At this point, you know lots about mirrors and lenses and can predict how they interact with light from objects to form images for observers. In the next part of the course, we consider applications of mirrors and lenses. We start with cameras and the eye, examples of systems that mostly use single (though sometimes multi-component lenses. We ll then move on to systems like telescopes and microscopes that use combinations of two or more mirrors/lenses. We also consider some of the non-ideal properties of lenses. 1. Chromatic aberration of lenses (4 points) A certain thin lens has an extremely large amount of chromatic aberation. Its focal length is 3 cm for blue light and 4 cm for red light. A 2 cm arrow is 6 cm in front of the lens and is illuminated with white light. Which image is closer to the lens? Red or blue? (Circle one) By ray tracing construct the images for red and blue light in the space below. 2. Chromatic aberrations of mirrors? (2 points) Do front surface mirrors have chromatic aberration? State what you think and explain why you have answered the question as you have. 3. Lens power (2 points) A lens has a focal length of 79 cm. What is its power in diopters? Physics 1230 HW8.1 Summer 2016
4. f-numbers and diameter. (2 points) You have an f/5.6 lens that has a 116 mm focal length. What is its effective diameter? 5. Exposure settings. (2 points) An exposure meter indicates that a correct setting for a photograph would use an f/5.6 diaphram setting and a shutter speed of 1/100 seconds. If you want to take the exposure so as to get a greater depth of field, you use a larger f-number, say f/11. Explain what your shutter speed should be. 6. Choosing a lens. (2 points) Suppose you have a camera and three lenses that fit it, 28 mm focal length, 50 mm focal length, and 150 mm focal length lenses. You want to photograph a fisherperson holding a fish out in front of them toward the camera, so the fish looks larger compared to the person and both are in focus. Explain which lens you would use and why. 7. The eye is like a camera. (2 points) Compare and contrast the eye with the basic camera. What part of the eye corresponds to each major component in the camera? Physics 1230 HW8.2 Summer 2016
8. (4 points) A converging lens is made of a type of glass (nglass = 1.6). The glass is placed in air (n=1) and the rays converge at the focal point, as shown below. When the lens is placed in water (nwater = 1.33) instead, where will the rays converge? Do you think it s the same point, at a further distance from the lens, or a shorter distance from the lens (circle one)? We re learn more about the eye soon; this fact means that your eye, which basically is a converging lens that acts like a camera, doesn t focus well when under water. 9. More about cameras: (5 pts total) Multiple choice questions: MC 1. You set up a lens and screen to make an image of a distant light. The light begins moving towards the lens. What must you do to keep the image sharp? Check all that apply. A. Move the screen towards the lens B. Move the screen away from the lens C. Use a lens with a different focal length (power) MC 2. When the lens is closest to the film you are focused on an object that is (a) nearby (b) far away Physics 1230 HW8.3 Summer 2016
MC 3. Which is true of a telephoto lens, which allows you to see a larger image of a small far away object? A. It has a long focal length and results in a bright image B. It has a long focal length and results in a dim image C. It has a short focal length and results in a bright image D. It has a short focal length and results in a dim image MC 4. Which of the following will tend to result in a BRIGHTER image? Check all that apply. Slow shutter speed Fast shutter speed Large aperture (small f-stop) Small aperture (large f-stop) MC 5. I use a lens to focus an image of a light on a piece of paper. What happens if I move the piece of paper towards the lens? A. The image disappears B. The image gets blurry C. The image gets larger D. The image gets dimmer Physics 1230 HW8.4 Summer 2016
10. (5 pts) Focusing a camera and the lens equation: A camera uses a converging lens to form a real image of the scene on the light-sensitive film. A) Your camera has a 35-80 mm variable focal length (zoom) lens. You want to take a photograph of a distant mountain, and you select your focal length to be 50 mm for the composition you want. How far should the lens be from the film surface for the mountains to be in focus (in mm)? B) If your friends are 1 meter away from you and you set the focal length on the camera to 50 mm, how far should the lens be from the film for the picture to be in focus (in mm)? Physics 1230 HW8.5 Summer 2016