Q.If the lower half of the reflecting surface of a concave mirror is covered with an opaque material, what effect will it have on the image formed by the mirror of an object placed in front of the mirror? Write. OR If a ray of light enters obliquely from an optically rarer medium to an optically denser medium, what will be the effect on the velocity and frequency of the light ray? Write.

Partial Mirror Obstruction – First Look

Imagine you are standing in front of a full-length mirror. You see your entire body. Now imagine someone places a large cardboard sheet in front of the mirror, covering the bottom half. You can still see your face and shoulders, but your legs are gone from the reflection. The mirror is partially obstructed — only a portion of it is visible to you.

That is the physical intuition. But in optics problems (especially for JEE/NEET), "partial mirror obstruction" usually refers to a different situation: the mirror itself is partially covered, not the object. A piece of opaque material is stuck on the mirror surface, blocking some of the reflecting area. The question is: does this affect the image you see?

The Surprising Answer

This is a classic exam trap. Students often think: "If I cover half the mirror, I'll see only half the object." That is wrong. Here is why.

Why the Image Remains Full-Sized

Consider a point object $O$ in front of a plane mirror. Light from $O$ spreads in all directions. Some rays hit the mirror, reflect, and enter your eye. Your brain traces these rays back in straight lines to locate the image $I$ behind the mirror.

Now cover the top half of the mirror. Rays that would have hit the top half are blocked. But rays from $O$ that hit the bottom half still reflect and reach your eye. As long as at least one ray from each point on the object reaches your eye after reflection, you see that point in the image. The image location $I$ is determined by geometry (law of reflection), not by how much mirror is available.

So if you cover half the mirror, you still see the entire object — just dimmer, because fewer rays reach your eye.

When Does Obstruction Actually Matter?

Partial obstruction does affect the image in two specific cases:

1. If the mirror becomes too small — If the uncovered portion is so tiny that no ray from some part of the object can reach your eye, that part of the image disappears. This is the field of view problem: you need a minimum mirror size to see the full object from a given eye position.

2. If the mirror is curved — For concave or convex mirrors, covering part of the mirror can change the image because different parts of the mirror contribute differently to image formation. But for a plane mirror, the image is virtual and formed by the entire mirror surface acting together — covering part just reduces light.

The Precise Statement

Common Exam Pitfall …