In the context of cathode ray discharge tubes, which one of the following statements about cathode rays is not correct?

Introduction / Context:
This question belongs to the early experimental study of electrons using cathode ray discharge tubes. Understanding cathode rays is important because they led to the discovery of the electron and provided evidence that atoms contain smaller charged particles. The question asks which statement about cathode rays is not correct, so you must recall their key properties and behaviour in the discharge tube.

Given Data / Assumptions:

• We consider a classic cathode ray discharge tube experiment.
• Cathode rays travel from cathode to anode when a high voltage is applied across electrodes in low pressure gas.
• Options present several statements describing origin, visibility, dependence on gas and behaviour in fields.
• We must pick the statement that is factually incorrect.

Concept / Approach:
Cathode rays are streams of electrons emitted from the cathode in a discharge tube. Their properties, such as charge to mass ratio and direction of motion, are independent of the type of gas in the tube or the metal used for electrodes. This universality showed that electrons are fundamental constituents of all atoms. Cathode rays travel from cathode to anode, are not directly visible, cause fluorescence when they strike certain materials and can be deflected by electric and magnetic fields.

Step-by-Step Solution:
Step 1: Check statement A: Cathode rays originate from the cathode and move towards the anode. This is correct and is observed in experiments. Step 2: Check statement B: Television picture tubes are modified cathode ray tubes in which an electron beam creates images on a fluorescent screen. This is correct. Step 3: Check statement C: The rays themselves are not visible; we see only the glow produced on the glass or fluorescent screen. This is correct. Step 4: Check statement E: Cathode rays are deflected by electric and magnetic fields, which proves they carry electric charge. This is also correct. Step 5: Check statement D: It claims that the characteristics of cathode rays depend on the nature of the gas present in the tube. Step 6: Experiments show that cathode rays always consist of electrons with the same charge and similar properties irrespective of the gas or cathode material. Step 7: Therefore, statement D is not correct and should be selected as the answer.

Verification / Alternative check:
In many textbooks, the universality of electron properties is emphasised by stating that the charge to mass ratio of cathode rays is the same for all gases and electrode materials. This was a key point for concluding that electrons are fundamental particles present in all atoms. If the gas type changed the basic character of cathode rays, this conclusion would not hold. Hence, any statement claiming that cathode ray characteristics depend on the gas must be false at this level.

Why Other Options Are Wrong:
Statements A, B and C describe standard experimental observations and are well supported by evidence. Statement E reflects an important property used to measure the charge to mass ratio of electrons. These are correct statements and therefore cannot be the answer to a question asking for the incorrect one. Only statement D contradicts the well established finding that cathode rays have the same basic properties regardless of the gas in the tube.

Common Pitfalls:
Students sometimes confuse the behaviour of cathode rays with that of positive rays (canal rays), whose characteristics do depend on the type of gas present. Remember that positive rays originate from ionised gas particles, so their mass and charge vary with the gas. Cathode rays, however, are streams of electrons, so their basic properties remain constant. Mixing up these two types of rays is a frequent source of error in such questions.

Final Answer:
The incorrect statement is that the characteristics of cathode rays depend upon the nature of the gas present in the cathode ray tube.