What are images used for?

Let $A, B, C$ and $D$ are written on cardboard as shown in the picture.

The cardboard is kept at a suitable distance behind a transparent empty glass of cylindrical shape. If the glass is now filled with water, one sees an inverted image of the pattern on the cardboard when looking through the glass. Ignoring the magnification effect, the image would appear as

Determine by drawing a ray-diagram, the number of images that will be formed for an object placed in between two plane mirrors if the inclination between the mirrors to ${60}^{°}$.

In the figure given below, there are two convex lenses $L_1$and $L_2$ having focal lengths of $f_1$ and $f_2$, respectively. The distance between $L_1$ and $L_2$ will be equal to the ______of the two foci.

Represent the union of two sets by Venn diagram for each of the following.

$X=\{x | x$ is a prime number between $80$ and $100\}$

$Y=\{y | y$ is an odd number between $90$ and $100\}$