CBSE Class 10 Science 2020 · Paper 31/5/1 · Question 30(a)

(a) The ray diagram shows a ray AO incident obliquely at O on one face of the glass slab, with the normal NN′ at O. On entering the glass the ray bends towards the normal and travels along OO′, the angle of refraction $r$ being smaller than the angle of incidence $i$. At O′ on the opposite face it bends away from the normal MM′ and emerges along O′B at the angle of emergence $e=i$, so the emergent ray is parallel to the incident ray but laterally displaced. Arrows mark the direction of light on the incident, refracted and emergent rays. (b) $n_{ga}=\frac{\text{speed of light in air}}{\text{speed of light in glass}}=\frac{3\times 10^8\text{m/s}}{2\times 10^8\text{m/s}}=1.5$. (c) $f=\frac{1}{P}=\frac{1}{-2.5\text{D}}=-0.4$ m $=-40$ cm; the negative sign shows a concave (diverging) lens.