Question

write the focal length of the lens used in this spectacles​

Answer 1

if we are suffering from myopia then this defect can be corrected by using the concave or diverging lens of appropriate focal length we can't measure the focal length it depends upon us how much eyesight is weak if we are suffering from hypermetropia then we have to use an eye lens of convex or converging of appropriate focal length

Answer 2

Convex are the most commonly used lenses for glasses. Focal length is not stated directly in a prescription for eyeglasses.To measure the focal length of a converging (convex) lens. To measure the focal length of a converging (convex) lens. Using the formula: 1/u + 1/v = 1/f, the focal length f of the lens can be found.concave lens is the opposite of a convex lens. Here one or both of the lenses surfaces are curved inwards. That is, the centre of the lens is closer to the plane than the edge. A concave lens is used to correct short-sightedness (myopia).The lens flexes to change its curvature and finish the focusing process. When an object is located at infinity, the focal length, or the distance from the cornea to the retina, of a normal relaxed eye is about 1.7 cm (17 mm).