Here's why:

* Cathode rays are streams of electrons. These electrons are emitted from the cathode (negative electrode) in a vacuum tube.

* They travel in straight lines from the cathode to the anode (positive electrode). This is because they are negatively charged particles and are attracted to the positively charged anode.

* The direction of motion can be influenced by magnetic fields, which cause the electrons to curve. However, in the absence of external fields, cathode rays move in a straight line.

Let me know if you have any other questions about cathode rays!