The technique or method used to absorb undesirable sounds by soft and porous surface is called:

At room temperature, electrons cannot escape the metal surface due to ………….. of atomic nucleus:

NOT gate is also called:

The 1st practical electric battery is known as:

Sound can travel only in presence of

Characteristic by which We can distinguishbetween two sounds of same loudness and pitch is called

Loudness of sound depends on

Frequency of silent whistle Lies between ___________

Optical centre is represented by

For a concave lens, the parallel rays appear to come from a. point behind the lens is called;

In lens number ·of curved surfaces at least;

Lenses are used in optical devices;