A body is said to be opaque to the radiation if (a, r and t are coefficient of absorption, reflection and transmission respectively)

In a thermodynamic system, $$\Delta U$$ represents the increases in its internal energy and dW is the work done by the system then correct statement out of the following is

A combination of two thin lenses in contact have power $$+10 \mathrm{D}$$. The power reduces to $$+6 \mathrm{D}$$ when the lenses are $$0.25 \mathrm{~m}$$ apart. The power of individual lens is

The reciprocal of the total effective resistance of LCR a.c. circuit is called