A plane mirror, _M_, is held in a vertical position resting upon a sheet of paper. (See Fig. 352.) Pins are set upright in the paper at _A_ and _B_. On placing the eye along the line _AC_ and looking toward the mirror an image of _B_ may be seen in the mirror due to the light reflected from its surface. Pins _C_ and _D_ are now set in the paper so that when one looks along the line _BD_ toward the mirror one may see all four pins apparently in one line. This indicates that the light from _A_ and _C_ passing along _CA_ toward _O_ is reflected back along the light _CBD_. By means of a ruler, draw lines through _BD_ and _AC_ till they intersect at _O_. Also draw _PO_ perpendicular to the mirror at _O_. Then the angles _AOP_ and _BOP_ will be found equal. These are called the angles of _incidence_ and _reflection_ respectively. _The law of reflection_ is therefore stated: _The angle of reflection is equal to the angle of incidence._ These angles are in the same plane, that of the paper. This law applies in all cases of reflection of light. It is similar to the law of reflection of sound (Art. 326.) Important Topics 1. Photometry, law of intensity, candle power, foot-candle. 2. Intensity of illumination. 3. Reflected light and law of reflection. 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? 9. The necessary illumination for reading is about 2 foot-candles. How far away may an 8-candle-power lamp be placed? 10. What is the illumination in foot-candles upon a surface 20 ft. from an arc lamp having an intensity of 1000 candle power?