Ray Optics and Optical Instruments · Question 7

A thin convex lens and a thin concave lens are kept in contact and are co-axial. Which of the following statements is correct for this combination of two lenses ?

1. behaves as concave lens if |fconvex|>|fconcave|
2. behaves as concave lens if |fconvex|<|fconcave|
3. behaves as convex lens if |fconvex|>|fconcave|
4. Focal length of the lens system will change if the positions of two lenses are interchanged

An object AB is placed 15 cm on the left of a convex lens P of focal length 10 cm. Another convex lens Q is now placed 15 cm right of lens P. If the focal length of lens Q is 15 cm, the final image is .

1. virtual, formed at 7.5 cm right of lens Q, with a size bigger than that of AB
2. real, formed at 7.5 cm right of lens Q, with a size same as that of AB
3. formed at infinity.
4. real, formed at 7 cm right of lens Q, with a size smaller than that of AB

A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm as shown in figure. The length of the image is cm. [figure]

1. 2.5
2. 5
3. 7.5
4. 7

A convex lens is made from glass material having refractive index of 1.4 with same radius of curvature on both sides. The ratio of its focal length and radius of curvature is .

1. 0.5
2. 2.5
3. 0.8
4. 1.25

For a thin symmetric prism made of glass (refractive index 1.5), the ratio of incident angle and minimum deviation will be .

1. 3 : 4
2. 3 : 2
3. 2 : 1
4. 1 : 2

Refer the figure given below. 1 and 2 are refractive indices of air and lens material. The height of image will be cm. [figure]

1. 1
2. 0.5
3. 1.2
4. 0.25

One side of an equilateral prism is painted by a transparent material of refractive index n2. The refractive index of prism is 1.6. The minimum value of n2 required for total internal reflection from painted face is . [figure]

1. 3 31.6
2. 3
3. 3.2 3
4. 4 35

If sunlight is focused on a paper using convex lens, it starts burning the paper in shortest time when the lens is kept at 30 cm above the paper. If the radius of curvature of the lens is 60 cm then the refractive index of the lens material is 10. The value of is .

The magnitudes of power of a biconvex lens (refractive index 1.5) and that of a plano-concave lens (refractive index = 1.7) are same. If the curvature of plano-concave lens exactly matches with the curvature of back surface of the biconvex lens, then ratio of radius of curvature of front and back surface of the biconvex lens is

1. 2 : 5
2. 5 : 2
3. 12 : 5
4. 5 : 12

A convex lens of refractive index 1.5 and focal length f = 18 cm is immersed in water. The difference in focal lengths of the given lens when it is in water and in air is f. The value of is . (refractive index of water = 4/3)