Unformatted text preview: Discussion Problems 3
Due in section week of Feb 9, 2008 Quiz 3
One of the most ubiquitous applications of lenses is corrective
glasses. First, one should know that the eyeball works like a converging lens. If you have good eyesight, then images
should form on the retina. Suppose a person has trouble seeing an object which is 10 m away. You nd that with a
corrective lens having a focal length -20cm = f1 (in gure) they can see well. Take the lens to sit 1cm away from the
eye, as shown in the picture. Problem 1. So you want to be an ophthalmologist? a. For instructional purposes, lets say you know the person's eyeball has a focal length f2 = 1cm. Draw a ray diagram
showing where the nal image will be. (Hint: rst draw the diagram for the image that would be formed by just
the lens and then use this as a source for the eyeball.)
b. Find the height and location of the image. How far in the eyeball is this person's retina? Figure 1: Lens with f1 are glasses, and the eyeball is represented by a lens of f2 .
Problem 2. Learn the Lingo. a. At the eye-doctor you don't hear much about focal length. To see why, note that relative to most objects, your
glasses sit very close to your eye. Given two lenses f1 and f2 immediately adjacent to each other, what is the
eective focal length?
b. Instead of focal length, doctors talk about a lens' power (for example, my prescription is -1.75, and bifocals can
have -2 on one part and +1 for the other). The power of a lens is dened in such a way that putting two lenses
together, it simply adds. In light of your answer from part a, how would you dene the lens' power? If the relevant
dimension of lens power is some power of meters, what prescription does the person from question 1 need? 1 1 ...
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This note was uploaded on 10/05/2009 for the course PHYSICS 7C taught by Professor Lin during the Spring '08 term at Berkeley.
- Spring '08
- Quantum Physics