Physics
Chapter 23: 6-10
Web Lecture
Optics: Lenses for Different Uses
Introduction
Strepsiades. Have you ever seen this stone in the chemist's shops, the beautiful and transparent one, from which they kindle fire?
Socrates. Do you mean the burning-glass?
Strepsiades. I do. Come what would you say, pray, if I were to take this, when the clerk was entering the suit, and were to stand at a distance, in the direction of the sun, thus, and melt out the letters of my suit?
— Aristophanes, The Clouds
The ancient Greeks understood ray optics theories — witness Euclid's work on Optics, Archimedes' practical machines built in defense of Syracuse. Even comedians could refer to the ability of a lens to focus enough sunlight to melt wax or start a fire, and expect their audiences to be familiar with this phenomenon.
Web Lecture (Under Revision)
Since hands-on activities are most helpful in understanding light ray behavior, you can and should play with lenses.
The Physics Classroom site offers both discussion of optics phenomena, practical demonstrations, and interactive simulations. Use the (Refraction and the Ray Model of Light to review concepts covered in your text. If you are having problems with a particular concept, start with that concept and make sure that you understand it, then visit the other pages.
Practice with the Concepts
Complete the Check your Understanding questions in each section you review.
Discussion Points
- When you look down into a swimming pool or a lake, are you likely to overestimate or underestimate its depth? Explain. How does the apparent depth vary with the viewing angle?
- A thin converging lens is moved closer to a nearby object. Does the real image formed change (a) in position, (b) in size?
- The thicker a double convex lens is in the center as compared to its edges, the shorter its focal length for a given lens diameter. Why?
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