Refraction of Light at spherical surfaces: Lenses
ICSE · Class 12 · Physics
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State the coordinate geometry sign convention used for lenses and spherical refracting surfaces.
Answer
Origin: optical centre. Principal axis: X-axis. Distances to the right of the optical centre are positive. Distances to the left are negative. Distances upward from the principal axis are positive. Di…
A spherical refracting surface separates a rarer medium of refractive index n1 from a denser medium of refractive index n2. State the refraction formula.
Answer
Refraction at spherical surface: n2/v - n1/u = (n2 - n1)/R. Here n1 is the refractive index of the rarer medium, n2 is the refractive index of the denser medium, u is object distance, v is image dista…
A spherical refracting surface has relative refractive index n = n2/n1. State the simplified formula.
Answer
Refraction at spherical surface using relative index: n/v - 1/u = (n - 1)/R. Here n = n2/n1.
Why does a concave spherical refracting surface always form a virtual image for an object in the rarer medium?
Answer
For a concave surface, R is negative. Using n2/v - n1/u = (n2 - n1)/R, the image distance v also comes out negative for negative u. So the image forms in the first medium and is virtual. The image poi…
Why does a convex spherical refracting surface form a real image for an object in the rarer medium?
Answer
For a convex surface, R is positive. The refracted rays actually meet on the other side of the surface, so the image is real. For small aperture, all rays from the object meet at the same point I.
An object lies in the rarer medium with n1 = 1, n2 = 1.5, u = -30 cm, and R = +20 cm. Find the image distance v for a spherical surface.
Answer
Given: n1 = 1, n2 = 1.5, u = -30 cm, R = +20 cm. Formula: n2/v - n1/u = (n2 - n1)/R. Substitute: 1.5/v - 1/(-30) = (1.5 - 1)/20. So, 1.5/v + 1/30 = 0.5/20 = 1/40. 1.5/v = 1/40 - 1/30 = (3 - 4)/120 = -…
State the power of a refracting spherical surface and explain the sign idea briefly.
Answer
Power of a refracting spherical surface = (n2 - n1)/R. A larger value means greater deviation of rays. The sign depends on the sign of R and on whether refraction is from rarer to denser or denser to …
What is the first focus of a lens?
Answer
The first focus is the point on the principal axis from which rays start in a convex lens, or towards which rays appear to go in a concave lens, so that after refraction they emerge parallel to the pr…
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