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Class 10 Light — Reflection and Refraction: cheat sheet
CBSE Science. Reflection by plane and spherical mirrors, refraction and refractive index, lenses, the mirror and lens formulas, and the power of a lens.
Key ideas
- Laws of reflection: the angle of incidence equals the angle of reflection, and both rays and the normal lie in one plane.
- For spherical mirrors, the focal length is half the radius of curvature.
- Sign convention: measure all distances from the pole (or optical centre); along the incident light is positive, against it negative; heights above the axis are positive.
- A concave mirror and a convex lens converge light; a convex mirror and a concave lens spread it out.
- Light bends towards the normal when it slows down entering a denser medium.
- Refractive index compares the speed of light in a vacuum (or air) with its speed in the medium.
- The power of a lens is 1 ÷ f with f in metres; convex lenses have positive power, concave lenses negative.
Remember
| Mirror formula | 1/v + 1/u = 1/f; f = R ÷ 2 |
| Mirror magnification | m = h′ ÷ h = −v ÷ u |
| Lens formula | 1/v − 1/u = 1/f |
| Lens magnification | m = h′ ÷ h = v ÷ u |
| Refractive index | n = c ÷ v = sin i ÷ sin r |
| Power of a lens | P = 1 ÷ f (f in metres), in dioptres (D) |
Watch out
- Forgetting the sign convention: an object in front of a mirror or lens has a negative u.
- Using the mirror formula for a lens (the lens formula has a minus sign).
- Using f in centimetres when finding the power of a lens.
Worked example
Where should an object be placed in front of a convex lens of focal length 9 cm to get a real image the same size as the object?
Working: m = v/u = −1, so v = −u; 1/v − 1/u = 1/9 gives u = −18 cm, v = 18 cm
Answer: 18 cm from the lens (u = −18 cm)
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