Two plane mirrors are inclined at an angle 20° to each other. Determine the number of images formed when an object is placed between them.

Analyze the question: Two plane mirrors are inclined at 20°, and we need the number of images of an object placed between them.

Concept: For two plane mirrors inclined at angle θ, the number of images is

n = (360° / θ) − 1

This applies when 360/θ is an even integer, and also when the object lies on the bisector of the mirrors. If 360/θ is an odd integer and the object is not on the bisector, the formula is different, but that does not apply here.

Solution:

n = 360° / 20° − 1 n = 18 − 1 n = 17

Since 360/20 = 18 is an even integer, the formula gives exactly 17 images whatever the position of the object between the mirrors.

Answer: 17 images, which is option A.

Tip: Always divide 360 by the angle first, then subtract 1. Quick checks:

• 90° gives 3 images
• 60° gives 5 images
• 45° gives 7 images
• 20° gives 17 images

Remember that the answer is always one less than 360/θ, because the object itself is not counted as an image.