What do you understand by an echo ?

Explanation

### Step-by-Step Text Solution:

1. **Definition of Echo**: An echo is a sound wave that is reflected back to the listener after it strikes a surface.

2. **Nature of Sound Waves**: Sound travels in waves. When a sound is produced, it creates sound waves that move through the air.

3. **Reflection of Sound Waves**: When these sound waves encounter a solid surface, such as a wall, they bounce back. This bouncing back of sound waves is known as reflection.

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