Light – Reflection and Refraction — RBSE Class 10 (Science)

A spoon shows you upside-down on one side and upright on the other; a straw looks bent in a glass of water; a magnifying glass gathers sunlight to a burning point. Light bends and bounces by exact rules, and once you know them you can predict every image — its position, size and nature. This is the most formula-and-diagram-driven chapter in Class 10 physics.

1. Reflection at spherical mirrors

A concave mirror curves inward (converging); a convex mirror curves outward (diverging). Key terms: pole (P), centre of curvature (C), focus (F), radius R and focal length f, with $f=\frac{R}{2}.$

Rules for ray diagrams (concave/convex):

• A ray parallel to the axis passes through (or appears from) F.
• A ray through F becomes parallel to the axis.
• A ray through C returns along itself.

Concave images change with object position (real/inverted, or virtual/erect when the object is between P and F). A convex mirror always gives a virtual, erect, diminished image → wide field of view (vehicle rear-view mirrors).

2. The mirror formula, magnification and sign convention

$\frac{1}{v}+\frac{1}{u}=\frac{1}{f},m=\frac{h^{\prime }}{h}=-\frac{v}{u}$

New Cartesian sign convention: distances measured from the pole; along the incident-light direction are +, opposite are −. Heights above the axis +, below −. Consequences: concave $f$ is −, convex $f$ is +. A negative m means a real, inverted image; positive m a virtual, erect image.

3. Refraction of light

Light bends when it passes between media of different optical density. Going into a denser medium it bends towards the normal; into a rarer medium, away from the normal.

Snell's law: $\frac{\sin i}{\sin r}=n_{21}$ (a constant). The refractive index of a medium is $n=\frac{\text{speed of light in vacuum }c}{\text{speed in the medium }v}.$ A higher $n$ means light travels slower and bends more (e.g. diamond $n\approx 2.42$).

4. Refraction through lenses

A convex (converging) lens is thick in the middle; a concave (diverging) lens is thin in the middle. Terms: optical centre O, principal focus F, focal length f.

Rules for ray diagrams:

• A ray parallel to the axis passes through F (convex) / appears from F (concave).
• A ray through O goes straight, undeviated.
• A ray through F emerges parallel to the axis.

A convex lens gives real images for distant objects and a magnified virtual image when the object is within f (a magnifying glass). A concave lens always gives a virtual, erect, diminished image.

5. Lens formula, magnification and power

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f},m=\frac{h^{\prime }}{h}=\frac{v}{u}$

The power of a lens measures its converging/diverging ability: $P=\frac{1}{f(\text{in metres})}\text{(unit: dioptre, D)}.$ Convex lens power is +, concave −. For lenses in contact, powers add: $P=P_1+P_2$.

6. Worked idea

An object is 10 cm in front of a concave mirror of focal length 15 cm. Where is the image? Sign convention: $u=-10$, $f=-15$. $\frac{1}{v}=\frac{1}{f}-\frac{1}{u}=-\frac{1}{15}+\frac{1}{10}=\frac{1}{30}$, so $v=+30$ cm — a virtual, erect, magnified image behind the mirror ($m=-v/u=+3$).

7. Closing thought

Two formula-pairs (mirror & lens) plus a strict sign convention solve every numerical; three ray rules draw every image. Learn concave-mirror image cases, remember convex mirror/concave lens always give small virtual images, and treat power in dioptres. In the RBSE board this chapter is a heavyweight — expect a ray diagram, a numerical, and MCQs worth 6–8 marks.