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Welcome to Class 10 Physics

Why does a torch make a beam, why does the sky look blue, what flows in a wire, and how does a current push a magnet? Class 10 Physics answers these with a handful of laws you can test at home.

1. Why physics, and why now?

Physics is the part of science that asks how things work and answers with rules precise enough to calculate with. In Class 9 you met motion, force, work, energy and sound: how things move and why. Class 10 turns to two invisible actors that run modern life. The first is light — how it bounces, bends and splits into colours, and how your eye turns it into vision. The second is electricity — the flow of charge in a wire, the heat and power it delivers, and the magnetism it creates.

These four chapters are short, but they are the foundation of a great deal: every mirror, lens, camera, pair of spectacles, phone charger, electric heater, fuse and electromagnet. They are also the starting point for Class 11 and 12 Physics and for the physics in JEE and NEET. The numbers are friendly — fractions, ratios and simple equations — and every idea can be checked with something in your kitchen or school laboratory.

📦 Class 9 ended with…

$$W=F\,s,\qquad P=\frac Wt,$$ $$\text{speed of sound}=\text{frequency}\times\text{wavelength}.$$

🚀 Class 10 begins with…

$$\frac1v+\frac1u=\frac1f,\qquad n=\frac cv,$$ $$V=IR,\qquad P=VI.$$

2. A story that needs the whole course

Pick up a mobile phone. Its camera has a stack of tiny lenses that bend light to form a sharp image on the sensor (Chapter 1). The screen mixes red, green and blue light, and your eye — itself a lens and a screen — blends them into every colour you see (Chapter 2). Charging it pushes an electric current through the battery; the charger's label tells you the potential difference and current, and from those the power and the charging time (Chapter 3). And the speaker makes sound because a current-carrying coil sitting next to a magnet feels a force (Chapter 4).

An older invention needs almost as much physics: the humble torch. A battery drives a current through a hot filament or an LED; a shiny concave bowl behind it turns the spreading light into a beam. Where exactly must the bulb sit? Try the lab below — and notice that the answer depends on the focal length, an idea you will master in Sections 1.1 and 1.2.

Interactive 1 — Design a Torch

A bulb sits on the axis of a concave reflector with focal length $12\ \text{cm}$. Slide the bulb (or press Play). Far from the mirror, the reflected light converges to a spot; inside the focus it spreads out; exactly at the focus $F$ it leaves as a parallel beam — the shape a torch or headlight needs. The reflected rays are computed from the mirror formula you will learn in Section 1.2.

20
bulb distance—
image of the bulb—
the beam is—
✏️ Example 1 — Charging a phone
A phone charger is labelled $5.0\ \text{V},\ 2.0\ \text{A}$, and the battery stores a charge of $4000\ \text{mAh}$ (milliampere-hours). Find (a) the power delivered by the charger, (b) the charge stored in coulombs, (c) the ideal charging time, and (d) the energy stored, in joules and in kilowatt-hours.
(a) Power $P=VI=5.0\times2.0=10\ \text{W}$ (Section 3.4).
(b) Charge $4000\ \text{mAh}=4.0\ \text{A}\times3600\ \text{s}$, and charge $=$ current $\times$ time (Section 3.1): $Q=4.0\times3600=1.44\times10^4\ \text{C}$.
(c) Time $t=\dfrac QI=\dfrac{14400}{2.0}=7200\ \text{s}=2.0\ \text{h}$ (real phones take a little longer because charging slows near full).
(d) Energy $W=VQ=5.0\times14400=7.2\times10^4\ \text{J}$. Since $1\ \text{kWh}=3.6\times10^6\ \text{J}$, that is $0.020\ \text{kWh}$ — a full charge costs a small fraction of one unit of electricity.

3. The four chapters at a glance

The course follows the physics chapters of the current NCERT Class 10 Science syllabus (NCERT chapters 9–12), numbered 1–4 here.

1 · Reflection & refraction→2 · The eye & colour→3 · Electricity→4 · Magnetic effects of current
Chapter 1

Light – Reflection and Refraction

Plane and spherical mirrors, ray diagrams, refraction and refractive index, lenses and their power.

$\dfrac1v+\dfrac1u=\dfrac1f,\quad P=\dfrac1f$
Open chapter → Chapter 2

The Human Eye and the Colourful World

How the eye focuses, defects of vision and their correction, prisms, rainbows, twinkling stars and the blue sky.

VIBGYOR
Open chapter → Chapter 3

Electricity

Charge, current and potential difference, Ohm's law, resistance and resistivity, circuits, heating and power.

$V=IR,\quad R=\rho\dfrac lA$
Open chapter → Chapter 4

Magnetic Effects of Electric Current

Field lines, the field of a wire, loop and solenoid, the force on a current, and safe domestic wiring.

Fleming's left-hand rule
Open chapter →
🗺️ How the chapters connect
  1. Chapter 1 builds the optics toolkit: the laws of reflection and refraction, ray diagrams, and two formulas — one for mirrors, one for lenses — with a single sign convention for both.
  2. Chapter 2 applies it to you. The eye is a lens of adjustable power; short and long sight are corrected with the lens power from Section 1.4. Refraction through a prism and through the air explains rainbows and twinkling stars; scattering explains the colour of the sky.
  3. Chapter 3 starts again with charge. Current, potential difference and resistance lead to Ohm's law, to series and parallel circuits, and to the heating effect that runs heaters and fuses — and to your electricity bill in kWh.
  4. Chapter 4 joins electricity and magnetism. A current makes a magnetic field, and a magnetic field pushes on a current. The chapter ends with how your home is wired safely.

Threads to watch for: one rule applied over and over (draw the normal; follow the current); every number carries a unit and a sign; and every result can be checked — by a ray diagram, by a meter, or in the live labs.

✏️ Example 2 — Two quick previews
(a) The reflector of a torch is a concave mirror with radius of curvature $6.0\ \text{cm}$. Where should the bulb be placed? (b) A $1.5\ \text{kW}$ heater runs for $2.0$ hours a day for $30$ days. At ₹$7$ per unit (kWh), what does it cost?
(a) Focus A parallel beam needs the bulb at the principal focus, and $f=\dfrac R2=3.0\ \text{cm}$ from the mirror (Section 1.1).
(b) Energy Energy $=$ power $\times$ time $=1.5\ \text{kW}\times(2.0\times30)\ \text{h}=90\ \text{kWh}$ (Section 3.4).
(b) Cost $90\times7=$ ₹$630$.
🧭 How to use this course
  • Read with a pen. Try every worked example before opening its solution, then compare step by step. Write the sign convention and the units every time — examiners do check.
  • Play with the labs. Each section has two interactives; drag, slide and predict what will happen before you press ▶ Play.
  • Do the exercises in order. They run from routine to challenging, and every part has a full solution.
  • Practise in the 🎯 Exam corner. Each section ends with original problems in the style of the board exam, NEET and JEE Main, with exam tips.
  • Use the quiz as a gate. Five questions end every section; if you score below 4, reread before moving on.
  • Look up words. Dotted words show their meaning when you tap them. The Glossary and Formula Sheet collect everything in one place.

4. Prerequisite diagnostic — are you ready?

Eight quick questions on the Class 9 science and school mathematics this course uses most. Answer all of them; for each one you miss you will see what to revise and where in this course the skill is needed.

Q1. $\dfrac1{10}+\dfrac1{15}=$
Q2. If $\dfrac1v=\dfrac1{20}-\dfrac1{30}$, then $v=$
Q3. Two angles of a triangle are $90^\circ$ and $35^\circ$. The third angle is
Q4. $25\ \text{cm}$ written in metres is
Q5. $\sin30^\circ=$
Q6. $\dfrac{3.0\times10^8}{2.0\times10^8}=$
Q7. A force of $10\ \text{N}$ moves an object $2.0\ \text{m}$ in $4.0\ \text{s}$. The power is
Q8. Rearranging $V=IR$ to make $R$ the subject gives

When you are ready, start with Chapter 1 — Light: Reflection and Refraction.

← Class 10 Physics — ContentsChapter 1 — Light – Reflection and Refraction →