CBSE Class 10 Science 2018 · Paper 31/1/C · Question 11(a)

Light - Reflection and Refraction 3 marks Medium

We wish to obtain an equal sized inverted image of a candle flame on a screen kept at distance of 4 m from the candle flame.

(a)Name the type of lens that should be used.
(b)What should be the focal length of the lens and at what distance from the candle flame the lens be placed.
(c)Draw a labelled diagram to show the image formation in this case.

Answer

A convex lens of focal length 1 m, placed 2 m from the flame, so flame and image are each at 2F.

(a) A convex (converging) lens. (b) For an equal-sized, real, inverted image the candle flame must be at

2 F_{1}

, and the image is then formed at

2 F_{2}

. So the flame-to-screen distance is

2 f + 2 f = 4 f = 4

m, which gives the focal length

f = 1

m (

100

cm). The lens must be placed

2

m from the candle flame, midway between the flame and the screen. (c) Ray diagram: the flame AB stands at

2 F_{1}

2

m in front of the convex lens. A ray from B parallel to the principal axis is refracted through

F_{2}

, and a ray from B through the optical centre O goes straight on. They meet at B′ on the screen at

2 F_{2}

2

m behind the lens, forming a real, inverted image A′B′ of the same size as the flame (see the figure).

🔒 Step-by-step solution