Light from a point source in air falls on a spherical glass surface (n = 1.5 and radius of curvature = 20 cm). The distance of the light source from the glass surface is 100 cm. Find the image distance.

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Accepted Answer

(4) 100 cm. Given: n₁ = 1 in air, n₂ = 1.5 in glass, R = +20 cm (centre of curvature on the outgoing side), source 100 cm in front so u = -100 cm. Idea: refraction at one spherical surface obeys (n₂)/v-(n₁)/u = (n₂-n₁)/R. (1.5)/v-1/(-100) = (1.5-1)/(20) (1.5)/v = (0.5)/(20)-1/(100) = 0.025-0.010 = 0.015 cm⁻¹ v = (1.5)/(0.015) = 100 cm. The plus sign puts the image on the far side of the surface, inside the glass, so it is real: the image distance is 100 cm.

Light from a point source in air falls on a spherical glass surface (n = 1.5 and radius of curvature = 20 cm). The distance of the light source from the glass surface is 100 cm. Find the image distance.

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