[faizan]

cathode rays were not produced at ordinary pressure inside the discharge tube?Why is it so?

[Arkcon]

What are cathode rays?  How are they produced?  What does "ordinary" pressure mean?  That is, what causes "pressure" inside the tube?

These are questions you can ask yourself, to help you understand the whole picture better.  You'll come up with a better answer, and be able to give a better response at test time, when they ask a similar, but not exactly the same, question.

[vmelkon]

You should read about what happens when you have a high voltage. For example, during lightning. For example, 10,000 V between 2 electrodes that are sufficiently close to create a discharge in the air (or if the electrodes are too far to create a discharge). For example, high voltage electric lines, 750,000 V.

[faizan]

cathode rays are electrons but why these rays are produced at low pressure?

[vmelkon]

If you have a vacuum, then the high energy electrons fly freely sort of like a car on a highway with no traffic.
If you have some air molecules, then the electrons are going to "*Ignore me, I am impatient*" into those molecules.

You have the same case for transmission electron microscopes. The same case for linear accelerators and cyclotrons. The same case for when the sun emits high energy protons and various nuclei into space.

Also, an important part of cathode ray tubes is the electon gun. You might want to read about that as well.

[vmelkon]

If you have a vacuum, then the high energy electrons fly freely sort of like a car on a highway with no traffic.
If you have some air molecules, then the electrons are going to "*Ignore me, I am impatient*" into those molecules.

You have the same case for transmission electron microscopes. The same case for linear accelerators and cyclotrons. The same case for when the sun emits high energy protons and various nuclei into space.

Also, an important part of cathode ray tubes is the electon gun. You might want to read about that as well.

and there is also this
http://en.wikipedia.org/wiki/Breakdown_voltage#Gases_and_vacuum
although the page doesn't give an explanation to how the breakdown happens in air.