⚡ Quick Revision
Last 24 hours before exam - Everything you need on one page
How to use: Read this page 24 hours before exam. Don't solve new problems. Just revise what you know. Sleep well. Ace the exam.
Formula Dump - All Formulas at a Glance
🪞 Mirrors
f = R/2
m = -v/u = h'/h
P = 1/f (Dioptre)
🔍 Lenses
1/f = (n₂/n₁ - 1)(1/R₁ - 1/R₂)
m = v/u = h'/h
P = P₁ + P₂ (contact)
🌊 Refraction
n = c/v = λ₀/λ
sin θc = 1/n
d_app = d_real / n
🌈 Prism
δ = i + e - A
n = sin[(A+δₘ)/2] / sin(A/2)
δ ≈ A(n-1) [small angle]
🔬 Microscope
Simple: m = D/f (at ∞)
Compound: m = -(L/f₀)(1 + D/fₑ)
Compound: m = -(L/f₀)(D/fₑ) [∞]
🔭 Telescope
L = f₀ + fₑ
R.P. = a / 1.22λ
Sign Convention - Quick Check
| Quantity | Positive (+) | Negative (-) |
|---|---|---|
| u (object) | Never in standard | Always (left of origin) |
| v (image) | Real (mirrors: left; lenses: right) | Virtual (mirrors: right; lenses: left) |
| f (focal length) | Converging: Concave mirror, Convex lens | Diverging: Convex mirror, Concave lens |
| m (magnification) | Erect image | Inverted image |
Critical Concepts - One-Liners
Mirrors
- Concave: Converging, can form real & virtual
- Convex: Diverging, always virtual, erect, diminished
- Plane mirror: v = -u, m = 1, laterally inverted
Lenses
- Convex: Converging, can form real & virtual
- Concave: Diverging, always virtual
- In contact: Powers add (P = P₁ + P₂)
- In liquid: f changes if n_liquid ≠ 1
Refraction & TIR
- TIR: Only denser to rarer, i > θc
- θc = sin⁻¹(1/n) for n > 1
- Frequency constant, speed & λ change
- Apparent depth always < real depth
Prisms & Instruments
- At min deviation: i = e, r₁ = r₂ = A/2
- Dispersion: Violet bends more (nᵥ > nᵣ)
- Microscope: Both lenses short f
- Telescope: Objective long f, eyepiece short f
Top 10 Mistakes - Avoid These!
- Using mirror formula for lens - Mirror: +, Lens: -
- Forgetting negative sign for u - Always u < 0 in standard
- Wrong focal length sign - Concave mirror f < 0, Convex lens f > 0
- Magnification formula confusion - Mirror: m = -v/u, Lens: m = v/u
- TIR in wrong direction - Only denser → rarer
- Mixing up real/virtual image signs - Check for mirrors vs lenses
- Lens power addition - Only for lenses in contact
- Not stating sign convention - Loses marks in CBSE
- Incomplete ray diagrams - Draw at least 2 rays
- Wrong minimum deviation formula - Must have both A and δm
Interactive Flashcards
Click to flip through key concepts (Hover or click card to reveal answer)
Q: Mirror Formula?
1/f = 1/v + 1/u
(Plus sign!)
Q: Lens Formula?
1/f = 1/v - 1/u
(Minus sign!)
Q: Critical Angle?
sin θc = 1/n
(Denser to rarer only)
Q: Telescope m?
m = -f₀/fₑ
(Normal adjustment)
Q: Power of combination?
P = P₁ + P₂
(Contact only!)
Q: Prism min deviation?
n = sin[(A+δm)/2] / sin(A/2)
i = e at min dev
Memory Tricks & Mnemonics
Mirror vs Lens Formula
Trick: "Mirror ADDS light (+), Lens SUBTRACTS from darkness (-)"
Mirror: 1/f = 1/v + 1/u
Lens: 1/f = 1/v - 1/u
Concave vs Convex
ConCAVE = CAVES inward (mirror bulges in, lens is thin at edges)
ConVEX = VEX means VERTEX/POINT outward
Concave mirror = Concave upward = Converging
Convex mirror = Convex outward = Diverging
Real vs Virtual Image
"REAL images are REAL on screen" - Can be projected
"VIRTUAL is just VISION" - Only visible when looking
Sign Convention
"Light goes LEFT to RIGHT, that's POSITIVE"
Object always on left → u always negative
Real image on right (with light) → v positive (lens)
Virtual image on left (against light) → v negative (lens)
Telescope vs Microscope
"Telescope sees FAR, needs FAR-reaching objective (long f)"
"Microscope sees SMALL, needs SMALL focal lengths (both short)"
One-Page Summary
Ray Optics in 100 Words
Light travels in straight lines (ray approximation). Mirrors use reflection (1/f = 1/v + 1/u), lenses use refraction (1/f = 1/v - 1/u). Sign convention is everything - u always negative, v depends on real/virtual. Concave mirrors and convex lenses converge (can form real images), convex mirrors and concave lenses diverge (only virtual). TIR happens denser→rarer when i > θc = sin⁻¹(1/n). Prisms deviate and disperse light. Microscopes use two short focal length lenses, telescopes use one long (objective) and one short (eyepiece). Master sign convention, ace the chapter.
Exam Day Checklist
Before Exam (Morning)
- ☐ Revise formula sheet (10 min)
- ☐ Recall sign convention (5 min)
- ☐ Don't solve new problems
- ☐ Stay calm, you've prepared
- ☐ Eat light breakfast
- ☐ Reach venue 30 min early
During Exam
- ☐ Read question twice
- ☐ Draw diagram first
- ☐ Write sign convention (CBSE)
- ☐ Show all steps
- ☐ Check units & signs
- ☐ Box final answers
Final Words
You've Got This! 💪
Ray Optics is one of the most scoring chapters.
You've learned the concepts.
You've practiced the problems.
You've understood the patterns.
Now go show what you know.
Accuracy > Speed. Understanding > Memory. You > Exam.