Exercises
1. Both sides of a card are equally illuminated when two lights are on opposite sides of it and 10 and 30 cm. respectively from it. what are their relative intensities?
2. What are the relative intensities of illumination from a gas light upon a book 6 ft. and 2 ft. respectively from the light?
3. Which is more expensive per candle power? How many times as expensive? A 50-watt 16-candle-power incandescent lamp at 10 cents per kilowatt-hour or a 100-candle-power Welsbach light burning 5 cu. ft. of gas per hour at 80 cents per 1000 cu. ft. of gas. (Find cost of each per hour, and then the cost of 1 candle power hour for each.)
4. Why are not ordinary shadows perfectly dark?
5. At what distance will a 16-candle-power lamp give the same illumination as a single candle at 10 in.?
6. If the sun is at an elevation of 30 degrees what is the angle of incidence at which it strikes the surface of water? What is the angle between the incident and the reflected rays?
7. What is the difference between the phenomena of reflection of light from a white sheet of writing paper and from a piece of clear window glass?
8. A horizontal ray of light, traveling due east, strikes a vertical mirror so that after reflection it is traveling due north. If the mirror be now turned 10 degrees about a vertical axis, the north edge moving east, what will be the direction of the reflected ray?