$A$ water drop in air refracts the light ray as:

Which of the following is correct regarding microscope and telescope?

$A$ point source of light is $60\, cm$ from a screen and is kept at the focus of a concave mirror which reflects light on the screen. The focal length of the mirror is $20\, cm$. The ratio of average intensities of the illumination on the screen when the mirror is present and when the mirror is removed is: (in $: 1$)

Ray optics is valid when characteristic dimensions are:

$A$ point object $O$ is moving towards a concave mirror as shown in the figure. Choose the correct option representing the direction of the velocity of the image ($F$ is the focus and $C$ is the centre of curvature).

By which type of mirror,the image obtained is always erect?