$A$ lamp is hanging at a height of $4\,m$ above a table. The lamp is lowered by $1\,m$. The percentage increase in illuminance will be.....$\%$

$A$ concave mirror has a radius of curvature of $40\, cm$. It is at the bottom of a glass that has water filled up to $5\, cm$ (see figure). If a small particle is floating on the surface of water,its image as seen from directly above the glass is at a distance $d$ from the surface of water. The value of $d$ is close to ......$cm$ (Refractive index of water $= 1.33$)

For a concave mirror,if a real image is formed,the graph between $\frac{1}{u}$ and $\frac{1}{v}$ is of the form:

An opaque card is held over the lower half of a converging lens as shown in the figure. Which picture best shows the image that appears on the screen?

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

$A$ 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 $: 1$)