Chapter 16.1 Cameras
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#10	#12

2. Sports photographers often use very long lenses on their cameras. How do the lengths of these lenses affect the photographs they produce?

The lenses are long because they have long focal lengths. When the focal length of a lens increases, the distance to an object in focus also increases. Along with this, the magnification of the distant object increases. So a longer focal length lens permits a 'larger-size' view of an object at greater distance.





6. On a bright sunny day you can use a magnifying glass to burn wood by focusing sunlight onto it. The focused sunlight forms a small circular spot of light that heats the wood until it burns. Why is the spot of light circular?

When you focus the sunlight onto the wood, the spot of light you get is an image of the sun on the surface of the wood. The image is circular because the sun is round.





8. Why is it easiest for the lens of your eye to form sharp images on your retina when the scene in front of you is bright and the iris of your eye is very small?

When the iris of your eye is small, the light coming into your eye from a single point of the scene in front of you is very restricted in angle. The change in angle for light entering diametrically opposed points of your iris is small. So all the light rays from a given point are nearly parallel as they enter your eye and come to a focus at the same place. This produces a large depth of focus.





10. As you zoom the lens of your video camera inward to take a close-up of one person, what characteristic of the lens changes and does it increase or decrease?

The focal length of the lens changes; it increases.





12. One part of a fiber optic communications system uses a lens to focus light from a semiconductor laser onto the end of an optical fiber. The tiny light source and fiber are on opposite sides of the lens and each is 1.0 cm away from the lens. Why must the lens have a focal length of 0.5 cm?

According to the lens equation, the sum of 1/(image distance) and 1/(object distance) equals 1/(focal length). For this case, the image and object distances are the same. This gives us a required focal length that is one-half of the object (or image) distance of 1.0 cm, and that is 0.5 cm.