Light always obeys the same two rules when it bounces off a mirror — and the human eye is really just a very sophisticated optical instrument that can fail in a couple of predictable ways.
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Every reflection, from a plane mirror to a curved one, obeys exactly the same two laws.
📌 The two laws of reflection
Law 1: the angle of incidence always EQUALS the angle of reflection.
Law 2: the incident ray, the reflected ray and the normal all lie in the same plane.
A plane mirror image is always virtual, erect, laterally inverted, and the same size and distance behind the mirror as the object is in front.
Angles are measured from the NORMAL (the line perpendicular to the mirror), never from the mirror's surface itself. This is the single most common mistake in reflection questions.
When two mirrors face each other at an angle, they can create MORE than one reflection of the same object.
📊 Number of images from two mirrors at angle θ
| Angle between mirrors | Number of images |
|---|---|
| 90° | 3 images |
| 60° | 5 images |
| 45° | 7 images |
| 0° (parallel mirrors) | Infinite images |
The formula: Number of images = (360° ÷ angle) − 1, when 360° ÷ angle gives a whole number. This is why a kaleidoscope, with mirrors at a small angle, creates so many repeated patterns.
The human eye is an optical instrument, and its two most common defects each have a matching type of corrective lens.
🏆 Eye defects and their correction
| Defect | Problem | Corrected with |
|---|---|---|
| Myopia (near-sightedness) | Distant objects appear blurred; image forms in front of the retina | A concave (diverging) lens |
| Hypermetropia (far-sightedness) | Near objects appear blurred; image forms behind the retina | A convex (converging) lens |
Pro tip: The eye lens changes its own shape to focus on near or far objects — this ability is called accommodation. It naturally weakens with age, which is a separate condition called presbyopia.
12 NSO-pattern questions on light and optical instruments, including two Achievers-level questions.
