Question

Which of the following rays undergoes deviation by a lens?

• A. Ray passing along the principal axis
• B. Ray passing through the optic centre
• C. Ray passing parallel to the principal axis
• D. None of the above

Answer 1

The Correct Option is C

A lens is an optical device made from transparent material with at least one curved surface that refracts light. The deviation of light by a lens depends on the path it takes relative to its optical properties. Let's analyze the options:

1. Ray passing along the principal axis: A ray traveling along the principal axis, which is the straight line passing through the optical center and is perpendicular to the lens surfaces, does not deviate as both surfaces are perpendicular to this path.
2. Ray passing through the optic center: A ray passing through the optic center of the lens continues in the same direction, without deviation, because the lens surfaces are parallel at this point, hence no angular refraction occurs.
3. Ray passing parallel to the principal axis: A ray parallel to the principal axis will deviate after refraction. In a converging lens (convex), this ray will refract towards the focal point on the opposite side of the lens. Thus, it undergoes deviation.
4. None of the above: This option does not apply, as we have identified cases (option 3) where deviation definitely occurs.

Based on our analysis, the correct answer is: Ray passing parallel to the principal axis. This ray undergoes deviation as it refracts due to the lens's curvature, focusing at the focal point.

Answer 2

In the case of a lens, different rays behave differently:

• A ray passing along the principal axis goes straight through without bending, so no deviation.
• A ray passing through the optical centre of the lens travels in a straight line without any deviation.
• A ray passing parallel to the principal axis is bent (or refracted) by the lens and passes through the focus on the opposite side. This ray undergoes deviation.

Therefore, the ray that undergoes deviation is the one that travels parallel to the principal axis.

Final Answer: Ray passing parallel to the principal axis