I guess we must use c=lambda*frequency...

Actually... no.



is used to find the frequency or wavelength of an electromagnetic wave if either the wavelength or the frequency is known.

Here, I guess that you are looking for the amount of heat that the water gains.

This is given by:



Q is the amount of heat in joules
m is the mass of the substance
c is the specific heat capacity of water (which you must be given)
and theta is the change in temperature in Kelvin.

Can you work this out now? :)