Light, Reflection and Refraction: Class 10 short notes
Light is the chapter with the most numericals in Class 10 Science. Nearly all of them use two formulas and one sign convention. Learn those, plus the image table for the concave mirror, and most of the chapter is done.
Short notes
- Laws of reflection: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal all lie in one plane.
- A plane mirror forms a virtual, erect image of the same size, as far behind the mirror as the object is in front, laterally inverted.
- A concave mirror converges light; a convex mirror diverges it. The radius of curvature is twice the focal length: R = 2f.
- Sign convention: measure from the pole (or optical centre). Distances in the direction of incident light are positive, against it negative. Heights above the axis are positive.
- Mirror formula: 1/v + 1/u = 1/f. Magnification m = h′/h = −v/u.
- A convex mirror always forms a virtual, erect, diminished image, so it's used as a rear-view mirror: it shows a wider view.
- Concave mirrors are used in torches and headlights, as shaving and dentists' mirrors, and in solar furnaces.
- Refraction: light bends when it passes from one medium to another because its speed changes. Going into a denser medium, it bends towards the normal.
- Snell's law: sin i / sin r is constant for a pair of media; this constant is the refractive index.
- Refractive index n = speed of light in vacuum / speed in the medium. Water is about 1.33, glass about 1.5, diamond 2.42.
- Lens formula: 1/v − 1/u = 1/f. Magnification m = h′/h = v/u.
- Power of a lens P = 1/f, with f in metres; the unit is the dioptre (D). A convex lens has positive power, a concave lens negative.
At a glance
| Object (concave mirror) | Image position | Image |
|---|---|---|
| At infinity | At F | Real, inverted, highly diminished |
| 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 |
Memory tricks
Mirror adds, lens subtracts
Mirror: 1/v + 1/u = 1/f. Lens: 1/v − 1/u = 1/f. And the minus sign moves too: mirror m = −v/u, lens m = v/u.
Only one virtual image for a concave mirror
Only when the object is closer than F (between P and F) is the image virtual and erect. That's why it works as a shaving mirror.
Power in metres
Always turn f into metres before P = 1/f. f = 25 cm means 0.25 m, so P = +4 D for a convex lens.
Practice questions
An object is placed at the centre of curvature of a concave mirror. The image is:
Show answer
At C, same size. At C, a concave mirror forms a real, inverted image of the same size, also at C.
A convex lens has a focal length of 50 cm. Its power is:
Show answer
+2 D. P = 1/f = 1/0.5 m = +2 D, positive for a convex lens.
Why is a convex mirror used as a rear-view mirror?
Show answer
It gives a wider field of view. It always forms erect, diminished images, so it shows a wider area behind the vehicle.
Light travels from air into glass. It:
Show answer
Bends towards the normal. Glass is optically denser, so light slows down and bends towards the normal.
Questions
How do I avoid sign mistakes in numericals?
Draw the axis first, put the object on the left, and give every distance a sign before using the formula. For mirrors and lenses, u is almost always negative.
Which ray diagrams should I practise?
All six object positions for a concave mirror, the convex mirror, and the main positions for convex and concave lenses.
Can TellBrief explain the numericals in my own physics chapter?
Yes. Add the chapter PDF or photos and choose CBSE once; you can then ask about any step in Hindi or English, and the answer points to the page.