31.

A thin prism having refracting angle 10° is made of glass of refractive index 1.42. This prism is combined with another thin prism of a glass of refractive index 1.7. This combination produces dispersion without deviation. The refracting angle of second prism should be

• 4°

• 6°

• 8°

• 8°

An astronomical refracting telescope will have large angular magnification and high angular resolution when it has an objective lens of

• Small focal length and large diameter

• Large focal length and small diameter

• Small focal length and small diameter

• Large focal length and large diameter

The refractive index of the material of a prism is √2 and the angle of the prism is 30°. One of the two refracting surfaces of the prism is made a mirror inwards, by a silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is

• 60°

• 45°

• zero

• 30°

An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be

• 30 cm away from the mirror

• 36 cm away from the mirror

• 36 cm towards the mirror

• 30 cm towards the mirror

The plane face of a plano-convex lens is silvered. If μ be the refractive index and R, the radius of curvature of the curved surface, then the system will behave like a concave mirror of curvature.

• μR

• R²/μ

• R/(μ -1)

• [(μ + 1)/(μ-1)]R

A student can distinctly see the object upto a distance 15 cm. He wants to see the blackboard at a distance of 3m. Focal length and power of lens used respectively will be

• -4.8cm, -3.3D

• -5.8cm, -4.3D

• -7.5cm, -6.3D

• -15.8cm, -6.33D

A thin glass prism (μ =15) in the position of minimum deviation deviates the monochromatic light ray by 10^o, the refracting angle of prism is

• 20^o

• 10^o

• 30^o

• 45^o

Two thin lenses of powers 12 D and-2D respectively are placed in contact, the power, focal length and nature
respectively will be

• 8D, 0.8m, convex

• 14D, 0.5m, convex

• 5D, 0.2m, convex

• 10D, 0.1m, convex

A prism of refractive index $\sqrt{2}$ has a refracting angle of 60°. At what angle a ray must be incident on it so that it suffers a minimum deviation.

• 45⁰

• 60⁰

• 90⁰

• 180⁰

The radius of the convex surface of planoconvex lens is 20 cm and the refractive index of the material of the lens is 1.5. The focal length is

• 30cm

• 50 cm

• 30 cm

• 40 cm