Question

the electron of cathod ray takes the place of knocked out electron.

Answer 1

cathode rays are also known as ‘electron beams’ (arguably a better name). They are essentially streams of electrons.

If we take gas inside a tube and have a beam of electrons passing through the gas, if the electrons are of sufficient energy, they can excite and/or ionise the gas.

Here, ionisation is when electrons are ‘knocked off’ the atom causing it to form a positive ion.

But ultimately, atoms can be ionised because the energy of the electrons in the cathode ray is sufficient to remove electrons from an atom’s orbital