Ray Optics and Optical Instruments

A thin biconvex lens is prepared from the glass ( =1.5) both curved surfaces of which have equal radii of 20 cm each. Left side surface of the lens is silvered from outside to make it reflecting. To have the position of image and object at the same place, the object should be placed, from the lens at a distance of cm.

1. 10
2. 12.5
3. 13
4. 13.5

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 biconvex lens is formed by using two thin planoconvex lenses, as shown in the figure. The refractive index and radius of curved surfaces are also mentioned in figure. When an object is placed on the left side of lens at a distance of 30 cm from the biconvex lens, the magnification of the image will be : [figure]

1. −2
2. −2.5
3. +2.5
4. +2

A thin convex lens of focal length 5 cm and a thin concave lens of focal length 4 cm are combined together (without any gap) and this combination has magnification m1 when an object is placed 10 cm before the convex lens. Keeping the positions of convex lens and object undisturbed a gap of 1 cm is introduced between the lenses by moving the concave lens away, which leads to a change in magnification of total lens system to m2. The value of | m1m2 | is .

1. 2527
2. 56
3. 527
4. 32

A convex lens of focal length 30 cm is placed in contact with a concave lens of focal length 20 cm. An object is placed at 20 cm to the left of this lens system. The distance of the image from the lens in cm is .

1. 607
2. 15
3. 45
4. 30

A concave-convex lens of refractive index 1.5 and the radii of curvature of its surfaces are 30 cm and 20 cm, respectively. The concave surface is upwards and is filled with a liquid of refractive index 1.3. The focal length of the liquid-glass combination will be

1. 70011 cm
2. 60011 cm
3. 80011 cm
4. 50011 cm

Two thin convex lenses of focal lengths 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is:

1. 5 D
2. 1 D
3. 20 D
4. 10 D