Question
Things to keep in mind to get the image formed by the lens.
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Answer
→To find the image of an object by a lens, we can, in principal, take any two rays emanating from a point on an object, trace their paths using the laws of refraction and find the point where the refracted rays meet (or appear to meet). In practice, however, it is convenient to choose any two of the following rays:
(i) A ray emanating from the object parallel to the principle axis of the lens after refraction passes through the second principal focus (in a convex lens) or appears to diverge (in a concave lens) from the first principal focus.
(ii) A ray of light, passing through the optical centre of the lens, emerges without any deviation after refraction.
(iii) A ray of light passing through the first principal focus of a convex lens (or appearing to meet principal axis at second focus point of a concave lens) emerges parallel to the principal axis after refraction.
→Figure (a) and (b) illustrate these rules for a convex and a concave lens.
(i) A ray emanating from the object parallel to the principle axis of the lens after refraction passes through the second principal focus (in a convex lens) or appears to diverge (in a concave lens) from the first principal focus.
(ii) A ray of light, passing through the optical centre of the lens, emerges without any deviation after refraction.
(iii) A ray of light passing through the first principal focus of a convex lens (or appearing to meet principal axis at second focus point of a concave lens) emerges parallel to the principal axis after refraction.
→Figure (a) and (b) illustrate these rules for a convex and a concave lens.
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A convex lens made of a material of refractive index $n_1$ is kept in a medium of refractive index $n_2$. Parallel rays of light are incident on the lens. Complete the path of rays of light emerging from the convex lens if : $(i)\ n_1 > n_2 \ (ii)\ n_1 = n_2 \ (iii)\ n_1< n_2.
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