Class 10 Light: Reflection and Refraction Notes | Class 10 Science
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eSaral > Class 10 Science Notes > Light: Reflection and Refraction
Light is one of the highest-scoring topics in the CBSE Class 10 Science board exam. Chapter 10 covers two connected phenomena — reflection (light bouncing off surfaces) and refraction (light bending as it changes medium) — and builds every concept from first principles. Students who understand the sign convention, master ray diagrams, and can apply the mirror formula and lens formula without hesitation routinely score full marks on this chapter.
These notes follow the exact CBSE syllabus sequence. Each section gives you the definition, the underlying law, worked examples, and a table or diagram reference so you can revise efficiently. For step-by-step solutions to textbook problems, check the NCERT Solutions on eSaral, which are prepared by IIT Bombay faculty AIR-41 rankers who know exactly which question types appear in board papers.
What is Light and How Does It Behave?
Light is electromagnetic radiation that the human eye can detect. It travels as a transverse wave and also behaves as a stream of energy packets called photons — a property known as wave-particle duality.
Key Physical Properties of Light
| Property | Value / Description |
|---|---|
| Speed in a vacuum | $3.0 \times 10^8\ \text{m/s}$ |
| Visible wavelength range | 400 nm (violet) – 700 nm (red) |
| Nature | Electromagnetic (transverse wave) |
| Medium required? | No — travels through vacuum |
| Energy packet | Photon |
What is Rectilinear Propagation of Light?
Rectilinear propagation means light travels in straight lines through a uniform medium. This explains sharp-edged shadows, the working of a pinhole camera, and the formation of solar and lunar eclipses. Light only changes direction in two situations: when it reflects off a surface or refracts as it crosses from one medium to another.
Wave-Particle Duality in Simple Terms
Thomas Young's 1801 double-slit experiment confirmed light's wave nature through interference. Albert Einstein's 1905 explanation of the photoelectric effect confirmed its particle nature. For Class 10, the practical takeaway is that light shows wave behaviour during reflection and refraction, while its photon nature matters more in senior classes and JEE Physics.
What Are the Laws of Reflection?
The two laws of reflection are: (1) the incident ray, reflected ray, and normal at the point of incidence are all in the same plane; and (2) the angle of incidence always equals the angle of reflection ($\angle i = \angle r$). Both angles are measured from the normal to the reflecting surface.
Regular vs. Diffused Reflection
| Feature | Regular Reflection | Diffused Reflection |
|---|---|---|
| Surface type | Smooth, polished | Rough, uneven |
| Reflected rays | Parallel — stay parallel | Parallel — scatter in all directions |
| Image formed | Clear and sharp | No distinct image |
| Examples | Mirror, calm water | Paper, wood, walls |
Both types obey the same two laws. In diffused reflection, each individual ray still follows $\angle i = \angle r$ — but because surface normals point in random directions, the outgoing rays scatter.
Plane Mirror Image Characteristics
- Image is virtual (cannot be projected on a screen)
- Image is erect (upright)
- Image is laterally inverted (left-right flipped)
- Image distance behind the mirror = object distance in front of the mirror
- Image size = object size (magnification = +1)
Principle of Reversibility of Light
If you reverse the direction of a reflected (or refracted) ray, it retraces its original path exactly. This principle is used in designing periscopes, kaleidoscopes, and multi-lens optical instruments.
Spherical Mirrors and Image Formation
Concave vs. Convex Mirrors — Key Differences
| Feature | Concave Mirror | Convex Mirror |
|---|---|---|
| Reflecting surface | Inner curved side | Outer curved side |
| Also called | Converging mirror | Diverging mirror |
| Focus position | In front of a mirror (real) | Behind the mirror (virtual) |
| Image type | Real or virtual | Always virtual, erect, diminished |
| Common uses | Torches, shaving mirrors, solar cookers | Rear-view mirrors |
Important Terms
- Pole (P): Geometric centre of the mirror.
- Centre of Curvature (C): Centre of the sphere.
- Radius of Curvature (R): Distance from P to C.
- Principal Focus (F): Point where rays converge/diverge.
- Focal Length (f): $f = \frac{R}{2}$
Mirror Formula
The mirror formula is:
$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$
where $u$ is object distance, $v$ is image distance and $f$ is focal length.
New Cartesian Sign Convention
- All distances measured from the pole (P)
- Direction of incident light = positive
- Opposite direction = negative
- Concave mirror: $f$ negative
- Convex mirror: $f$ positive
Magnification Formula
$m = \frac{-v}{u} = \frac{h'}{h}$
| Magnification Value | Meaning |
|---|---|
| $m > 0$ | Virtual, erect image |
| $m < 0$ | Real, inverted image |
| $|m| > 1$ | Enlarged image |
| $|m| < 1$ | Diminished image |
| $|m| = 1$ | Same size image |
How Does Refraction of Light Work?
Refraction is the bending of light when it passes from one transparent medium to another. This bending occurs because light changes speed at the boundary between media.
Laws of Refraction (Snell's Law)
- Incident ray, refracted ray and normal lie in the same plane.
- For a given pair of media:
$n = \frac{\sin i}{\sin r}$
What is Refractive Index?
$n = \frac{c}{v} = \frac{3.0 \times 10^8}{v}$
| Medium | Approximate Refractive Index |
|---|---|
| Vacuum / Air | 1.0 |
| Water | 1.33 |
| Glass | 1.52 |
| Diamond | 2.42 |
Real-Life Examples of Refraction
- The pencil appears bent in water.
- Stars appear to twinkle.
- The swimming pool appears shallower.
- Mirages on hot roads.
Check NCERT Solutions for Class 11 Physics
Lenses: Types, Image Formation and Power
Convex vs. Concave Lenses
| Feature | Convex Lens | Concave Lens |
|---|---|---|
| Shape | Thicker at the centre | Thinner at the centre |
| Effect | Converges rays | Diverges rays |
| Focus | Real focus | Virtual focus |
| Common Use | Magnifying glass, camera | Myopia correction |
Lens Formula
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
Magnification for Lenses
$m = \frac{v}{u}$
Power of a Lens
Power is the reciprocal of focal length:
$P = \frac{1}{f(\text{in metres})}$
Unit: Dioptre (D)
For lenses in contact:
$P_{total} = P_1 + P_2$
Check NCERT Solutions for Class 12 Physics
Frequently Asked Questions
Find answers to common questions.
What is refractive index and how is it calculated?
Refractive index (n) measures how much a medium reduces the speed of light compared to a vacuum. It is calculated as n = c/v, where c = 3.0 × 10⁸ m/s (speed in vacuum) and v is the speed in the medium. Alternatively, n = sin i/sin r using Snell's law. Diamond has a high refractive index of 2.42, which causes its brilliant sparkle.
What is the difference between a concave and a convex mirror?
A concave mirror has its reflective surface on the inner curved side and converges parallel light rays to a real focus in front of the mirror. A convex mirror has its reflective surface on the outer curved side and diverges rays, forming only virtual, erect, and diminished images. Rear-view mirrors in vehicles are convex; torch reflectors are concave
What are the two laws of reflection?
The first law states that the incident ray, reflected ray, and normal at the point of incidence lie in the same plane. The second law states that the angle of incidence equals the angle of reflection (∠i = ∠r). Both laws hold for every type of reflecting surface — flat, concave, or convex.
What is the mirror formula in Class 10?
The mirror formula is 1/v + 1/u = 1/f, where v is the image distance, u is the object distance, and f is the focal length — all measured from the pole of the mirror using the New Cartesian Sign Convention. This formula applies to both concave and convex mirrors, with sign differences based on the convention
What is the relationship between focal length and radius of curvature?
For any spherical mirror, the focal length (f) is exactly half the radius of curvature (R): f = R/2. This relationship is derived by applying the laws of reflection to a ray parallel to the principal axis. In numericals, if you are given R = 20 cm, the focal length is immediately f = 10 cm.
Why does a pencil appear bent in water?
A pencil appears bent because light rays from the submerged part travel from water (denser medium, n = 1.33) to air (rarer medium, n = 1.0). At the water-air boundary, these rays bend away from the normal. Your eye traces them back as straight lines, making the pencil appear displaced upward — a direct result of refraction.
What is the power of a lens?
Power of a lens is the reciprocal of its focal length measured in metres: P = 1/f. The unit is the dioptre (D). A convex lens has positive power, a concave lens has negative power. An optometrist's prescription of −2.5 D means a concave lens with focal length −0.4 m is needed to correct short-sightedness