Cathode rays are streams of electrons emitted from the negatively charged electrode or cathode when an electric discharge takes place in a vaccum tube. They are called cathode rays as they are emitted from the cathode.
To produce the cathode rays, a glass tube fitted with two electrodes at its open end, is used. Electrodes are connected to a D.C. source of high voltage. The electrode which is connected to the positive terminal of the electric source is called anode, and the one connected to the negative terminal is called cathode. The glass tube is connected to a vacuum pump. When the pressure inside the tube falls to about 
mm of mercury and the high voltage supply to the electrodes is switched on, a particular type of rays emanate from the cathode. These are the cathode rays which produce fluorescent effect in the tube. These rays move towards anode. Experiments have proved that the properties of these rays do not depend upon the gas present in the tube. The charge on the electrons and their mass remain the same. These rays have some specific properties as follows:
- These rays travel in straight lines.
- Their direction is always perpendicular to the surface of the cathode.
- They possess mechanical energy so exert pressure.
- When these rays fall on certain substances, they produce a fluorescent effect.
- When these rays hit some substance, the temperature of the material rises.
- They can penetrate through thin metallic foils.
- They can ionize the gases on which they fall.
- The velocity of these rays lies between
to 
of that of the velocity of light. - These rays get deflected by the magnetic field.
- They are also affected by electric fields.
Cathode rays are very useful to us. When they fall on a metal like platinum or tungsten they produce X-rays. X-rays are very useful in science, industry and medical sciences. Cathode ray tube is also used as an indicator in radar systems in which electric signals can be seen on a fluorescent screen.
