CBSE Class 10 Science 2018 · Paper 31(B) · Question 20(a)

(a) Spherical lens: a piece of transparent material (such as glass) bound by two surfaces, of which one or both are spherical. Two types: (1) convex or converging lens - thicker at the middle than at the edges; (2) concave or diverging lens - thinner at the middle than at the edges. (i) Principal axis: the imaginary straight line passing through the two centres of curvature ($C_1$ and $C_2$) of the lens; it passes through the optical centre. (ii) Optical centre: the central point O of the lens; a ray of light through it passes without any deviation. (iii) Aperture: the effective diameter of the circular outline of the spherical lens. (b) Nature of the lens: the image is obtained on a screen, so it is real, and only a convex lens forms a real image; the lens is convex (converging). Focal length: the lens is midway, so $u=-30$ cm and $v=+30$ cm; $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{30}+\frac{1}{30}=\frac{1}{15}$, so $f=+15$ cm (the candle is at $2F$: $4f=60$ cm). Image: real and inverted, and of the same size as the flame ($m=\frac{v}{u}=-1$).