Modern Physics is a very important constituent of the Physics portion of the IIT JEE mains and advanced and NEET. It is interesting as well as usually fetches many questions in the JEE. It includes topics like Nuclear Fission and Fusion which are easy to master. These topics are quite fascinating but involve concepts that must be understood properly.
Modern Physics for IIT JEE refers to the Physics based on the two major branches: relativity and quantum mechanics. Classical Physics refers to the traditional Physics which was based on the concepts before coming up with Modern Physics. There were various theoretical and experimental paradoxes that forced thinking out of the traditional path. Modern physics is generally encountered when dealing with extreme conditions.