A man at the back of a crowd watches a parade by holding a plane mirror just above his head. The parade passes…

The question is poorly worded, so the first step is to interpret the geometry.

Analyze the question

The man holds a plane mirror just above his head, and the mirror is 0.25 m in front of him. The parade is 6 m behind his head. We want the distance from the man to the image of the parade.

Concept

A plane mirror forms a virtual image as far behind the mirror as the object is in front of it (image distance = object distance).

Solution

• The object (parade) is behind the man, and the mirror is 0.25 m in front of the man.
• Distance from the mirror to the parade = 6 m + 0.25 m = 6.25 m.
• The image is 6.25 m behind the mirror, on the side away from the parade, which is in front of the man.
• Distance from the man to the image = 6.25 m + 0.25 m = 6.50 m.

So the image appears 6.50 m from the man.

Check: the mirror is 0.25 m in front of the man, and the image lies 6.25 m beyond the mirror, so the total is 0.25 + 6.25 = 6.50 m.

Answer: A (6.50 m)

Tip: For plane mirror problems, draw a quick line with the object, the mirror and the observer marked on it. Find the object-to-mirror distance first, then place the image the same distance on the other side of the mirror. Then measure from the observer to the image. Do not just reuse the object distance.