Name _________________________________ Section ________ Physics 208 Spring 2008 Lab 2: Lenses and the eye A. Lenses You have a Pasco track and light source, three color-coded converging lenses, and a white screen on which to view images. Position the light source accurately at 0 cm on the track, and the white screen at 30 cm. Put one of the red color-coded lens on the track between the screen and the light source. A1. Find all the positions of one of the red lenses that give a sharp image on the screen (there is more than one position, but there may not be four) Lens position (cm) Image distance (cm) Object distance (cm) Magnification A2. From the data of A2, calculate the focal length of the red lens. Note: the two red lenses have nominally the same focal length, but may differ by several millimeters.2 A3. Keep the white screen at 30 cm, take out the red lens, and put in the green lens. Write down all the positions of the lens that give a sharp image on the screen. Explain. A4. Now move the screen to 100 cm, and write down the positions of the lens that gives a sharp image on the screen. A5. Calculate the focal length of the green lens. A6. Position the green lens at the point that results in the largest focused image. Slide a piece of paper across the surface of the lens, gradually blocking more and more of the lens. What do you see on the screen? Explain.3 A7. Magnifying glass (simple magnifier). Use each lens as a magnifying glass. That is, hold the lens in your hand close to the words on this page so that they appear larger when you look through the magnifying glass. Move the magnifying glass and your eye until the words are as big as possible, but still in focus. The image may be distorted due to aberrations in the lenses. Are the words on the page closer to, or farther from the lens than the focal length? Is the image formed by the magnifying glass upright, or inverted? Is the image formed by the magnifying glass real or virtual? Explain. Which lens can magnify the most when used as a magnifying glass, long focal length or short focal length? Remember this section! In the next part, the eyepiece lens of a microscope and telescope works in exactly this way.4 TWO-LENS SYSTEMS The next sections involve two-lens systems. These will make a lot more sense if you remember these things: 1. In most cases, the first lens forms a real image. 2. The magnification of the first lens is the image distance divided by the object distance. 3. The image formed by the first lens is used as the object for the second lens 4. A real image will appear on a screen placed at the image location. A virtual image will not, but your eye can focus the rays from a virtual image on your retina. Compound Microscope A8. In this section, you will make a microscope from the lenses you have available. A compound microscope is an optical instrument using two lenses that makes a nearby sample appear larger. The objective (lens closest to the sample) is used to make a larger, real image of the sample. The eyepiece (lens closest to your eye) is used as a magnifying glass to look more closely at the image from the objective. To get large magnification, where should you put the objective relative to the sample? Explain Given the physical constraints of your setup, which of your available focal lengths should you use for the objective to get the largest image? Explain. Which of your available focal lengths should you use for the eyepiece to get the largest overall magnification? Explain.5 A9. Choose two of your lenses and make a microscope on the optical track, using the light source as a sample. Position the lenses to make the light source appear magnified. Remember that the objective lens will form a real image that you can see on a screen. The eyepiece is used as a magnifying glass to look at this real image. a: Draw a diagram of your microscope at right, drawing each lens at the position you have it on the track. Also indicate the focal point of each lens. b: Use the white screen to locate the image formed by the objective. Indicate this point on the diagram at right. Also use ray-tracing to locate the image from the objective. Note that ray tracing is extremely sensitive to small errors, so use your measured image location as your primary reference. Use ray-tracing to locate the image formed by the eyepiece. Are these images real or virtual? Image Type Objective Eyepiece \ Optical axis Light source 0 cm 10 cm 20 cm 30 cm 40 cm 50 cm 4 5 6 7 8 9 10 11Data 1B6 Telescope A10. In the following section, you make a telescope from the lenses you have available. A telescope is an optical instrument that uses two lenses to make a distant object appear closer. The objective lens points toward the distant object (such as the building across the street) and forms an image on the opposite side of the objective lens. The eyepiece is used as a simple magnifier to look more closely at the image produced by the objective. Which of your available focal lengths should you use for the objective to make the largest image? Why? Which of your available focal lengths should you use for the eyepiece to get the largest overall magnification? Why?7 A11. Pick two of your lenses to make a telescope. Since a telescope is used to view objects far away, you should remove the light source and aim the telescope out the window (propping up one end of the track on the eye-model light source works well) or at a light board. a: Draw a diagram of your telescope at right, labeling the position (in cm) of each lens. Also indicate the focal point of each lens. b: Don’t do ray-tracing for the telescope, just indicate the positions of the lenses, their focal points, and the location of the image formed by the objective (use the white screen). Image Type Objective Eyepiece Optical axis 0 cm 10 cm 20 cm 30 cm 40 cm 50 cm 4 5 6 7 8 9 10 11Data 1B8 B. The eye Your eye also has multiple “lenses”, although they are referred to as the “cornea” and the “crystalline lens”. Both of them do some refracting – these are two closely-spaced lenses. If you wear eyeglasses, that is a third lens! The image
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