Reflection of light — laws of reflection and image formation in plane mirror

Question

State the two laws of reflection. Describe the properties of an image formed by a plane mirror.

(NCERT Class 7, Chapter 15 — Light)

Solution — Step by Step

Law 1: The angle of incidence ( $\angle i$ ) is equal to the angle of reflection ( $\angle r$ ).

\angle i = \angle r

Law 2: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

Both angles are measured from the normal (the perpendicular to the mirror surface at the point where light hits), not from the mirror surface itself.

The image in a plane mirror has five important properties:

Virtual — the image cannot be obtained on a screen; it only appears to be behind the mirror
Erect — the image is upright (not inverted top-to-bottom)
Same size — the image is exactly the same size as the object
Laterally inverted — left and right are swapped (your left hand appears as the image’s right hand)
Same distance behind the mirror as the object is in front — if you stand 2 m from the mirror, your image appears 2 m behind it

To draw the image of a point object:

Draw at least two rays from the object to the mirror
Apply the law of reflection ( $\angle i = \angle r$ ) to each ray
Extend the reflected rays backwards (behind the mirror) with dotted lines
Where these dotted lines meet is the position of the virtual image

Why This Works

When light bounces off a smooth, flat surface (like a plane mirror), it follows a predictable rule — the angle going in equals the angle coming out. Our brain traces the reflected rays backwards in straight lines to figure out “where the light came from.” Since the rays appear to come from behind the mirror, we see the image there — even though no light actually exists behind the mirror. That’s why the image is called virtual.

Lateral inversion happens because the mirror reverses the direction perpendicular to its surface (front-back), not left-right. But since we mentally “rotate” our image to face us, the front-back reversal appears as a left-right swap.

Alternative Method — The mirror formula approach

For a plane mirror, the radius of curvature is infinity ( $R = \infty$ ), so the focal length is also infinity. Using the mirror formula $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$ , we get $v = -u$ . The negative sign confirms the image is behind the mirror (virtual), at the same distance as the object.

For board exams, always draw the normal as a dotted line perpendicular to the mirror surface. Mark $\angle i$ and $\angle r$ clearly. The most common reason students lose marks in ray diagrams is not drawing or labelling the normal.

Common Mistake

Students measure the angle of incidence from the mirror surface instead of from the normal. If light hits a mirror at 30° to the surface, the angle of incidence is actually 60° (since $90° - 30° = 60°$ ). Always measure from the normal — this is a frequent trap in MCQs.

Question

Solution — Step by Step

Why This Works

Alternative Method — The mirror formula approach

Common Mistake

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