When blue light is used instead of red light in a convex lens,its focal length will ......... .

An object placed at a distance of $16 \ cm$ from a convex lens produces an image of magnification $m$ $(m > 1)$. If the object is moved towards the lens by $8 \ cm$, then again an image of magnification $m$ is obtained. The numerical value of the focal length of the lens is (in $cm$)

Photographs of the ground are taken from an aircraft flying at an altitude of $2000\; m$ by a camera with a lens of focal length $50\; cm$. The size of the film in the camera is $18\; cm \times 18\; cm$. The area of the ground that can be photographed by the camera is

The slit of a collimator is illuminated by a source as shown in the adjoining figures. The distance between the slit $S$ and the collimating lens $L$ is equal to the focal length of the lens. The correct direction of the emergent beam will be as shown in figure:

$A$ point object $O$ is placed on the principal axis of a convex lens of focal length $20 \, cm$ at a distance of $40 \, cm$ to the left of it. The diameter of the lens is $10 \, cm$. If the eye is placed $60 \, cm$ to the right of the lens at a distance $h$ below the principal axis,then the maximum value of $h$ to see the image will be.......$cm$.

$A$ diminished image of an object is to be obtained on a large screen $1\, m$ from it. This can be achieved by