Sometimes, in deserts, an inverted image of a tree is seen which gives a false impression of water under the tree. This is called a mirage.
Mirage is formed due to the refraction of light. In a desert, during the day-time, the hot sand rapidly heats the layers of air in contact with it. As a result, the layers of air near the ground are rarer and the successive upper layers are denser than those below them.
The light rays from sun after reflection from the top of a tree travel from denser (cold upper layers of air) to rarer medium (warmer lower layer of air) and undergo refraction bending away from the normal. On successive refraction, the angle of refraction increases and the angle of incidence of ray going from denser to rarer medium also increases till a stage is reached when the angle of refraction becomes 90°. On further increase in angle of incidence the light ray is not refracted but is reflected. This reflected ray now travels from rarer to denser layer and bends towards the normal at each refraction. On reaching the eye of the observer, an inverted image of the tree is seen. Thus, it gives a false impression of a pool of water in front of the tree.
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.