Two thin equiconvex lenses each of focal length 0.2 m are placed coaxially with their optic centres 0.5 m apart. Then the focal length of the combination is

• − 0.4 m

• 0.4 m

• − 0.1 m

• 0.1 m

A prism of a certain angle deviates the red and blue rays by 8° and 12° respectively. Another prism of the same angle deviates the red and blue rays by 10° and 14° respectively. The prisms are small angled and made of different materials. The dispersive powers of the materials of the prisms are in the ratio

• 5 : 6

• 9 : 11

• 6 : 5

• 11 : 9

A 20 cm length of a certain solution causes right handed rotation of 38°. A 30 cm length of another solution causes left handed rotation of 24°. The optical rotation caused by 30 cm length of a mixture of the above solutions in the volume ratio 1 : 2 is 

• left handed rotation of 14°

• right handed rotation of 14°

• left handed rotation of 3°

• right handed rotation of 3°

Blue colour of sea water is due to

• interference of sunlight reflected from the water surface

• scattering of sunlight by the water molecules

• image of sky in water

• refraction of sunlight

A ray of light enters from a rarer to a denser medium. The angle of incidence is i. Then the reflected and refracted rays are mutually perpendicular to each other. The critical angle for the pair of media is

• sin^-1 (tan i)

• tan^-1 (sin i)

• sin^-1 (cot i)

• cos^-1 (tan i)

A fish in water (refractive index n) looks at a bird vertically above in the air. If y is the height of the bird and x is the depth of the fish from the surface, then the distance of the bird as estimated by the fish is

• $\mathrm{x} + \mathrm{y} \left(1 - \frac{1}{\mathrm{n}}\right)$

• x + ny

• $\mathrm{x} + \mathrm{y} \left(1 + \frac{1}{\mathrm{n}}\right)$

• $\mathrm{y} + \mathrm{x} \left(1 - \frac{1}{\mathrm{n}}\right)$

Figure shows a mixture of blue, green and red coloured rays incident normally on a right angled prism. The critical angles of the material of the prism for red, green and blue are 46°, 44° and 43° respectively. The arrangement will separate

• red colour from blue and green

• blue colour from red and green

• green colour from red and blue

• all the three colours

A convex and a concave lens separated by distance d are then put in contact. The focal length of the combination

• decreases

• increases

• becomes 0

• remains the same

A convex lens is made of 3 layers of glass of 3 different materials as in the figures. A point object is placed on its axis. The number of images of the object are

• 1

• 2

• 3

• 4

A convex lens made of glass has focal length 0.15 m in air. Ifthe refractive index of glass is $\frac{3}{2}$ and that of water is $\frac{4}{3}$, the focal length of lens when immersed in water is

• 0.45 m

• 0.15 m

• 0.30 m

• 0.6 m