PhysicsHigh Weightage ★★★★Class 12
Optics
Reflection, refraction, lenses, mirrors, and wave optics (interference, diffraction) — expect 4–5 EAPCET questions across ray and wave optics.
4–5Questions in EAPCET
~4%Paper Weightage
12Core Formulas
4Mistake Traps
Concept Core
Mirrors, lenses, Snell's law, and Young's double slit — the complete optics framework.
Mirror Formula & Sign Convention
All distances measured from the pole of the mirror/centre of lens. Distances in direction of incident light are positive (+). Against incident light are negative (−).
Mirror formula: 1/v + 1/u = 1/f
f = R/2 (f = focal length; R = radius of curvature)
Magnification: m = −v/u
Concave mirror: f is negative (converging). Convex mirror: f is positive (diverging).
Refraction & Snell's Law
n₁ sinθ₁ = n₂ sinθ₂ (Snell's Law)
n = c/v = speed of light in vacuum / speed in medium
Total internal reflection: when light goes from dense to rare medium at angle ≥ critical angle θ_c. sin θ_c = n₂/n₁ (where n₁ > n₂)
Lens Formula & Power
Lens formula: 1/v − 1/u = 1/f
Lens maker's formula: 1/f = (n−1)(1/R₁ − 1/R₂)
Power: P = 1/f (in metres) Unit: Dioptre (D)
m = v/u (no minus sign for lens)
Lenses in contact: P = P₁ + P₂ or 1/f = 1/f₁ + 1/f₂
Young's Double Slit Experiment (YDSE)
Fringe width β = λD/d
Bright fringes: path diff = nλ (n = 0,1,2...)
Dark fringes: path diff = (2n−1)λ/2
d = slit separation, D = screen distance, λ = wavelength. Fringe width increases with larger D or λ, smaller d.
Single Slit Diffraction
Central maximum has double width of secondary maxima:
First dark fringe: a sinθ = λ → θ ≈ λ/a
Central max width (angular): 2λ/a
Smaller slit (a) → broader diffraction pattern. Slit width a ≫ λ → geometric optics (negligible diffraction).
Optical Instruments — Human Eye
Near point: 25 cm (least distance of distinct vision, D)
Simple microscope (magnifying glass): m = 1 + D/f
Compound microscope: m = (L/f₀) × (D/fₑ) (L = tube length)
Telescope: m = f₀/fₑ (objective focal length / eyepiece focal length)
Formula Vault
All optics formulas — reflection, refraction, lenses, and wave optics.
Mirror Formula
1/v + 1/u = 1/f = 2/R
Sign convention: distances from pole
Mirror Magnification
m = −v/u
Negative = inverted image
Snell's Law
n₁ sinθ₁ = n₂ sinθ₂
n = c/v for each medium
Critical Angle
sin θ_c = n₂/n₁ = 1/n
Total internal reflection above θ_c
Lens Formula
1/v − 1/u = 1/f
Lens: no minus in magnification
Lens Power
P = 1/f (metres) in Dioptres
P₁+P₂ for lenses in contact
YDSE Fringe Width
β = λD/d
d=slit sep; D=screen dist
Bright Fringe Condition
Δ = nλ (n = 0,±1,±2,...)
Path difference = integer multiple of λ
Dark Fringe Condition
Δ = (2n−1)λ/2
Path difference = half-odd multiple of λ
Resolving Power (Telescope)
θ_min = 1.22λ/D
D = aperture diameter; Rayleigh criterion
Worked Examples
5 problems — mirror, lens, Snell's law, YDSE fringe width, and a classic sign-convention trap.
EasyObject 30 cm in front of concave mirror (f=20cm) — find image▾
An object is placed 30 cm in front of a concave mirror of focal length 20 cm. Find image position.
1
Sign convention: u = −30 cm (object in front), f = −20 cm (concave mirror)
2
Mirror formula: 1/v + 1/u = 1/f
3
1/v = 1/f − 1/u = 1/(−20) − 1/(−30) = −1/20 + 1/30 = −3/60 + 2/60 = −1/60
4
v = −60 cm (negative = real image in front of mirror)
✓ Image at 60 cm in front of mirror (real, inverted)
EasyFind critical angle for glass (n = 1.5) to air▾
Find the critical angle for total internal reflection when light goes from glass (n=1.5) to air.
1
sin θ_c = n_air/n_glass = 1/1.5 = 2/3
2
θ_c = sin⁻¹(2/3) ≈ 41.8°
✓ Critical angle = ≈ 41.8°
MediumYDSE: λ=600nm, d=0.1mm, D=1m — find fringe width▾
In YDSE, λ=600 nm, slit separation d=0.1 mm, screen distance D=1 m. Find fringe width.
1
β = λD/d = (600×10⁻⁹ × 1) / (0.1×10⁻³)
2
= 600×10⁻⁹ / 10⁻⁴ = 6×10⁻³ m = 6 mm
✓ Fringe width β = 6 mm
EAPCET LevelCombination of two lenses: f₁=20cm, f₂=−30cm — find effective focal length▾
Two thin lenses of focal lengths 20 cm and −30 cm are in contact. Find the effective focal length and power.
1
1/f = 1/f₁ + 1/f₂ = 1/20 + 1/(−30) = 3/60 − 2/60 = 1/60
3
P = 1/f = 1/0.6 = +1.67 D
✓ f_eff = 60 cm, Power = +1.67 D (converging overall)
Trap QuestionMirror formula vs Lens formula sign differences — where students go wrong▾
An object 15 cm in front of a convex lens (f=10cm). A student uses 1/v+1/u = 1/f (mirror formula). What error occurs?
1
The trap: Mirror formula: 1/v + 1/u = 1/f. Lens formula: 1/v − 1/u = 1/f. The sign in the middle is different.
2
Using mirror formula for lens: 1/v = 1/f − 1/u = 1/10 − 1/(−15) = 1/10 + 1/15 = 5/30 = 1/6 → v = 6 cm ✗
3
Using correct lens formula: 1/v = 1/f + 1/u = 1/10 + 1/(−15) = 3/30 − 2/30 = 1/30 → v = 30 cm ✓
✓ Lens formula: 1/v − 1/u = 1/f; Mirror formula: 1/v + 1/u = 1/f — different sign!
Mistake DNA
4 optics errors from EAPCET distractor analysis.
📐
Using Mirror Formula for Lens (or Vice Versa)
Mirror: 1/v + 1/u = 1/f. Lens: 1/v − 1/u = 1/f. The central sign is different — confusing these is very common.
❌ Wrong
Lens problem:
1/v + 1/u = 1/f ✗
(that's mirror formula!)
✓ Correct
Lens: 1/v − 1/u = 1/f ✓
Mirror: 1/v + 1/u = 1/f ✓
Memory: Lens = L = minus
Memorise: LENS uses MINUS in the middle (1/v − 1/u). MIRROR uses PLUS (1/v + 1/u). This single distinction prevents the most common optics error.
🪞
Sign Convention: Forgetting u is Negative for Object in Front
In Cartesian sign convention, the incident light travels left to right. Object in front of mirror/lens: u is negative.
❌ Wrong
Object 20cm in front:
u = +20 cm ✗
✓ Correct
Incident light travels left
to right. Object in front:
u = −20 cm ✓
(against incident direction)
The sign convention: incident light direction is positive. Object is BEHIND the incoming light, so its distance is negative. Real object → u always negative.
🔵
YDSE: Using d and D Incorrectly
In β = λD/d: D is the distance from slits to screen, d is the slit separation. Students swap these.
❌ Wrong
β = λd/D ✗
(d and D swapped;
larger D should give
bigger fringes)
✓ Correct
β = λD/d ✓
D = screen distance (large)
d = slit separation (small)
Bigger D → wider fringes
Intuition check: moving the screen farther (bigger D) spreads the fringes → β increases with D. Wider slit spacing (bigger d) compresses fringes → β decreases with d.
🌈
Total Internal Reflection: Wrong Direction
TIR occurs when light goes from denser to rarer medium (n₁ > n₂), NOT the reverse.
❌ Wrong
Light from air(n=1) to
glass(n=1.5): TIR possible ✗
(going to denser medium)
✓ Correct
TIR: denser → rarer ✓
Glass(1.5) → air(1.0) ✓
At angle ≥ θ_c = sin⁻¹(n₂/n₁)
TIR requires the light to be going from a denser medium to a rarer one at a sufficiently large angle. The critical angle formula sin θ_c = n₂/n₁ only applies when n₁ > n₂.
Chapter Intelligence
Optics is split into ray optics and wave optics — both are tested in EAPCET.
EAPCET Weightage (2019–2024)
Mirrors and lenses (formula)~9 Refraction and Snell's law~7 YDSE (fringe width, conditions)~6 Total internal reflection~5 High-Yield PYQ Patterns
Image position using mirror/lens formulaCritical angle calculationYDSE fringe width λD/dEffective focal length of combinationMagnification m = −v/u (mirror)Power of lens in dioptresSnell's law angle calculation
Exam Strategy
- Sign convention: always write u as negative for real objects. f negative for concave mirror. Apply formulae with signs included.
- Mirror formula: 1/v + 1/u = 1/f. Lens formula: 1/v − 1/u = 1/f. These are different — don't confuse them.
- YDSE: β = λD/d is the fringe width. Bright = path diff nλ; Dark = (2n−1)λ/2. These appear as direct formula questions every year.
- Power of lens: P = 1/f(metres) in Dioptres. Convex lens = positive P. Concave lens = negative P. Lenses in contact: P_total = P₁ + P₂.
- Optics connects to Wave Physics (interference is a wave phenomenon) and Modern Physics (wave nature of light via photoelectric effect).