Question

Out of the five incident rays shown in the figure find the three rays that are obeying the laws of refraction and may be used for locating the position of image formed by a convex lens.

A. 1, 2 & 3
B. 2, 3 & 4
C. 3, 4 & 5
D. 1, 2 & 4

Answer 1

Hint: The ray which is incident on the lens parallel to the principal axis gets focused on the focal point of the lens. The ray which is incident on the lens passing through the focal point of the lens becomes parallel to the principal axis after refraction through the lens. The ray of light which is incident on the lens at the optical center of the lens gets refracted through the lens without any deviation.

Complete step by step solution:
According to law of refraction,
${{\eta }_{1}}\sin \left( i \right)={{\eta }_{2}}\sin \left( r \right)$
Where,
${{\eta }_{1}}$ is the refractive index of the medium in which ray of the light is travelling,
${{\eta }_{2}}$ is the refractive index of the material of the lens.
$i$ is the angle of incidence
$r$ is the angle of refraction.

Using the law of refraction, when light travels from the rarer medium to the denser medium then it bends towards the normal to the refracting surface and when it travels from denser medium to the rarer medium then it moves away from the normal.
Ray 1 follows the law of refraction but this ray can’t be used for locating the image formed by the lens, because it is not one of the rays which is used for locating the location of the image.
Ray 2 is incident parallel to the principal axis of the lens and gets focused on the focal point of the lens. It follows the law of refraction and also can be used for locating the position of the image.
Ray 3 is incident on the optical point of the lens and gets refracted without any deviation. It follows the law of refraction and also can be used to locate the position of the image formed.
Ray 4 is incident on the lens passing through the focal point of the lens and becomes parallel to the principal axis of the lens. It follows the law of refraction and also can be used for locating the position of the image.
Ray 5 follows the law of refraction but this ray can’t be used for locating the image formed by the lens, because it is not one of the rays which is used for locating the location of the image.

Hence, the answer is 2, 3 and 4.

Note: The incident ray parallel to the principal axis passes through the focus of the lens after refraction and vice-versa.