Ans. In the beginning, no one was sure about the nature of cathode-rays. It was J.J. Thomson who carried out many experiments and concluded that cathode-rays are negatively charged electrons. The three reasons to support this evidence are as follows:
1. They are attracted towards positively charged plate.
2. They are deflected in magnetic field opposite to the direction of positive charge.
3. Their charge to mass ratio (e/m) is equal to e/m of electrons.