MyAiSchool Light — Reflection and Refraction (Ch. 9) sits in the Natural Phenomena unit, 12 marks, and almost always brings one ray diagram and one numerical. Master three things and the chapter is yours: the three drawing rules, the image-position tables for mirrors and lenses, and the sign convention that makes the formulas work. All three are below, with the 12 questions that repeat every year — solved.
Last updated: 13 September 2026
What does the paper actually ask from this chapter?
| Comes almost every year | Marks | What wins the mark |
|---|---|---|
| One ray diagram (mirror or lens, a specific object position) | 2–3 | Arrows on every ray; image drawn where rays actually meet |
| One numerical on the mirror formula, lens formula or power | 2–3 | Sign convention written out before you substitute |
| One reasoning question (“why is a convex mirror used as…”) | 1–2 | Name the property, then the consequence |
| Refractive index / speed of light | 1–2 | State the formula, then substitute with units |
The three rules behind every ray diagram
You never memorise diagrams; you draw any of them from three rays, and you only need two. The full treatment is in Reflection of Light and Spherical Mirrors.
- Parallel ray: a ray parallel to the principal axis passes through the focus after reflection (concave mirror / convex lens) or appears to come from the focus (convex mirror / concave lens).
- Focus ray: a ray through the focus leaves parallel to the principal axis. The reverse of rule 1.
- Centre ray: a ray through the centre of curvature of a mirror comes straight back; a ray through the optical centre of a lens goes straight through, undeviated.
Two habits examiners award marks for: arrows on every ray showing direction, and the image drawn where the two rays — or their backward extensions — actually meet. Use dotted lines for virtual images and for extensions. A diagram with no arrows loses a mark even when the geometry is right.
Concave mirror — the six cases
| Object at | Image at | Nature | Size |
|---|---|---|---|
| Infinity | Focus F | Real, inverted | Point-sized |
| Beyond C | Between F and C | Real, inverted | Diminished |
| At C | At C | Real, inverted | Same size |
| Between C and F | Beyond C | Real, inverted | Enlarged |
| At F | At infinity | Real, inverted | Highly enlarged |
| Between P and F | Behind the mirror | Virtual, erect | Enlarged |
Convex mirror has only one story: the image is always behind the mirror, between P and F — virtual, erect, diminished, whatever the object position. That single fact answers two mark-questions a year.
Lenses — the cases that get asked
| Object at | Image position | Nature and size |
|---|---|---|
| Beyond 2F₁ | Between F₂ and 2F₂ | Real, inverted, diminished |
| At 2F₁ | At 2F₂ | Real, inverted, same size |
| Between F₁ and 2F₁ | Beyond 2F₂ | Real, inverted, enlarged |
| At F₁ | At infinity | Real, inverted, highly enlarged |
| Between F₁ and O | Same side as the object | Virtual, erect, enlarged — the magnifying glass |
Concave lens: always between the optical centre and F₁ on the object’s side — virtual, erect, diminished, for every object position. One rule, no cases to learn.
The formulas — and the sign convention that makes them work
| Quantity | Formula | Watch out for |
|---|---|---|
| Mirror formula | 1/v + 1/u = 1/f | Plus sign — different from the lens formula |
| Lens formula | 1/v − 1/u = 1/f | Minus sign; mixing the two is the classic slip |
| Magnification (mirror) | m = h′/h = −v/u | The leading minus |
| Magnification (lens) | m = h′/h = v/u | No minus here |
| Power of a lens | P = 1/f, f in metres | Convert cm to m first; unit is the dioptre (D) |
| Refractive index | n = c/v | c = 3 × 10⁸ m/s in vacuum |
| Snell’s law | sin i / sin r = n₂₁ | Constant for a given pair of media only |
New Cartesian sign convention, in three lines: measure every distance from the pole (or optical centre); distances in the direction of the incident light are positive, against it negative; heights above the axis are positive. In practice: u is negative for a real object; f is negative for a concave mirror and positive for a convex lens.
The five mistakes that cost marks here
| Mistake | What it costs | The fix |
|---|---|---|
| Dropped minus sign in a numerical | The whole numerical | Write u, v and f with signs on a separate line before substituting |
| Mirror formula used for a lens | 2–3 marks | Mirror adds, lens subtracts. Say it out loud once per question |
| No arrows on the rays | 1 mark from a correct diagram | Arrow every ray as you draw it, not at the end |
| Solid lines for a virtual image | 1 mark | Dotted for virtual images and for backward extensions |
| Focal length left in centimetres for power | The answer, by a factor of 100 | P = 1/f needs f in metres. 25 cm is 0.25 m |
None of these are conceptual failures — they are marks lost while knowing the answer, which makes them the cheapest ones to win back.
Is Light actually done, or does it just feel done? Take the free 15-minute Score Map for Class 10 Science — 20 questions tagged chapter by chapter, and a map showing exactly where the marks are leaking before the mid-terms. No fees, no card.
The 12 most-asked questions, with the answers that score
1. Under what condition does a concave mirror form a virtual image?
When the object is placed between the pole and the focus. The image is virtual, erect and enlarged — the shaving-mirror case.
2. Why is a convex mirror preferred as a rear-view mirror?
It always forms an erect, diminished image and therefore gives a much wider field of view than a plane mirror of the same size.
3. Why is a concave mirror used in torches and vehicle headlights?
A source placed at the focus gives a powerful parallel beam after reflection.
4. An object is placed 10 cm from a concave mirror of focal length 15 cm. Find the image.
u = −10 cm, f = −15 cm → 1/v = 1/f − 1/u = −1/15 + 1/10 = 1/30 → v = +30 cm. The image is 30 cm behind the mirror: virtual, erect, and m = −v/u = +3, so three times enlarged.
5. Light enters from air into glass of refractive index 1.5. What is the speed of light in the glass? (c = 3 × 10⁸ m/s)
v = c/n = 3 × 10⁸ ÷ 1.5 = 2 × 10⁸ m/s.
6. A ray passes through a rectangular glass slab. Why is the emergent ray parallel to the incident ray?
Refraction at the two parallel faces is equal and opposite — the bending at entry is undone at exit, leaving only a sideways (lateral) displacement.
7. Define 1 dioptre of power.
The power of a lens of focal length 1 metre. P = 1/f with f in metres; positive for convex, negative for concave.
8. Find the power of a concave lens of focal length 25 cm.
f = −0.25 m → P = 1/(−0.25) = −4 D.
9. Where should an object be placed so a convex lens forms an image of the same size?
At 2F₁. The image forms at 2F₂ — real, inverted, same size. Draw this one with both rays; it is the most-asked lens diagram.
10. A concave lens always forms what kind of image?
Virtual, erect and diminished, between the optical centre and the focus, on the same side as the object — for every object position.
11. The magnification produced by a mirror is +1. What does it mean?
The image is virtual and erect (positive) and of the same size as the object (magnitude 1) — this is a plane mirror.
12. An object 5 cm high is placed 25 cm from a convex lens of focal length 10 cm. Find the position and size of the image.
u = −25 cm, f = +10 cm → 1/v = 1/f + 1/u = 1/10 − 1/25 = 3/50 → v ≈ +16.7 cm on the far side; m = v/u = −0.67, so the image is real, inverted and about 3.3 cm high.
The 20-minute job for today
- Draw the six concave-mirror cases from memory using the three rules — then check them against the table above.
- Solve questions 4, 8 and 12 on paper, with the sign convention written out before you substitute.
- Work the NCERT in-text and exercise questions for this chapter — that is where the paper comes from.
What to revise after Light
Light feeds directly into the next chapter, and both sit in the same unit:
- The Human Eye and the Colourful World — the same lens rules applied to vision defects. Myopia and hypermetropia numericals reuse P = 1/f exactly.
- Electricity — the heaviest chapter in the Effects of Current unit, and the other big numerical chapter in the paper.
For the wider picture of which chapters carry the marks, see the five chapters that carry 40% of your mid-term paper.
Frequently asked questions
When does a concave mirror form a virtual image?
Only when the object is between the pole and the focus. The image is then virtual, erect and enlarged.
What is the difference between the mirror and lens formulas?
The mirror formula adds the reciprocals (1/v + 1/u = 1/f); the lens formula subtracts (1/v − 1/u = 1/f). Mixing them is the most common numerical error in this chapter.
How many ray diagrams do I need to learn?
None, if you know the three rules. Six concave-mirror cases and five convex-lens cases can all be drawn from them.
What image does a concave lens form?
Always virtual, erect and diminished, between the optical centre and the focus, on the same side as the object.
How many marks is Light worth?
It sits in the Natural Phenomena unit, which carries 12 marks along with The Human Eye and the Colourful World. Confirm against the current CBSE Science curriculum document for your session.
Which is the most-asked numerical from this chapter?
A mirror or lens formula question where the object position makes the image virtual — because that is where the sign convention decides the answer.
Want the next chapter pack the moment it drops? Join the myAiSchool Class 10 WhatsApp channel — one pack, one quiz and one worksheet every week, free. And this Sunday 20 September, 11 AM: the free live masterclass on the five chapters that decide your mid-term marks.