An object is placed in front of two plane mirrors inclined at 60o to each other. Determine the number of the images formed

Analyze the question: An object sits between two plane mirrors that are inclined at an angle θ = 60°. We need the number of images formed.

Concept: For two inclined plane mirrors, the number of images is given by

n = (360° / θ) − 1

This formula works when 360°/θ is an even number (the object can be anywhere between the mirrors). When 360°/θ is an odd number, the formula still holds if the object is placed on the bisector. In most exam questions, we just apply the formula.

Solution:

1. θ = 60°
2. 360° / 60° = 6
3. n = 6 − 1 = 5

So 5 images are formed.

Check: The images lie on a circle centered at the point where the mirrors meet. Together with the object, there are 6 equally spaced positions around the circle (360° ÷ 60° = 6), so 5 of them are images.

Answer: Option C (5).

Exam tip: Remember the pattern "360 over θ, minus 1."

• 90° gives 3 images
• 60° gives 5 images
• 45° gives 7 images
• 30° gives 11 images

Also, as θ gets smaller, more images form. Parallel mirrors (θ = 0) give infinitely many images.