Minimum distance to hear an echo is 17 metre. If the sound is reflected back on less than 0.1 second, no echo is heard. The effect of sound on ear remains for 0.1 second.

Horse shoe (U shaped ) magnets can tretain magnetism for a long time. Keeper is used with magnet while storing.

Fluorescent tube gets warmed up and the mercury volatises, the discharge produces Ultra violet radiations and it falls on the fluorescent material (Silicates of Zinc and Cadmium) to emit light.