Question

3. An optometrist measures the power of a lens in diopters (D). Define diopters When your eye is relaxed, does the lens have its largest or shortest focal length? In this state, can you see more clearly at a distance or up close? Is the lens at its fattest or thinnest? 4.

Answer 1

3. Power of lens :- The ability of a lens to converge or diverge light rays depends on its focal length.  Example   A convex lens of short focal length bends the light rays through large angles, by focussing them closer to the optical centre.   Concave lens of very short focal length causes higher divergence than the one with longer focal length. The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power.  Definition:   The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power Pof a lens of focal length f is given by  

Power of lens (P) Focal length of the lens (F)

The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D. If f is expressed in metres, then, power is expressed in dioptres.   Dioptre   1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1m-1. You may note that the power of a convex lens is positive and that of aconcave lens is negative.   Optician prescribes corrective lenses indicating their powers. Let us say the lens prescribed has power equal to + 4.0 D. This means the lens prescribed is convex.

4 Ans.

When the eye is relaxed and the interior lens is the least rounded, the lens has its maximum focal length for distant viewing . As the muscle tension around the ring of muscle is increased and the supporting fibers are thereby loosened, the interior lens rounds out to its minimum focal length.