If you increase the frequency, for example from 10 kHz to 25 kHz, will the brightness of the LED change?

Question

Electronics

In this case, the duty cycle remains the same.

Answer 1

If you have an ideal abstract spherical LED in a vacuum and a strictly rectangular signal, then the brightness will not change, because fill factor has not changed.
And in the real physical world, the signal will also have rounded corners (and possibly extra peaks) and all sorts of parasitic capacitances on the tracks and outputs of the LED will begin to play a role. Depending on a bunch of parameters, the brightness will either increase slightly, or vice versa. So it goes.
Either way, the change in brightness will be small and it's likely that no one will notice.