CBSE 2023 · Region 5 · Set 1 · Q34 · 4 marks
A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. Considering image formation by a single spherical surface successively at the two surfaces of a lens, lens maker's formula is obtained. It is useful to design lenses of desired focal length using surfaces of suitable radii of curvature. This formula helps us obtain a relation between $\displaystyle \mathrm{u}, \mathrm{v}$ and f for a lens. Lenses form images of objects and they are used in a number of optical devices, for example microscopes and telescopes.(i)An object AB is kept in front of a composite convex lens, as shown in figure. Will the lens produce one image ? If not, explain.
(ii)A real image of an object formed by a convex lens is observed on a screen. If the screen is removed, will the image still be formed ? Explain.A double convex lens is made of glass of refractive index $\displaystyle 1.55$ with both faces of the same radius of curvature. Find the radius of curvature required if focal length is $\displaystyle 20$ cm .Two convex lenses A and B of focal lengths $\displaystyle 15$ cm and $\displaystyle 10$ cm respectively are placed coaxially ' $\displaystyle d$ ' distance apart. A point object is kept at a distance of $\displaystyle 30$ cm in front of lens A . Find the value of 'd' so that the rays emerging from lens B are parallel to its principal axis.
A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. Considering image formation by a single spherical surface successively at the two surfaces of a lens, lens maker's formula is obtained. It is useful to design lenses of desired focal length using surfaces of suitable radii of curvature. This formula helps us obtain a relation between $\displaystyle \mathrm{u}, \mathrm{v}$ and f for a lens. Lenses form images of objects and they are used in a number of optical devices, for example microscopes and telescopes.
(i)
An object AB is kept in front of a composite convex lens, as shown in figure. Will the lens produce one image ? If not, explain.
(ii)
A real image of an object formed by a convex lens is observed on a screen. If the screen is removed, will the image still be formed ? Explain.
A double convex lens is made of glass of refractive index $\displaystyle 1.55$ with both faces of the same radius of curvature. Find the radius of curvature required if focal length is $\displaystyle 20$ cm .
Two convex lenses A and B of focal lengths $\displaystyle 15$ cm and $\displaystyle 10$ cm respectively are placed coaxially ' $\displaystyle d$ ' distance apart. A point object is kept at a distance of $\displaystyle 30$ cm in front of lens A . Find the value of 'd' so that the rays emerging from lens B are parallel to its principal axis.
Marking-scheme solution
(iii)
For
OR &
(i) No, The lens is made up of two materials of different refractive indices. It has two focal lengths.
(ii)
Yes Rays are still intersecting/ converging at the location of image.
(iii)
\(\displaystyle \frac{1}{f}=(n-1)\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}\right]\)
\[R_{1}=+R, R_{2}=-R
\]
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CBSE Class 12 Physics past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.