The central part of a convex lens is thicker than the edges. Because of its converging nature, this lens is called a converging lens. When light enters the lens, it undergoes refraction and eventually converges at a point known as the main focus, or F. The focal length is the distance between the primary focus and the lens’s centre. The power of the length is the reciprocal of the focal length. It is measured in diopters. The displacement technique of length is used to determine the focal length and power of a thin convex lens.
Place the convex lens in front of an object and record the distance at which the image formed. Let’s preserve the item and the screen in their current positions. Now slowly move the lens nearer the screen until the thing is clearly visible. After that, the image begins to fade. At that time, secure the lens. This is based on a simple idea. Even if we swap the object’s distance from the lens with the image’s distance from the lens, we’ll still obtain an image. The displacement length is then calculated as the distance between the first and second positions of the lens.
f = D2−d2/4D
f = 12−(0.4)2 /4×1
f = 1−0.16/4
f = 0.21 m
Therefore, the power is given as P = 1/f …………….(2)
Substituting the value of f in equation (2)
P = 1/.21
P = 4.761diopter