Question

The optical prescription for a pair of spectacles is:
Right eye : $-3.50 \mathrm{D}$ Left eye : $-4.00 \mathrm{D}$
(a) Are these lenses thinner at the middle or at the edges?
(b) Which lens has a greater focal length?
(c) Which is the weaker eye?

Answer 1

Hint: We know that near-sightedness (myopia) is a common vision condition in which we can see objects near to us clearly, but objects farther away are blurry. It occurs when the shape of our eye causes light rays to bend (refract) incorrectly, focusing images in front of our retina instead of on your retina. A near-sighted person sees near objects clearly, while objects in the distance are blurred. Farsightedness is the result of the visual image being focused behind the retina rather than directly on it. A farsighted person sees faraway objects clearly, while objects that are near are blurred.

Complete step by step answer
We know that when light hits the retina (a light-sensitive layer of tissue at the back of the eye), special cells called photoreceptors turn the light into electrical signals. These electrical signals travel from the retina through the optic nerve to the brain. Then the brain turns the signals into the images we see. Rods and cones convert the light from our retinas into electrical impulses, which are sent by the optic nerve to the brain, where an image is produced. The macula is the part of the retina that gives us central vision. It's how we see form, colour, and detail in our direct line of sight.
(i) These lenses have negative powers, meaning the person is near-sighted. Hence, he has Myopia eye defect.
(ii) As the lenses have negative powers, so the focal length is negative. Hence, they are concave in nature. And concave lenses are thinner in the middle
(iii) We know, Power of lens, $\mathrm{P}=\dfrac{1}{\mathrm{f}}$
As power and focal length are inversely proportional to each other, so the lens with lower power will have greater focal length. That is the right eye lens, -3.5D lens has greater focal length.

Note: We know that the power of a lens is defined as the reciprocal of the focal length. Lens power is measured in dioptres (D). Converging (convex) lenses have positive focal lengths, so they also have positive power values. Diverging (concave) lenses have negative focal lengths, so they also have negative power values. Power of a lens is the ability to converge or diverge the ray of light falling on it. Power of a lens is defined as reciprocal of focal length of the lens. A lens of small focal length has large power of converging or diverging a parallel beam of light.