Light – Reflection and Refraction is the highest-scoring physics chapter of Class 10 Science: ray diagrams, one numerical and one sign-convention question appear nearly every year. Here are the notes you need.
Reflection and spherical mirrors
Laws of reflection: angle of incidence = angle of reflection; incident ray, reflected ray and normal lie in one plane. Key terms: pole (P), centre of curvature (C), focus (F), radius R = 2f.
Rules for drawing ray diagrams (mirrors)
- A ray parallel to the principal axis passes through F after reflection (concave) or appears to come from F (convex).
- A ray through F becomes parallel to the axis.
- A ray through C returns along the same path.
Image by a concave mirror (learn this table)
| Object position | Image position | Nature |
|---|---|---|
| At infinity | At F | Real, inverted, point-size |
| Beyond C | Between F and C | Real, inverted, small |
| At C | At C | Real, inverted, same size |
| Between C and F | Beyond C | Real, inverted, enlarged |
| At F | Infinity | Real, inverted, highly enlarged |
| Between P and F | Behind mirror | Virtual, erect, enlarged |
A convex mirror always gives a virtual, erect, diminished image behind the mirror — that is why it is used as a rear-view mirror.
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Mirror formula and magnification
1/v + 1/u = 1/f; magnification m = h′/h = −v/u. Sign convention: distances measured from the pole; left of mirror negative; heights above axis positive. So u is always negative, f is negative for concave and positive for convex.
Numerical: Object 10 cm in front of a concave mirror of focal length 15 cm. u = −10, f = −15. 1/v = 1/f − 1/u = −1/15 + 1/10 = 1/30 → v = +30 cm (behind mirror, virtual). m = −30/(−10) = +3 (erect, 3 times enlarged).
Refraction
Bending of light when it enters another medium. Snell’s law: sin i / sin r = constant (n). Refractive index n = c/v; n21 = n2/n1. Light bends towards the normal when entering a denser medium.
Lenses
Lens formula 1/v − 1/u = 1/f; m = v/u. Power P = 1/f (f in metres), unit dioptre (D). Convex lens: f positive, converges. Concave lens: f negative, always gives virtual, erect, small image.
Numerical: A lens has power −2.5 D. f = 1/P = −0.4 m = −40 cm → concave lens.
Common mistakes
- Mixing the mirror and lens formula (+ vs −).
- Forgetting u is negative.
- Drawing arrows on the wrong side of the ray.
Related: Motion Class 9 notes for the graph skills used in physics.
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Frequently asked questions
What is the difference between mirror formula and lens formula?
Mirror: 1/v + 1/u = 1/f. Lens: 1/v − 1/u = 1/f. Magnification is −v/u for mirrors and v/u for lenses.
Why is a convex mirror used in vehicles?
It always forms a small, erect, virtual image and gives a wide field of view.
What is 1 dioptre?
The power of a lens whose focal length is 1 metre.
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