Why is the signal on my oscilloscope so jittery/ugly?

My pulse generator is directly connected through a 50 Ohm 50 centimeter cable and set up with this signal:

But my oscilloscope is showing this:

What is the cause of this?

After a short while I thought it might be my desk light, and it is! Turning the light of and the signal instantly clears to a sharp line.

I believe this is caused by a switching power supply which is built into the light. This was the light that was fixed almost directly beneath my oscilloscope, a Muller Licht Linex 55 NW: