The law of reflection states that the angle of incidence equals the

angle of reflection, measured from the normal. The correct answer is angle of reflection, measured from the normal. Both angles are measured from the normal to the mirror, not from the mirror surface itself. Read more: GenSci0051 — Geometric Optics: Mirrors, Reflection and Refraction.

Geometric Optics: Mirrors, Reflection and Refraction

A spoon in a glass of water looks bent. A car’s side mirror makes traffic look farther away than it is. Both tricks come from the same branch of physics: geometric optics, the rules for how light bounces off mirrors and bends through different materials.

A concave mirror’s ray construction: rays from object point A converge at image point A’ after reflecting off the mirror surface. Fffred~commonswiki, Public Domain, via Wikimedia Commons.

MCQ Questions

GenSci0051

0° critical angle

Diamond’s critical angle for light escaping into air — the lowest of any common material, which is why cut diamonds sparkle.

Air

n = 1.00

The reference value; every other refractive index is measured against it

Water

n = 1.33

Critical angle into air: about 48.6°

Crown Glass

n = 1.52

Critical angle into air: about 41.1°

Diamond

n = 2.42

Critical angle into air: about 24.4°

The exam angle: a lower critical angle means light gets trapped inside the material more easily by total internal reflection. That is the entire reason a cut diamond looks brighter than a piece of glass shaped the same way.

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📑 Contents

1Reflection and Mirror Formation
2Refraction and Snell's Law
3Test Yourself
4Total Internal Reflection and Real Instruments
5Exam Point of View

🏛️ Must Know

Reflection and Mirror Formation

Law of Reflection Light hitting a mirror bounces off so the angle of incidence equals the angle of reflection. Both angles are measured from the normal, a line at 90° to the mirror surface.
Concave Mirror A concave mirror curves inward, like the inside of a bowl. It can form a real, inverted image when the object sits beyond its focus.
Convex Mirror A convex mirror curves outward. It always forms a virtual, upright, and smaller image, no matter where the object is placed.
Mirror Formula The mirror formula links object distance u, image distance v, and focal length f: 1/v + 1/u = 1/f. It works for both concave and convex mirrors, with a sign convention for distances.
Magnification Magnification m equals −v/u. A negative m means the image is inverted; a positive m means it is upright.

📘 Good to Know

Refraction and Snell's Law

Refraction Refraction is light bending as it crosses from one transparent material into another. It happens because light travels at different speeds in different materials.
Refractive Index The refractive index n of a material compares the speed of light in a vacuum to its speed inside that material. A higher n means light slows down more, and bends more sharply.
Snell's Law Snell's law states n1 sinθ1 = n2 sinθ2, linking the angles on either side of the boundary to each material's refractive index.
Bending Direction Light bends towards the normal when it enters a denser material. It bends away from the normal when it exits into a less dense one.
Real Numbers Air has a refractive index of about 1.00, water about 1.33, and ordinary crown glass about 1.52.

Test Yourself

🌟 Great to Know

Total Internal Reflection and Real Instruments

Critical Angle The critical angle is the angle of incidence, inside a denser material, at which the refracted ray grazes along the boundary at exactly 90°.
Total Internal Reflection Past the critical angle, light stops refracting out at all. It reflects entirely back inside the denser material. This only happens going from denser to rarer, never the other way.
Diamond's Sparkle Diamond's refractive index of 2.42 gives it a critical angle of only about 24.4°. Almost any light entering a cut diamond bounces around inside it before it can escape, which is why it sparkles so much.
Optical Fibres Optical fibres carry light signals over long distances using repeated total internal reflection along a thin glass core, with almost no loss of light.
Reflection vs Refraction in Instruments A periscope, made of plane mirrors, works purely by reflection. The law of reflection does not depend on the medium between the mirrors. So filling a periscope's tube with a liquid leaves the image unchanged. A lens-based instrument depends on refraction instead, and would be affected.

📝 Exam Point of View

NDA & NA (I) 2026 GAT — Three Mirror and Optics Questions

Question NDA & NA (I) 2026, General Ability Test, Q64: an object beyond a concave mirror's centre of curvature starts accelerating towards the mirror. The correct answer is that the image accelerates towards the centre of curvature.
Why Use the mirror formula to see why. As object distance u shrinks from very large towards the centre of curvature, image distance v grows from the focus towards it too. The two meet exactly at the centre of curvature.
Link See the full question, NDA & NA (I) 2026 GAT, Q64.
Question NDA & NA (I) 2026 GAT Q68: a periscope's tube, using two plane mirrors at 45°, is filled with a liquid of refractive index 1.5. The correct answer is that the image seen stays unchanged.
Why The trap is assuming refraction matters here. A periscope works entirely by reflection, and the law of reflection does not depend on the refractive index of the medium between the mirrors.
Link See the full question, NDA & NA (I) 2026 GAT, Q68.
Question NDA & NA (I) 2026 GAT Q69: a mirror always forms its image between the pole and the focus, whatever the object's position. The correct answer is that it must be a convex mirror.
Why Only a convex mirror keeps its image confined between the pole and the focus for every object position. Its focus and centre of curvature both sit behind the mirror, so no other mirror type does this.
Link See the full question, NDA & NA (I) 2026 GAT, Q69.