Formation of Images by Two Plane Mirrors
(B) Very Short Answer Type
1. What is the most common use of a plane mirror?
Looking glass.
2. How does an optician increase the effective length of his room using mirrors? Where does he place the signboard?
By fixing a plane mirror on one wall and placing the signboard behind the customer to make the room appear larger.
3. What relationship do the images formed by two inclined plane mirrors have with respect to the object placed between them?
The images lie on the circumference of a circle whose centre is the point of intersection of the two mirrors, and whose radius equals the object’s distance from that point.
(C) Short Answer Type
1. Two plane mirrors are placed making an angle θ between them. Write an expression for the number of images formed and state the condition.
Number of images, n = (360°/θ) - 1. If (360°/θ) is even or the object is placed symmetrically, total images = n. If (360°/θ) is odd and object placed asymmetrically, total images = n
2. When the angle between two plane mirrors is gradually increased from 0° to 180°, how does the number of images change?
The number of images decreases as the angle between mirrors increases.
3. State two uses of a plane mirror.
1. Used as a looking glass.
2. Used by barbers, dentists, and opticians for clear images.
3. Used in periscopes and kaleidoscopes.
(D) Long Answer Type
1. How many images are formed when two plane mirrors are kept at right angles (90°) to each other? Show with a ray diagram.
Three images are formed when two plane mirrors are at right angles to each other.
2. Two plane mirrors are arranged parallel and facing each other. How many images are formed for a point object placed between them?
An infinite number of images are formed as light reflects back and forth endlessly between the two mirrors.
(E) Numericals
1. State the number of images of an object placed between the two plane mirrors when the mirrors are inclined at:
(a) 90°, (b) 60°
(a) 3 images
(b) 5 images
2. An object is placed (i) asymmetrically and (ii) symmetrically between two plane mirrors inclined at 50°. Find the number of images formed.
(i) 7 images (asymmetrically placed)
(ii) 6 images (symmetrically placed)
3. For mirrors inclined at 45°, find the number of images formed.
n = (360° / 45°) - 1 = 8 - 1 = 7 images.
