Page 79 - Physics - XII

Q37. What are the diff erences between the image formed by convex lens and a concave lens?
(NCERT)
Ans. The image formed by the concave lens is virtual, erect, and diminished.

The image formed by the convex lens is real, inverted, and the size of the image increases as the
object moves near the lens.

Q38. How does the focal length of a thick convex lens diff er from that of a thin lens? (NCERT)
Ans. A thin lens is one whose thickness is negligible as compared to the image and object distances. So,
a thin lens has a larger focal length than the thick lens.

Q39. How can you recognise a convex lens, a circular glass slab, and a concave lens without touching

them? (NCERT)
Ans. When the glass piece is put on a printed page and the virtual image of the printed page is seen. Then,
the magnifi cation of the image is judged.

(i) The glass piece is a glass slab if the image has same size as that of object.

(ii) The glass piece is a convex if the image is magnifi ed.
(iii) The glass piece is a concave if the image is diminished.

Q40. Where does the centre of curvature of the plane surface of a plano-convex lens lie? (NCERT)
Ans. The centre of curvature of the plane surface of a plano-convex lens lies at the centre of the lens.

Q41. Defi ne the principal axis of a plano-convex lens. (NCERT)
Ans. The principal axis of a plano-convex lens is defi ned as that straight line which is perpendicular to
the plane surface, but passes through the centre of curvature of the curved surface.
Q42. How does the focal length of a convex lens change if it is dipped in water? (NCERT)
Ans. The focal length of lens changes when immersed in water as the lens works according to the
refraction of light. The change in focal length can be found by using lens maker's formula.

1 g 1 1

f w 1 r 1 r 2

where, µ and µ are refractive indices of glass and water. As we know that, µ is 1.33. If the
g w w
refractive index of lens is greater than that of water, then the focal length is reduced.

Q43. What is the relation between focal length and radius of curvature of a plano-convex lens?
(NCERT)
Ans. The relation between the focal length and radius of curvature is given by

1 ( 1) 1 1

f R 1 R 2

where, µ= refractive index,
R and R = radii of curvatures
2
1
Q44. Can a virtual image produced by a lens be inverted? (NCERT)
Ans. No, a virtual image produced by a lens cannot be inverted.

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