Cathode rays – Introduction and Properties

Cathode rays – Introduction and Properties

Cathode Rays Introduction :

Cathode rays are the invisible rays, emerging normally form the cathode of a discharge tube kept at a presence of (10-2 to 10-3 )mm Hg and under a very high potential difference of the order  (10-15) kV, supplied from an induction coil.


When the gas pressure in a discharge tube is kept around 10-2 to 10-3 mm of Hg, and potential difference of about  10-15 kV is applied between is electrodes by means of an induction coil, then the whole tube is filled with darkness (crook’s dark space) and the wall of the tube facing the cathode is illuminated by fluorescence whole color depends upon the composition of the tube. Due to falling of a particular type of invisible rays on the glass, the fluorescence produces. The rays emerge from the cathode and are called cathode rays.

These cathode rays are independent of the nature of the gas and their propagation is independent of the position of anode.

Cathode Rays
Cathode Rays

Properties of Cathode Rays

Here are the some of their properties:

    • Cathode rays travel in straight line

      They have rectilinear propagation as that of light and always travel in straight path.

    • Cathode rays heat the material they fall on

      If we place a platinum strip at the centre of curvature of a concave cathode, it becomes red hot. It is because the cathode rays have very high kinetic energy due to their high velocity. When they fall on platinum, their heat energy converts into kinetic energy.

    • Cathode rays exert mechanical pressure

      They have high momentum, so that they exert pressure on striking a surface. If we place a light paddle wheel of mica in their path such that rays fall on half part of the wheel, then the wheel rotates which proves that they have momentum. This fact that they possess K.E. or momentum establishes that these rays are the moving particles of mass.

    • Cathode rays can produce physical and chemical changes

      They rays affect the photographic plate and turn the color of lithium chloride into violet.

    • Cathode rays can ionize gases

      If they collide with atoms of gases, they can eject electrons from them.

    • Cathode rays can produce x-rays

      When they fall on hard metals having high melting point (e.g tungsten, platinum, molybdenum etc), they produce x-ray.

    • Cathode rays produce fluorescence

      When they fall on glass, zinc sulphide or barium platinocyanide, these substances emit coloured light. The color depends on the nature of the substance.

    • Cathode rays penetrate through metal foils

      If we place aluminium foil normally in the path of cathode rays, they can penetrate through it and rays emerge from other side.

    • Cathode rays deflect in electric and magnetic field

      If we keep two plates parallel to the path of cathode rays, inside or outside the tube, and apply potential difference between them, the rays deflect towards the positive plate.

      Similarly, When we bring one end of the bar magnet near the discharge tube, the cathode rays deflect from their path. The polarity of end of magnet determines the direction of their deflection.

    • Cathode rays carry negative charge

      The direction of their deflection in electric and magnetic field show that they are bunch of negatively charged moving particles. These particles are ‘electron’.

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