Focus is a point on the principal axis where the incident rays parallel to the principal axis meet after reflection. Highly diminished image is formed at focus if the object is
placed at infinity.
Why is echo not heard in our living rooms? To hear echo, the minimum distance between the source and the reflector must be 17m. When this distance is more than 17m, the reflected sound can be distinguished from the original. As the length of our rooms is less than 17m, we don't hear echo.
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