The distance between the object and the screen is $100 \, cm$. $A$ lens produces an image on the screen when it is placed at either of two positions $40 \, cm$ apart. The power of the lens is (approximately): (in $, D$)

$A$ point source is located at a distance of $20 \ cm$ from the front surface of a symmetrical glass biconvex lens with equal radii of curvature $5 \ cm$. The distance at which the image is formed from the rear surface of this lens is $[$Given refractive index of the glass is $1.5]$

An object is placed at a point distant $x$ from the focus of a convex lens and its image is formed at $I$ as shown in the figure. The distances $x$ and $x'$ satisfy the relation:

$A$ white light is falling on a bi-convex lens. Which of the following options represents the correct qualitative behaviour of the focussing of this light?

$A$ plane wave passes through a convex lens. The geometrical shape of the wavefront that emerges is

$A$ student measures the focal length of a convex lens by placing an object pin at a distance '$u$' from the lens and measuring the distance '$v$' of the image pin. The graph between '$u$' and '$v$' plotted by the student should look like: