When a plane mirror is rotated through a certain angle,the reflected ray turns through twice as much and the size of the image

When light rays parallel to the principal axis fall on a convex lens,they refract and converge at a point on the principal axis. What is this point called?

Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of $15\, cm$ from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?

Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

$A$ ray of light $AB$ parallel to the principal axis is incident on a convex lens as shown in the figure. Copy the figure.
$(i)$ Draw the refracted ray.
$(ii)$ Name the point through which the refracted ray passes on the principal axis.

Light waves are said to travel in a straight line because ...