The focus (F) of a concave mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, meet (converge) after reflection from the mirror.
The focus (F) of a convex mirror is a point on the principal axis at which the light rays incident parallel to the principal axis, appear to meet after reflection from the mirror.
The distance of the focus from the pole of the mirror is called the focal length of the mirror.
Chapter Overview: Energy
Energy is the capacity to do work, measured in joules (J). It exists in various forms: kinetic (motion), potential (position), heat, light, sound, chemical (bonds), electrical (current), and nuclear (atomic nucleus). Energy can be transformed from one form to another — for example, an electric bulb converts electrical energy to light and heat, a solar panel converts light to electrical energy, and our body converts chemical energy (food) to kinetic and heat energy. The law of conservation of energy states that energy cannot be created or destroyed, only transformed; total energy remains constant. Energy sources are classified as renewable (solar, wind, hydro, geothermal, biomass, tidal — replenished naturally, low pollution) and non-renewable (coal, petroleum, natural gas, nuclear fuels — exhaustible, cause pollution). The growing energy crisis requires conservation: using LED bulbs, public transport, solar heaters, energy-efficient appliances, and reducing wastage.
Key Definitions & Concepts
| Term | Definition / Details |
|---|---|
| Energy | Capacity to do work. SI unit: joule (J) |
| Kinetic Energy | Energy of a moving object |
| Potential Energy | Energy stored due to position or configuration |
| Conservation of Energy | Energy cannot be created or destroyed, only transformed |
| Renewable Source | Energy source replenished naturally: solar, wind, hydro, biomass |
| Non-Renewable Source | Exhaustible source: coal, petroleum, natural gas, nuclear fuels |
| Energy Transformation | Conversion of energy from one form to another |
Must-Know Points for Exams
- Energy = capacity to do work. Unit: joule (J).
- 8 forms: kinetic, potential, heat, light, sound, chemical, electrical, nuclear.
- Law of conservation: total energy of an isolated system is constant.
- Renewable sources: inexhaustible, low pollution. Non-renewable: exhaustible, cause pollution.
- Nuclear fuels (uranium) are NON-renewable despite being very powerful.
- Conserve energy: LED bulbs, public transport, solar heaters, switch off when not in use.
Quick Self-Test
- Define the main concept of this chapter in one sentence.
- List the key types or categories discussed in this chapter.
- Give three real-life examples related to the main concept.
- Draw and label the key diagram of this chapter from memory.
- State the main law or principle covered in this chapter.