EDU BRIGHT PAGES | Class 10 Test Series #6

1. Which one of the following materials cannot be used to make a lens?

Water Glass Plastic Clay

Clay is opaque and does not allow light to pass through, so it cannot be used to make a lens.

2. The image formed by a concave mirror is virtual, erect and larger than the object. Where is the object placed?

Between focus and centre of curvature At the centre of curvature Beyond the centre of curvature Between pole and principal focus

A concave mirror produces a virtual, erect and magnified image when the object is placed between the pole and the focus.

3. Where should an object be placed in front of a convex lens to get a real image of the same size?

At principal focus At twice the focal length At infinity Between optical centre and focus

When an object is placed at 2F of a convex lens, the image formed is real, inverted and of the same size.

4. A spherical mirror and a thin spherical lens both have focal length –15 cm. They are likely to be:

Both concave Both convex Mirror concave, lens convex Mirror convex, lens concave

Negative focal length indicates a concave mirror and a concave lens.

5. No matter how far you stand from a mirror, your image appears erect. The mirror is:

Only plane Only concave Only convex Either plane or convex

Plane and convex mirrors always form erect images irrespective of object distance.

6. Which lens is preferred for reading small letters in a dictionary?

Convex lens of focal length 50 cm Concave lens of focal length 50 cm Convex lens of focal length 5 cm Concave lens of focal length 5 cm

A convex lens with a shorter focal length has greater power and provides higher magnification, making it suitable for reading small letters.

7. We wish to obtain an erect image of an object using a mirror. Which mirror should be used?

Concave mirror only Convex mirror only Plane mirror only Plane mirror or convex mirror

Both plane mirrors and convex mirrors always form erect images irrespective of the position of the object.

8. An object is placed at twice the focal length of a convex lens. The image formed will be:

Virtual, erect and enlarged Real, inverted and of same size Real, inverted and enlarged Virtual and diminished

For a convex lens, when the object is placed at 2F, the image is formed at 2F on the other side. It is real, inverted and of the same size.

9. An optician uses a combination of lenses instead of a single thick lens to correct vision. Which of the following is the main scientific advantage of using multiple thin lenses in contact rather than one lens of the same total power?

It increases only the brightness of the image It reduces image defects like chromatic and spherical aberration It changes the colour of the image It makes the image larger without affecting clarity

A system of thin lenses in contact can be designed so that different lenses partly cancel each other's aberrations, such as chromatic and spherical aberrations, giving a sharper, better-corrected image than a single thick lens of the same power.[web:2][web:3]

10. A lens system used in a microscope consists of three lenses placed in contact. Two lenses have powers of +4 D and −1 D. If the net power of the system is +5 D, what is the power of the third lens, and what does its nature suggest?

+2 D, convex lens +0 D, plane glass +3 D, convex lens −3 D, concave lens

The net power of thin lenses in contact is the algebraic sum of their individual powers: (P = P_1 + P_2 + P_3 P=P1+P2+P3 5 = 4 + (-1) + P_3 p3 = 2D A positive power corresponds to a converging or convex lens, so the third lens is a convex lens.[web:14]

11. The power of a lens is +4 D. Which of the following statements is correct?

The lens is concave and its focal length is +0.25 m The lens is convex and its focal length is +0.25 m The lens is convex and its focal length is −0.25 m The lens is concave and its focal length is −0.25 m

Power of a lens is given by (P = 1/f) where (f) is in metres. For P = +4, (f = 1/4 = +0.25m). A positive focal length and positive power indicate a converging or convex lens, so the correct statement is that it is a convex lens with focal length +0.25 m.[web:14]

12. A concave lens has a focal length of −50 cm. What is the power of the lens?

+2 D −2 D +0.5 D −0.5 D

Convert focal length into metres:f = -50\100 = -0.50m Power P = 1/f = 1/-0.50 = -2D The negative power confirms that it is a diverging or concave lens

13. Which factor correctly represents the ability of a lens to converge or diverge light rays?

Aperture of the lens Thickness of the lens Power of the lens Diameter of the lens

The ability of a lens to bend (converge or diverge) light is measured by its power P, which is the reciprocal of focal length. A shorter focal length means higher power and stronger convergence or divergence of light rays

14. For a convex lens, an object is placed at a distance of −20 cm and the image is formed at +40 cm. What is the magnification?

−2 +2 −0.5 +0.5

For lenses, magnification is given by (m = v/u). Here,u = -20cm v = +40cm, so (m = 40/(-20) = -2). The negative sign indicates the image is inverted, and the magnitude 2 shows the image is twice the size of the object.

15. Which of the following combinations will give a lens system of zero power?

+2 D and +2 D −1 D and −1 D +2 D and −2 D +1 D and +2 D

Net power of thin lenses in contact is the algebraic sum: \(P_{\text{net}} = P_1 + P_2 For +2 D and −2 D, P = +2 + (-2) = 0 so the system has zero power and behaves like plane glass, causing no net convergence or divergence.[web:6][web:15]

16. If the magnification produced by a lens is negative, the image formed is:

Virtual and erect Real and erect Real and inverted Virtual and inverted

For lenses, a negative magnification indicates that the image is inverted with respect to the object. An inverted image formed by a lens is real, so negative magnification corresponds to a real and inverted image.