What is an echo? State the conditions for hearing an echo and show that the minimum distance needed is 17.2 m.

1. 1 · Define. An echo is sound heard again after reflection from a large, distant surface.
2. 2 · State the laws of reflection. ∠i = ∠r; the incident sound, the reflected sound and the normal lie in one plane.
3. 3 · Draw. [Diagram: a person, a wall at distance d, arrows to and from the wall, the normal with equal angles i and r marked]
4. 4 · Explain persistence of hearing. The ear keeps a sound for about 0.1 s, so the echo must return at least 0.1 s later.
5. 5 · Derive the minimum distance.
   • The total path is 2d, so v = 2d/t and d = vt/2.
   • d = (344 × 0.1) ÷ 2 = 17.2 m.
6. 6 · List the conditions.
   • Reflector at least 17.2 m away.
   • A large, hard reflecting surface.
   • Sound loud enough to be heard after reflection.
7. 7 · Real-life link. A shout near a rocky hill or in an empty mandapam gives an echo; a small room does not.
8. 8 · Close. So an echo needs time (0.1 s) and distance (17.2 m).

Scoring tip: Always divide by 2 in echo sums: the sound makes a round trip.