Page 97 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 85
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At suf ciently low pressure (about 10 atm), this glow lls the whole
tube. The colour of the glow depends upon the nature of the gas in
the tube and on the colour of the glass used for making the discharge
tube.
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ƒ When the pressure is lowered to about 10 atmosphere, light emission
by the residual air in the discharge tube stops. But, the walls of the
discharge tube opposite to the cathode start glowing. At this stage, a
stream called cathode rays is emitted from the cathode.
Thus, we see that when electrical discharge is passed through gases
at very low pressures, cathode rays are produced.
You must have seen bright advertising signboards of different colours
at shopping malls and around your city/town. These signboards consist of
many discharge tubes in which neon gas or a mixture of neon gas with some
other gas is lled at very low pressure. When very high electrical potential
is applied across these tubes, glows of different colours are produced. The
colour of the glow depends upon the nature of the gas in the tube and colour
of the glass used in making the discharge tube. For example, the colours
obtained when neon gas or its mixtures are used are:

Colour of the glass Observed colour
Gas/Gas mixture
of discharge tube of the glow
Neon Colourless Orange-red
Neon Light red Deep-red
Neon + Argon + Hg (v) Colourless Light blue

Neon + Argon + Hg (v) Green Light green
Neon + Argon + Hg (v) Purple Dark blue
Sodium vapour street lamps and television tubes are also based on the
same phenomenon of electrical discharge through gases at low pressure.

Cathode Rays

How are cathode rays produced
When a very high electrical potential (~10,000 volts) is applied across a
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gas taken in a discharge tube at a very low pressure (about 10 atm or
0.001 torr; 1 torr = 1 mm of Hg column), some radiations are emitted from
the cathode. These radiations are called cathode rays.

Cathode ray tube

Fig. 4.2 Emission of cathode rays Cathode rays are produced
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when very high voltage is applied on a gas at very low (about 10 atm) pressure
The emission of cathode rays in a discharge tube is shown in Fig. 4.2.
At this stage, the glass (walls) of the discharge tube opposite to cathode
starts glowing with a faint greenish light.