When a plane wavefront is incident on a convex lens,the emerging wavefront becomes . . . . . . .

$A$ convex lens focuses an object $20 \ cm$ from it on a screen placed $5 \ cm$ away from it. $A$ glass plate (refractive index $\mu = \frac{7}{5}$) of thickness $t = 1.4 \ cm$ is inserted between the lens and the screen. What is the new distance of the object from the lens so that its image is again focused on the screen (in $cm$)?

$A$ lens having focal length $f$ and aperture of diameter $d$ forms an image of intensity $I.$ If an aperture of diameter $\frac{d}{2}$ in the central region of the lens is covered by a black paper,the focal length of the lens and the intensity of the image will be,respectively:

$A$ lens is made of flint glass (refractive index $= 1.5$). When the lens is immersed in a liquid of refractive index $1.25$,the focal length

The focal lengths of a converging lens are $f_V$ and $f_R$ for violet and red light,respectively. Which of the following relations is correct?

$A$ convex lens having refractive index $1.6$ has a focal length of $12 \text{ cm}$ when it is in air. The focal length of that lens when placed in water is (refractive index of water = $1.28$) (in $\text{ mm}$)