Question 6
Describe an experiment to verify the laws of reflection of light.
The laws of reflection of light can be verified by the following experiment:
- Take a drawing board and a white sheet of paper. Fix the white sheet of paper on the drawing board. Draw a straight line MM1 on the paper.
- On the line MM1 take a point O at its middle and draw a line OA such that ∠MOA is less than 90°. Draw a normal ON on MM1 at point O.
- Take a small plane mirror and place along the line MM1. Use plasticine to fix the mirror on the paper so that it stands upright.
- Fix pins P and Q vertically on the line OA. Set your eye on the other side of the normal ON along the direction as shown in below figure.
- You will see images P', Q' of pins P and Q in the mirror.
- Set your eye in line with the two images P', Q' and fix two more pins R and S on the paper in front of the mirror such that both the pins R and S are in line with P', Q'.
- Remove the mirror and draw small circles around pins P, Q, R and S. Draw a line OB joining the points R and S.
- AO is incident ray, OB is reflected ray and ON is normal at the point of incidence O.
- Using a protractor measure angle of incidence i (∠AON), angle of reflection r (∠BON). We will find two angles are equal.
This experiment verifies the two laws of reflection:
- Angle of incidence = Angle of reflection.
- Incident ray, reflected ray and normal all lie in the same plane of paper which is the second law of reflection.
ICSE Class 7 Physics — Energy: Complete Study Guide
Energy is a central concept in ICSE Class 7 Physics, carrying approximately 12 marks. This chapter introduces students to the universal principle that energy drives all processes in nature and technology. From the food we eat to the devices we use, every activity involves energy transformation.
Students learn nine forms of energy: kinetic, potential, heat, light, sound, chemical, electrical, nuclear, and mechanical. The distinction between kinetic energy (energy of motion) and potential energy (stored energy due to position or shape) is fundamental. Understanding energy transformation chains — how a car converts chemical energy in fuel to heat to mechanical motion — helps students see physics at work in everyday life. The law of conservation of energy, which states that energy can neither be created nor destroyed but only transformed, is one of the most important principles in all of science.
The chapter also addresses the crucial real-world topic of energy sources — renewable (solar, wind, hydro) versus non-renewable (coal, oil, gas) — and the importance of energy conservation in the face of growing global demand and environmental concerns.
| Concept | Key Points |
|---|---|
| Kinetic Energy | Energy of motion; depends on mass and speed |
| Potential Energy | Stored energy; gravitational (height) or elastic (stretched/compressed) |
| Conservation of Energy | Total energy remains constant; only changes form |
| SI Unit | Joule (J) |
Must-Know Concepts
- Energy = ability to do work. SI unit: Joule (J)
- KE depends on mass and speed; PE depends on height/mass or stretch/compression
- Energy transformation happens in every device: electrical → light (bulb), chemical → heat (burning)
- In a pendulum: PE is max at extremes, KE is max at centre; total is constant
- Renewable sources can be replenished; non-renewable took millions of years to form
Common Mistakes
- Saying energy is "used up" or "destroyed" — it only changes form
- Forgetting elastic PE — it is not only about height; stretched objects also have PE
- Incomplete transformation chains — always include heat and sound as by-products
- Confusing energy sources (solar, coal) with energy forms (light, chemical)
Exam Tips
- Draw energy transformation chains using arrows for full marks
- Use the pendulum or falling ball as your go-to example for conservation of energy
- For renewable vs non-renewable, give at least 3 examples of each in a table
- When discussing "energy crisis," clarify that it means shortage of useful forms, not destruction of energy
What is the law of conservation of energy?
Energy can neither be created nor destroyed. It can only be transformed from one form to another. The total energy of an isolated system remains constant. For example, in a swinging pendulum, potential energy converts to kinetic energy and back, but the total remains the same.
Why does a bouncing ball not return to its original height?
When a ball bounces, some of its kinetic energy is converted to heat (from deformation) and sound (the bouncing noise). Less energy is available to convert back to potential energy, so the ball reaches a lesser height. Total energy is still conserved — it has just changed to less useful forms.
What is the difference between renewable and non-renewable energy sources?
Renewable sources (solar, wind, hydro, biomass) can be replenished naturally in a short time and will not run out. Non-renewable sources (coal, petroleum, natural gas, nuclear fuel) took millions of years to form, exist in limited quantities, and will eventually be exhausted.