Class 10 Light: Reflection and Refraction Notes | Class 10 Science

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

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

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)

Exam Tip: Virtual means the image cannot be caught on a screen. A plane mirror image is always virtual and erect.

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

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

$m = \frac{-v}{u} = \frac{h'}{h}$

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)

1. Incident ray, refracted ray and normal lie in the same plane.
2. For a given pair of media:
   $n = \frac{\sin i}{\sin r}$

What is Refractive Index?

$n = \frac{c}{v} = \frac{3.0 \times 10^8}{v}$

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

Lens Formula

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

Magnification for Lenses

$m = \frac{v}{u}$

Important: Mirror formula uses addition, while lens formula uses subtraction.

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

Board Exam Tip: Memorize the sign convention, mirror formula, lens formula, magnification formula, refractive index formula, and power formula. These account for most numerical questions asked in this chapter.