In a Carnot engine, the absolute temperature of the source is $25 \%$ more than the absolute temperature of the sink. The efficiency of the engine is

The molar specific heat capacity of a diatomic gas at constant pressure is $C$. The molar specific heat capadtr of a monoatomic gas at constant volume is

Two stretched strings $A$ and $B$ when vibrated together produce 4 beats per second. If the tension applied to the string $A$ increased. The number of beats produced per second is increased to 7. If the frequency of string $B$ is 480 Hz initially, the frequency of string $A$ is

The focal length of a thin converging lens in air is 20 cm . When thefens is immersed in a liquid, it behaves like a concave lens of power 1 D . If the refractive index of the material of the lens is 1.5 . The refractive index of the liquid is