LIGHT AND LENSES
A. OBJECTIVES The purpose of this exercise is to show how light behaves as it is bent while passing through glass lenses. This behavior is termed “refraction.” B. BACKGROUND Lenses cause light rays to converge at a focusing point. An image formed by such a line is called a REAL IMAGE. It can be projected on a screen. The enlarged image you see with a magnifying glass when you hold it close to your eye is called a VIRTUAL IMAGE. It is not being projected on the back of your eye, but your mind is interpreting the image to be enlarged. A real image is inverted (upside down), but a virtual image is upright. C. EQUIPMENT You need two magnifying glasses. They could be 2″ and 3″ in diameter, respectively, but must be different sizes. D. PROCEDURES Hold the lens at arm’s length, and observe through it. Is the picture you see erect or upside down? Bring the lens close to your eye, and see the image change. Is the image you see now erect or inverted? Which one is the real image? Go to a room that is at least 15 meters long (if you can’t find one that long, do your best). Stand on the other side of the room or way down a long hallway from a lamp at night or a window in the daytime. Hold the 3″ lens up so that the right source is over your shoulder, and hold a piece of white paper beyond the lens so light shines on it from the lens. Adjust the distance between the lens and paper until an image appears. Is the image inverted or upright? Is it real or virtual? Measure the distance from the lens to the paper (you may need help for this). This is the focal length of the lens, as long as you were able to get more than 15 meters from the light source. The focal length is used to determine how long a telescope has to be in order to be in focus and to determine the distance that film must be located away from a camera lens. Go up close to the light (or window), and repeat the operation. What happened to the image? The distance from the lens to the screen is no longer an accurate measurement of the focal length of that lens. With help, measure the distance from lens to screen and also the distance from lens to the light bulb (or window). Divide the distance to the image by the distance to the light source. This number is the magnification. That is, it tells how many times the image is magnified from the original object.
Repeat this procedure for the smaller lens. Add the focal lengths of the large and small lenses together. This is the length of a telescope that is in focus when the two lenses are placed at opposite ends of the tube. Finally, hold the smaller lens close to your eye and the larger lens about an arm length away. Look through both lenses at once, and determine whether or not you can see a clear magnified image. If you succeeded, you have made a simple refracting telescope. E. RESULTS AND CONCLUSIONS Record your measurements and observations, and discuss them in your summary. Curved mirrors as well as lenses are used to gather light in many telescopes. F. REVIEW AND PRACTICE 1. List some common objects that use lenses or mirrors for projecting or magnifying images. When you load slides into a projector tray, you are supposed to put them in upside down. Why?
laboratory report should contain the following sections: (1) Hypothesis, (2) Procedures,
(3) Observations and Results, and (4) Conclusions. Make certain you include all four headings with at least a short paragraph for each. In addition, tables, graphs, and answers to questions may be necessary in the latter two sections.
HYPOTHESIS
Scientific research should contain a preliminary statement of the expected outcome of the experiment. This can include predictions of the specific experiment or the general anticipated result. If you are merely doing an observation and have no idea of the outcome, you cannot make an actual hypothesis. Instead, make a short statement of the purpose of the observation. However, if you have preconceived ideas of the outcome, include them in this section, and then see how they compare to the results.
PROCEDURES
Even though you are told what to do, write a paragraph of the specific steps you actually took in doing the experiment or observation. Because you are coming up with your own equipment, your procedures will be of particular interest.
OBSERVATIONS AND RESULTS
This is where you should make a detailed statement of the outcome of your experiment. Record all your pertinent observations in a clear, readable form. Arrange your data in tables (such as measurements and calculations you make). Answer any questions asked in this Study Guide, marking these clearly so that they can be easily found.
CONCLUSIONS
Your conclusions should include a comparison between the outcome of the experiment and your initial predictions made in the hypothesis. In cases where you are attempting to recreate a physical constant, compare your number to the accepted value, using the formula for experimental error:
Experimental Error Equation
If you find a large difference in your results from the expected value or if your anticipated observations are not the same as your actual observations, try to identify possible sources of error or reasons for the difference in the hypothesis and results
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