Which one of the following devices makes use of the electrons to strike certain substances to produce fluorescence?

$A$ metal plate gets heated when cathode rays strike against it due to:

Positive rays were discovered by

Three particles having their charges in the ratio of $1 : 3 : 5$ produce the same spot on the screen in Thomson's experiment. Their masses are in the ratio of

An electron is:

$(a)$ Estimate the speed with which electrons emitted from a heated emitter of an evacuated tube impinge on the collector maintained at a potential difference of $500\;V$ with respect to the emitter. Ignore the small initial speeds of the electrons. The specific charge of the electron,$i.e.$,the $e/m$ is given to be $1.76 \times 10^{11}\; C\; kg^{-1}$.
$(b)$ Use the same formula you employ in $(a)$ to obtain electron speed for a collector potential of $10\;MV$. Do you see what is wrong? In what way is the formula to be modified?