$A$ hollow convex lens of glass will behave like a

Define magnification for a lens and derive its expression.

The diameter of a plano-convex lens is $6 \, cm$ and its thickness at the centre is $3 \, mm$. If the speed of light in the material of the lens is $2 \times 10^8 \, m/s$,the focal length of the lens is.......$cm$.

$A$ point object is placed on the axis of a thin convex lens of focal length $0.05 \ m$ at a distance of $0.2 \ m$ from the lens and its image is formed on the axis. If the object is now made to oscillate along the axis with a small amplitude of $A \ cm,$ then what is the amplitude of oscillation of the image? [You may assume $\frac{1}{1+x} \approx 1-x,$ where $x << 1$]

$A$ lens is placed between a source of light and a wall. It forms images of area $A_1$ and $A_2$ on the wall for its two different positions. The area of the source of light is

The image of an illuminated square is obtained on a screen with the help of a converging lens. The distance of the square from the lens is $40\,cm$. The area of the image is $9$ times that of the square. The focal length of the lens is........$cm$.