Light Reflection and Refraction Questions for CTET PAPER II

Laws of Reflection

When light hits a smooth surface and bounces back, it follows two fixed laws. These laws apply to all types of mirrors — plane, concave, or convex. The normal is an imaginary line drawn perpendicular to the mirror surface at the point where light hits.

• Law 1: The angle of incidence equals the angle of reflection (angle i = angle r).
• Law 2: The incident ray, reflected ray, and normal all lie in the same plane.
• Both angles are measured from the normal, NOT from the mirror surface.
• Plane mirror forms a virtual, erect, laterally inverted image of the same size.
• Image distance = Object distance in a plane mirror (image is as far behind mirror as object is in front).

Spherical Mirrors: Concave and Convex

A spherical mirror is a part of a hollow sphere. Concave mirror curves inward (like a cave — remember: CAVEd in = CONcave). Convex mirror curves outward (bulges out). The principal focus (F) is where parallel rays meet after reflection. For concave mirror F is in front; for convex mirror F is behind the mirror (virtual focus).

• Concave mirror: converging mirror. Used in torches, shaving mirrors, headlights, doctor's head mirror.
• Convex mirror: diverging mirror. Used as rear-view mirrors in vehicles — gives wider field of view.
• Radius of Curvature R = 2 x Focal Length f. So f = R/2.
• Concave mirror can form real+inverted OR virtual+erect images depending on object position.
• Convex mirror ALWAYS forms virtual, erect, diminished images — no matter where the object is.
• Object at F of concave mirror: image forms at infinity (parallel beam — used in torches).

Refraction of Light

When light travels from one medium to another (say air to glass), it changes speed and bends. This bending is called refraction. Light bends toward the normal when entering a denser medium (speed decreases) and away from normal when entering a rarer medium. This is why a pencil in water looks bent.

• Snell's Law: n1 sin(i) = n2 sin(r), where n = refractive index of medium.
• Refractive index n = speed of light in vacuum (c) divided by speed in medium (v). So n = c/v.
• Higher refractive index = optically denser = light slows more = bends more toward normal.
• Real depth vs apparent depth: objects in water appear closer than they are. n = Real depth / Apparent depth.
• Total Internal Reflection occurs when light goes from denser to rarer medium at angle greater than critical angle. Used in optical fibres.

Lenses: Convex and Concave

A convex (converging) lens is thicker in the middle. A concave (diverging) lens is thinner in the middle. Convex lenses are used in magnifying glasses, cameras, and to correct hypermetropia (far-sightedness). Concave lenses are used to correct myopia (near-sightedness). Power of a lens tells how strongly it bends light.

• Convex lens: converging. Can form real+inverted OR virtual+erect images depending on object position.
• Concave lens: ALWAYS forms virtual, erect, diminished images — same behavior as convex mirror.
• Power of lens P = 1/f (f in metres). Unit is Dioptre (D). Convex = positive power; Concave = negative power.
• Lens formula same structure as mirror formula: 1/v - 1/u = 1/f.
• Magnification for lens: m = v/u (note: no negative sign unlike mirror formula).
• Object at 2F of convex lens: image is real, inverted, same size — used in photocopiers.

• Confusing mirror formula sign for magnification (m = -v/u) with lens magnification (m = v/u). The negative sign is ONLY in the mirror version.
• Taking focal length of convex mirror or concave lens as negative — remember convex mirror and concave lens both have positive focal length in real usage context; always apply New Cartesian Sign Convention carefully where distances in direction of incident light are positive.
• Saying convex mirror or concave lens can form real images — they NEVER do. Only concave mirror and convex lens can form real images.
• Mixing up uses: rear-view mirror is CONVEX (not concave). Shaving/makeup mirror is CONCAVE. Getting these swapped in MCQs is a common trap.
• Forgetting that Power is in Dioptres only when f is in metres. If f is given in cm, divide by 100 first.

• CAVEd in = CONcave (concave mirror caves inward). CONvex bulges out like a CONvex belly.
• DAVID rule for concave mirror image positions: Beyond C = real+inverted+diminished; At C = real+inverted+same size; Between C and F = real+inverted+enlarged; At F = infinity; Between F and P = virtual+erect+enlarged.
• 3Cs for Convex Mirror and Concave Lens: they are Converging-opposite types that ALWAYS give Virtual, erect, diminished images — same behavior, just different tools.
• Power mnemonic: Plus Power = Plus lens = convex. Minus Power = Minus (concave) lens. Simple sign match.

• CTET frequently asks application-based questions: which mirror is used in which device. Expect at least one such direct question — memorize the uses table (concave: torch, headlight, doctor mirror; convex: rear-view, security/shop mirrors).
• Numericals in CTET are kept simple — usually one-step application of mirror/lens formula or power calculation. Practice substituting values with sign convention in under 60 seconds.
• Questions on nature of image (real/virtual, erect/inverted, enlarged/diminished) are very common. For each combination of object position and mirror/lens type, know the outcome without calculating.
• Refraction questions at CTET Class 6-8 level focus on concept of bending of light, apparent depth formula, and total internal reflection (optical fibre application) — not heavy Snell's Law numericals.
• Correction of eye defects (myopia uses concave lens, hypermetropia uses convex lens) is a high-frequency crossover question that appears in both Science content and sometimes in CDP-related health topics — do not skip it.