# Understanding Ellen's Eyeglass Prescription

## Explanation:

**Ellen has a prescription of -1.0 D for her eyeglasses.** This prescription indicates that Ellen has myopia, also known as nearsightedness. In myopia, a person can see near objects clearly but has difficulty seeing objects that are far away. The negative sign in Ellen's prescription suggests that diverging lenses are used to correct her vision by spreading out the light before it reaches her eyes, thus helping her to focus on distant objects more effectively.

**The power of eyeglasses is measured in diopters, which describes the inverse of the focal length in meters.** For example, in the case of a myopic person with a prescription of -4.00 D, their far point (the maximum distance at which they can see objects clearly) would need to be calculated. Also, the power required for correcting the vision of someone with laser vision correction, such as a patient who is severely myopic and has a far point of 5.00 cm, can be determined as follows:

**To find out by how many diopters the power should be reduced to obtain normal distant vision,** we use the formula Power (D) = 1 / focal length (m). Since the far point is given as 5.00 cm, or 0.05 m, the calculation would be 1 / 0.05 m = +20.0 D. This indicates that the power of the eye would need to be reduced by 20 diopters to correct the vision to normal.