Hmm, not necessarily. Back in the days when this problem was originally offered incandescent bulbs were by far the most popular light source for facilities, and if you look at a graph[1] of such a bulb's output, you'll find the fast majority of it is below the threshold of visible light -- the real primary property of incandescent bulbs is IR (heat) production. As a physicist Feynman would be well aware of this, and hence I think he would approved of the low-tech intended solution over the suggested alternatives.

If you posit fluorescent lights, the traditional solution would still work. LED lights weren't stipulated as a special condition and even they are stated, they're not 100% efficient and hence might well work with the traditional solution.

[http://www.sciencebuddies.org/science-fair-projects/project_...]