Lenses

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 in dioptres of the combination is:

1. 25

2. 50

3. infinite

4. zero

Two thin lenses of focal lengths f₁ and f₂ are in contact and coaxial. The power of the combination is:

1. $\sqrt{\frac{{\mathrm{f}}_1}{{\mathrm{f}}_2}}$

2. $\sqrt{\frac{{\mathrm{f}}_2}{{\mathrm{f}}_1}}$

3. $\frac{{\mathrm{f}}_1+{\mathrm{f}}_2}{{\mathrm{f}}_1{\mathrm{f}}_2}$

4. None of the above

A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of a focal length of cm. The diameter of the sun is m and its mean distance from the earth is m. What is the diameter of the sun's image on the paper?

1.	m	2.	m
3.	m	4.	m

A lens having focal length and aperture of diameter forms an image of intensity . An aperture of diameter in central region of lens is covered by a black paper. The focal length of lens and intensity of the image now will be respectively:
1. and
2. and
3. and
4. and

A biconvex lens has a radius of curvature of magnitude . Which one of the following options, best describes, the image formed by an object of height cm placed from the lens?

1.	virtual, upright, height cm
2.	real, inverted, height cm
3.	real, inverted, height cm
4.	virtual, upright, height cm

Two identical glass equiconvex lenses of focal length each are kept in contact. The space between the two lenses is filled with water . The focal length of the combination is:

1.		2.
3.		4.

A plane-convex lens of unknown material and unknown focal length is given. With the help of a spherometer, we can measure the

1.	focal length of the lens.
2.	radius of curvature of the curved surface.
3.	aperture of the lens.
4.	refractive index of the material.

The power of a biconvex lens is dioptre and the radius of curvature of each surface is cm. The refractive index of the material of the lens is:

1.		2.
3.		4.

A convex lens of focal length and a concave lens of focal length are kept along the same axis with a distance between them. If a parallel beam of light falls on leaves as a parallel beam, then the distance in will be:

1.		2.
3.		4.

A point object is placed at a distance of from a convex lens of focal length . If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of from it, the final image would be formed at a distance of:

1.	from the plane mirror, it would be a virtual image.
2.	from the plane mirror, it would be a virtual image.
3.	from the lens, it would be a real image.
4.	from the lens, it would be a real image.

A biconvex lens has radii of curvature, each. If the refractive index of the material of the lens is the power of the lens is:

1.	infinity	2.
3.		4.

Two thin lenses are of the same focal lengths but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be:
1. infinite
2. zero
3.
4.

In a certain camera, a combination of four similar thin convex lenses are arranged axially in contact. Then the power of the combination and the total magnification in comparison to the power and magnification for each lens will be, respectively -
1. and
2. and
3. and
4. and