A red coloured object illuminated by mercury vapour lamp, when seen through a green filter, will appear

• red

• blue

• black

• white

Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is $\frac{3}{2}$ is

• 2 × 10^-4 s

• 2 × 10⁴ s

• 2 × 10^-11 s

• 2 × 10¹¹ s

Two thin lenses of focal length 20 cm and 25 cm are in contact. The effective power of the combination is

• 4.5 D

• 18 D

• 9 D

• 2.5 D

The magnification of the image when an object is placed at a distance x from the principal focus of a mirror of focal length f

• $\frac{\mathrm{x}}{\mathrm{f}}$

• $1 + \frac{\mathrm{f}}{\mathrm{x}}$

• $\frac{\mathrm{f}}{\mathrm{x}}$

• $1 - \frac{\mathrm{f}}{\mathrm{x}}$

If n₁ and n₂ are the refractive indices of the core and the cladding respectively of an optic fibre, then

• n₁ = n₂

• n₁ < n₂

• n₂  <  n₁

• n₂ = 2n₁

66.

A fish at a depth of 12 cm in water is viewed by an observer on the bank of a lake. To what height the image of the fish is raised ? (Refractive index of lake water = 4/3). 

• 9 cm

• 12 cm

• 3.8 cm

• 3 cm

A ray of light is incident at 50° on the middle of one of the two mirrors arranged at an angle of 60° between them. The ray then touches the second mirror, gets reflected back to the first mirror, making an angle of incidence

• 50°

• 60°

• 70°

• 80°

A lamp (point source) is hanging along the axis of a circular table of radius r. At what height should the lamp be placed above the table so that the illuminance at the edge of the able is $\frac{1}{8}$ of that at its centre ?

• $\frac{\mathrm{r}}{2}$

• $\frac{\mathrm{r}}{\sqrt{2}}$

• $\frac{\mathrm{r}}{\sqrt{3}}$

• $\frac{\mathrm{r}}{\sqrt{7}}$

The focal length (f) of a spherical (concave or convex) mirror of radius of curvature R is

• $\frac{\mathrm{R}}{2}$

• R

• $\left(\frac{3}{2}\right)\mathrm{R}$

• 2R

In a compound microscope, the intermediate image is

• virtual, erect and magnified

• real, erect and magnified

• real, inverted and magnified

• virtual, erect and reduced