CBSE Class 10 Science 2017 · Paper 531/1 · Question 14

(a) A convex (converging) lens. The image is obtained on a screen, so it is real, and of the two kinds of lens only a convex lens can form a real image of a real object; a concave lens always forms a virtual image. The lens formula agrees: $\frac{1}{f}=\frac{1}{20}-\frac{1}{-80}=\frac{5}{80}$, so $f=+16$ cm, which is positive. (b) (i) The flame and the screen are on opposite sides of the lens, so the distance between the object and its image is $80+20=100$ cm. (ii) $m=\frac{v}{u}=\frac{+20}{-80}=-0.25$: the image is real, inverted and one-fourth the size of the flame. (c) The flame is beyond $2F_1$. A ray from its tip parallel to the axis passes through $F_2$ after refraction, and a ray through the optical centre goes on undeviated; they meet $20$ cm behind the lens, between $F_2$ and $2F_2$, where the screen is (see the ray diagram).