What is the Power of a Common Magnifying Glass?

The situation here is the same as those shown in Figure 25.27 and Figure 25.28. The Sun is so far away that the Sun’s rays are nearly parallel when they reach Earth. The magnifying glass is a convex (or converging) lens, focusing the nearly parallel rays of sunlight. Thus the focal length of the lens is the distance from the lens to the spot, and its power is the inverse of this distance (in m).

The focal length of the lens is the distance from the center of the lens to the spot, given to be 8.00 cm. Thus,

To find the power of the lens, we must first convert the focal length to meters; then, we substitute this value into the equation for power. This gives

This is a relatively powerful lens. The power of a lens in diopters should not be confused with the familiar concept of power in watts. It is an unfortunate fact that the word “power” is used for two completely different concepts. If you examine a prescription for eyeglasses, you will note lens powers given in diopters. If you examine the label on a motor, you will note energy consumption rate given as a power in watts.