Define radiation. Give any two examples where heat is transferred by radiation. What is the name of invisible heat rays which transfer heat by radiation ?

Explanation

### Step-by-Step Solution

1. **Define Radiation**:
- Radiation is the process of transferring heat from one place to another or from one object to another without the need for a medium. This means that heat can travel through a vacuum (empty space) without requiring air or any other substance to carry it.

2. **Examples of Heat Transfer by Radiation**:
- **Example 1**: The heat we feel from the Sun. When we stand outside in sunlight, we can feel warmth on our skin due to the radiation emitted by the Sun. This heat travels through the vacuum of space to reach us.
- **Example 2**: The warmth we feel from a burning piece of wood. When wood is burned, it emits heat that radiates outwards, warming the surrounding area without needing a medium like air to transfer that heat.
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