Here's why:
* Discovery: J.J. Thomson's experiments with cathode ray tubes in the late 19th century demonstrated that cathode rays are deflected by magnetic fields. This deflection was consistent with the behavior of negatively charged particles.
* Further experiments: Further experiments using electric fields confirmed the negative charge of cathode rays. The rays were attracted to the positive plate and repelled by the negative plate, indicating their negative charge.
It's important to note that cathode rays are not the same as light, even though they travel in straight lines. Light is made up of photons, which have no charge, while cathode rays are streams of electrons, which have a negative charge.
