Question

define focus, why is the tungsten used almost exclusively for the filament of electric lamps?

Answer 1

Tungsten is used as :-
1) High resistance so more light
2) High melting point so it doesnt melt due to heat produced

Answer 2

For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points—one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the focal length.
Elliptical mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other,
This is called focus.
ure tungsten has some amazing properties including the high melting point , lowest vapor pressure, and greatest tensile strength out of all the metals.Because of these properties it is the most commonly used material for light bulb filaments.

Hope it helps...
plz refer the image....