RAY OPTICS&OPTICAL INSTRUMENTS

We combine two lenses, one is convex and other is concave having focal lengths /, and f2 and their combined focal length is F. Combination of the lenses will behave like concave lens, if

• f1 > f2

• f1 = f2

• f1 < f2.

• f1 ≤ f2

The focal length of a biconvex lens of radii of each surface 50 cm and refractive index 1.5, is

• 40.4 cm

• 75 cm

• 50 cm

• 80 cm

Tom’ lenses of focal lengths ± 15 cm and ± 150 cm are available for making a telescope. To produce the largest magnification, the focal length of the eyepiece should be

• + 15 cm

• + 150 cm

• – 150 cm

• – 15 cm

A convex lens and a concave lens, each having the same focal length of 25 cm, are put in contact to form a combination of lenses. The power of the combination (in dioptres) is

• zero

• 25

• 50

• infinity

The refractive index of the material of an equilateral prism is √3. What is the angle of minimum deviation?

• 45°

• 60°

• 37°

• 30°

A convex lens of refractive index 3/2 has a power of 2.5 D in air. If it is placed in a liquid of refractive index 2 then the new power of the lens is

• – 1.25 D

• – 1.5 D

• 1.25 D

• 1.5 D

A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will

• become zero

• become infinite

• become small, but non-zero

• remain unchanged

Two beams of red and violet color are made to pass separately through a prism (angle of the prism is 60°). In the position of minimum deviation, the angle of refraction will be

• 30° for both the colors

• greater for the violet color

• greater for the red color

• equal but not 30° for both the colors

An astronomical refractive telescope has an objective of focal length 20 m and an eyepiece of focal length 2 cm. Then

• the magnification is 1000

• the length of the telescope tube is 20.02 m

• the image formed of inverted

• all of these

A ray of light incident at an angle θ on a refracting face of a prism emerges from the other face normally. If the angle of the prism is 5° and the prism is made of a material of refractive index 1.5, the angle of incidence is

• 7.5°

• 5°

• 15°

• 2.5°