That it was an image of themselves was shown by the movements of the image. For example, if the person closed one eye the image closed one eye. Later polished surfaces were developed which allowed people to view images of themselves with hand held devices, now called "hand mirrors". Much later, glass devices were developed which changed your view of the world as you looked through them. In particular they could make clear images of objects that were very close to the eye, thereby giving you a magnified view of the object. The purpose of this laboratory exercise is to see some of these phenomena for yourself and to try to explain what you see. Exercise 1. Playing with a hand-mirror The first exercise is to take the hand-mirror provided (a square piece of silvered glass in a holder) and hold it 30 cm in front of your face so that you can see your image. (Have a partner make sure the distance of the mirror from your nose is 30 cm.) Then hold a ruler horizontally (sideways) at your nose. Using only one eye, (if you have two functioning eyes, keep one closed) observe the length of the ruler that can be seen in the mirror? (Enter your answer in your report book.) Then hold the ruler vertically at your nose. What is the length of the ruler that you can now see in the mirror?? (Enter your answer in your report book.) How do these measurements compare with the size of the mirror? (Enter your answer in your report book.) Repeat the experiment with the mirror 60 cm from your nose. Explain your results. (Again, use your report book.)
P101 Laboratory Exercise - 2002 Version 2 Exercise 2. Locating the position of a mirror image. The next exercise is to mount the hand-mirror in the holder provided for the optical bench and look at the image of a small light placed in front of the mirror. MIRROR To do this properly, place the mirror as deep as possible in the holder and move the holder to near the center of the bench. Then place the light in its holder so that the image of the light is near the top edge of the mirror. EDGE OF MIRROR IMAGE OF Then place the pointer in another holder mounted behind the mirror and adjust its height so that when you look into the mirror to view the image of the light the tip of the pointer appears just above the edge of the mirror. PO INTER MIRROR Now turn the mirror until the image of the light lines up with the pointer. POINTER EDGE OF MIRROR IMAGE OF
Experiment No. 4 - Optics 3 Now while viewing both the tip of the pointer and the image of the light, move your head from side to side. Does the pointer stay aligned with the image of the light? If it does not then adjust the distance of the pointer from the mirror until it does. Keep doing this adjustment until it keeps in exact alignment. (This method of locating things is called the "parallax method". As pointed out in the lectures, this is the method you normally use, without realizing it, to locate most objects in space.) How does the distance of the pointer from the mirror now compare with this distance of the light from the mirror? (Enter your answer in your report book.) Why should this be? (Enter your answer in your report book.) Exercise 3. Playing with converging lenses As a preliminary to this set of experiments, hold a piece of paper with some printing on it (such as these lab manual notes) in front of your face and bring it as close to your face as you can and still have a clear view of the print. (Don't cheat as that will foul up the results of the rest of the tests.) If you normally wear corrective lenses leave them on. Have your partner measure the distance of the paper from your eyes. This is your "near point". Now place the 10 cm focal length lens very close to your preferred eye and look through it at the printed page. Again, if you wear them, leave your glasses on. Repeat the measurement for your new "near point". Now with the lens still in front of your eye move the paper as far away as possible for it to be still clear when looking through the lens. Have your partner measure the distance of the paper from your eye. This is now your "far point". (Your normal "far point", presuming that you have normal vision or that your abnormal vision has been corrected by glasses, should be at infinity.) Enter your results in the table below. Lens used Near point Far point Bare eyeball 10 cm lens 20 cm lens 30 cm lens
P101 Laboratory Exercise - 2002 Version 4 Now repeat this experiment with the 20-cm lens and the 30-cm lens and enter your results in the table. Explain your results in terms of the power of the converging lenses that give you these results. (Enter your answer in your report book.) Exercise 4. The image from a converging lens Mount the light in a holder at the left end of the optical bench. Place the 10-cm focal length lens in one of the lens holders and place the holder so that the lens is 15 cm from the light. Put the cardboard screen frame in another holder on the optical bench and move it back and forth relative to the lens until a sharp image of the light is formed. LENS IN HOLDER CARDBOARD SCREEN POINTER 15 CM Measure the distance of the image from the lens. Explain the relationship between the object distance, the image distance and the focal length of the lens. (Enter your answer in your report book.) Take the cardboard screen frame out of the optical bench holder and put the pointer in it instead. Now look into the lens with your eye about 30 cm to the right of the pointer. Try to find the image of the light. It will be an enlarged upside-down version of the real thing, the most prominent feature being the curved filament. Adjust the tilt of the light mount and the height of the pointer until the tip of the pointer points to the center of the image of the filament. While looking at the pointer tip and the image, move your head from side to side. The pointer and the image should appear to move together. Now move the pointer closer to the lens and again move your head side to side. Does the pointer move as much as the image or more? (Enter your answer in your report book.) Repeat by moving the pointer farther from the lens. Does the pointer move as much as the image or more? (Enter your answer in your report book.) Explain your results by the theory of parallax. (Again, use your report book.)
Experiment No. 4 - Optics 5 Replace the pointer by the cardboard screen and move the lens to 20 cm from the light. What is now the distance of the image from the lens? (Enter your result in your report book and explain it.) Repeat with the lens 30cm from the light. What is now the distance of the image from the lens? (Enter your result in your report book and explain it.) Now without changing the position of the lens replace it by the one of 20-cm focal length. Again move the cardboard screen back and forth from the lens to the far end of the optical bench. Describe and explain the behaviour of the spot of light on the cardboard. Again keeping the lens holder at 30 cm from the light, remove the 20-cm lens and replace it by the 30-cm lens. Repeat as for the 20-cm lens. (Describe and explain the behaviour of the spot of light on the cardboard.) Exercise 5. The behaviour of a diverging lens With the lamp still in the same place, move the lens holder to the right and insert the tube, mounted in one of the lens holders, just in front of the light. TUBE IN LENS HOLDER Moving the cardboard screen just to the far end of the tube will give you a round spot of light on the screen. Measure the diameter of this spot. (Enter your result in your report book.) Now move the screen to 50 cm from the end of the tube. Again measure the diameter of the spot. (Enter your result in your report book.) Now place lens D in the lens holder and move it so it covers the exit of the tube (see diagram below). Adjust the light, the tube and the lens so that they are all centered. (This may take some tilting of the light mount, the tube mount, the lens mount, or all three.) Finally place the cardboard screen to the right of the lens. TUBE IN LENS HOLDER LENS IN HOLDER CARDBOARD SCREEN
P101 Laboratory Exercise - 2002 Version 6 Now move the cardboard screen along the optical bench. Describe and explain the behaviour of the spot of light on the screen. What is the diameter of the spot at 50 cm from the lens? Repeat using lens E in place of lens D. (That is describe and explain the behaviour of the spot of light on the screen, in particular how it is different for the two lenses.) What is now the diameter of the spot at 50 cm from the lens? What is the focal length of lens D? (Show your calculation.) What is the focal length of lens E? (Show your calculation.)