Light Reflection Refraction
Key Concepts
| # | Concept | Explanation |
|---|---|---|
| 1 | Law of Reflection | Angle of incidence (∠i) = Angle of reflection (∠r); incident ray, reflected ray & normal lie in same plane. |
| 2 | Spherical Mirrors | Concave → converges light; Convex → diverges light; pole (P), focus (F), centre of curvature (C), radius (R), focal length (f) = R/2. |
| 3 | Mirror Formula | 1/f = 1/v + 1/u (Cartesian sign: real −ve, virtual +ve); magnification m = –v/u. |
| 4 | Refraction | Bending of light when it passes obliquely from one medium to another; governed by Snell’s law: n₁ sin i = n₂ sin r. |
| 5 | Refractive Index (n) | n = c/v (speed in vacuum ÷ speed in medium); n > 1 for denser medium; n = real depth ÷ apparent depth. |
| 6 | Critical Angle (θc) | sin θc = n₂/n₁ (n₁ > n₂); beyond θc total internal reflection (TIR) occurs—basis of optical fibre. |
| 7 | Lens Formula | Same as mirror formula but f is +ve for convex & −ve for concave; power P (D) = 1/f (metre). |
| 8 | Prism & Dispersion | White light splits into VIBGYOR because n varies with λ (violet deviates most, red least). |
15 Practice MCQs
1. A ray of light strikes a plane mirror at 30° to the mirror surface. The angle of reflection is
A) 30° B) 60° C) 90° D) 0°Answer: B) 60°
Solution: Angle to normal = 90° – 30° = 60° = ∠i ⇒ ∠r = 60°.
Shortcut: Always drop perpendicular; angle with normal decides.
Tag: Law of reflection
2. The radius of curvature of a concave mirror is 40 cm. Its focal length is
A) 40 cm B) 20 cm C) 80 cm D) 10 cmAnswer: B) 20 cm
Solution: f = R/2 = 40/2 = 20 cm.
Shortcut: Remember “f is half R” for mirrors.
Tag: Spherical mirror
3. An object is placed 15 cm in front of a concave mirror of f = 10 cm. The image is
A) virtual & erect B) real & inverted C) real & erect D) virtual & invertedAnswer: B) real & inverted
Solution: u = –15 cm, f = –10 cm ⇒ 1/v = 1/f – 1/u = –1/10 + 1/15 = –1/30 ⇒ v = –30 cm (real).
Shortcut: If |u| > |f| & concave → real.
Tag: Mirror formula
4. Speed of light in medium is 2 × 10⁸ m s⁻¹. Refractive index of medium is
A) 1.0 B) 1.33 C) 1.5 D) 2.0Answer: C) 1.5
Solution: n = c/v = 3×10⁸ / 2×10⁸ = 1.5.
Shortcut: n = 3 ÷ (speed in 10⁸) gives quick value.
Tag: Refractive index
5. A coin at bottom of 6 cm water (n = 4/3) appears at depth
A) 8 cm B) 4.5 cm C) 6 cm D) 3 cmAnswer: B) 4.5 cm
Solution: Apparent depth = real depth / n = 6 / (4/3) = 4.5 cm.
Shortcut: Multiply real depth by 0.75 (≈ 3/4) for water.
Tag: Apparent depth
6. Critical angle for glass-air interface (n_glass = 1.5) is about
A) 30° B) 42° C) 60° D) 90°Answer: B) 42°
Solution: sin θc = 1/1.5 = 0.666 ⇒ θc ≈ 42°.
Shortcut: Rough inverse of n gives angle; 1.5 → ~42°.
Tag: Critical angle
7. Which colour travels slowest in glass?
A) Red B) Yellow C) Violet D) GreenAnswer: C) Violet
Solution: Higher n for shorter λ ⇒ violet bends most & speed = c/n least.
Shortcut: “Violet Viciously Slow”.
Tag: Dispersion
8. Power of a convex lens of f = 25 cm is
A) +4 D B) +2.5 D C) +0.25 D D) –4 DAnswer: A) +4 D
Solution: P = 1/0.25 m = +4 D.
Shortcut: 100 ÷ cm gives D (100/25 = 4).
Tag: Lens power
9. A virtual, erect & magnified image is formed by
A) concave mirror with object beyond C B) convex mirror C) concave mirror with object between F & P D) plane mirrorAnswer: C) concave mirror with object between F & P
Solution: Only this range gives virtual, erect & |m| > 1.
Shortcut: “Inside F → virtual magnified shave mirror”.
Tag: Image characteristics
10. When light goes from air to diamond (n = 2.42) it
A) bends away from normal B) bends towards normal C) does not bend D) reflects totallyAnswer: B) bends towards normal
Solution: n increases ⇒ speed decreases ⇒ bends towards normal.
Shortcut: “Dense → Normal hug”.
Tag: Refraction direction
11. A diver inside water looks at the setting Sun; to him the Sun appears
A) higher than real B) lower than real C) at real position D) invisibleAnswer: A) higher than real
Solution: Light bends away from normal exiting water; diver extrapolates along bent ray → elevated image.
Shortcut: Always “bend away → looks high”.
Tag: Apparent position
12. An object is placed 20 cm from a convex mirror of f = 30 cm. Image distance is
A) –12 cm B) +12 cm C) –7.5 cm D) +7.5 cmAnswer: B) +12 cm
Solution: 1/v = 1/f – 1/u = 1/30 – 1/(–20) = 1/30 + 1/20 = 5/60 ⇒ v = +12 cm (virtual).
Shortcut: For convex mirror v always +ve & < |f|.
Tag: Convex mirror
13. Magnification produced by plane mirror is always
A) –1 B) +1 C) 0 D) ∞Answer: B) +1
Solution: Image size = object size & erect ⇒ m = +1.
Shortcut: “Plane plain +1”.
Tag: Plane mirror
14. Optical fibres work on the principle of
A) scattering B) diffraction C) total internal reflection D) interferenceAnswer: C) total internal reflection
Solution: Light trapped by repeated TIR along fibre.
Shortcut: “TIR → Fibre wire”.
Tag: Total internal reflection
15. A convex lens acts as a simple magnifying glass when object is placed
A) at 2F B) at F C) between F & optical centre D) beyond 2FAnswer: C) between F & optical centre
Solution: Only here image virtual, erect & |m| > 1.
Shortcut: “Inside F → magnify”.
Tag: Magnifying glass
Speed Tricks
| Situation | Shortcut | Example |
|---|---|---|
| Mirror focal length | f = R ⁄ 2 (same unit) | R = 60 cm ⇒ f = 30 cm |
| Apparent depth in water | Multiply real depth by ¾ | 8 cm water → looks 6 cm |
| Power from cm | D = 100 ÷ cm | 20 cm lens → 5 D |
| Critical angle quick | θc ≈ 42° for n = 1.5 | Glass-air |
| Lens / mirror sign | “Real is Negative” | Real image → v −ve, inverted → m −ve |
Quick Revision
| Point | Detail |
|---|---|
| 1 | ∠i = ∠r & all three (incident, reflected, normal) are coplanar. |
| 2 | Concave mirror → converging; can give real/virtual image depending on object position. |
| 3 | Convex mirror always forms virtual, erect, diminished image—used as rear-view. |
| 4 | n = c/v; higher n ⇒ slower light & more bending towards normal. |
| 5 | Real depth > apparent depth; shift = t(1 – 1/n). |
| 6 | TIR possible only when light moves denser → rarer & ∠i > θc. |
| 7 | For lens, convex → +f & +P; concave → –f & –P. |
| 8 | Magnification |
| 9 | Violet has highest n & least speed in glass → deviates most. |
| 10 | Combine lenses: P_total = P₁ + P₂ (in dioptre). |
BETA
