Geometrical Optics
Try it: Measuring your eyeglass prescription|
Your eyeglasses prescription is simply the power (in diopters) of your
lenses. If you do not suffer from astigmatism, your eyeglass lenses are
spherical lenses and you need only measure the focal length of each
lens to determine its power. Here one method is suggested, but if you
can think of others, try them to see if they all give the same result.
Ponder: How can you tell by sight whether your lens is converging or diverging? How can you tell by touch? Converging Lens: Since objects are imaged on the focal plane, use your lens to form the sharpest image of the sun on a piece of paper, as you would with a burning glass. Measure the distance from the lens to the paper to obtain the focal length. Suppose the distance is 62.5 cm = 0.625 m. The power of the lens, and therefore its prescription is then 1/0.625 = 1.6 diopters. Diverging Lens:
Find a magnifying glass and measure its focal length by the technique
described above. Place it next to your eyeglass lens and measure the
focal length, and hence the power of the combination. (If it is not
converging, find a stronger magnifying glass.) |