# How would this "lampblack" substance turn modulated light into comprehensible sound? **URL:** \ **Category:** Factual Questions\ **Created:** [December 14, 2015, 4:03am UTC](https://boards.straightdope.com/t/how-would-this-lampblack-substance-turn-modulated-light-into-comprehensible-sound/740393 "2015-12-14T04:03:50Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [December 14, 2015, 4:03am UTC](https://boards.straightdope.com/t/how-would-this-lampblack-substance-turn-modulated-light-into-comprehensible-sound/740393/1 "2015-12-14T04:03:50Z")
Today I learned that Alexander Graham Bell [invented a form of wireless telephony](https://en.wikisource.org/wiki/Popular_Science_Monthly/Volume_19/June_1881/Production_of_Sound_by_Radiant_Energy_I) which created its signal by using a vibrating mirror to modulate light, then translated the signal back into sound by having the light shine on a substance called “lampblack.” As far as I can tell, lampblack is basically a very fine soot. You can read more about this in the link by ctrl-f-ing “lampblack”. My question is, how would this substance change light into sound at all, much less change modulated light into the very sounds that produced the modulation. It seems, if I’m reading it correctly, that Bell didn’t have to do anything particularly fancy–just shine the light on the stuff and boom, a sound reproducing the original sound, audible from all around the room. Anyone know anything about how this worked, or how it would work? ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [December 14, 2015, 4:07am UTC](https://boards.straightdope.com/t/how-would-this-lampblack-substance-turn-modulated-light-into-comprehensible-sound/740393/2 "2015-12-14T04:07:11Z")
Well down towards the last mention of lampblack I do find this speculation by Bell as to why it worked: > [@](#): > > The loud sounds produced from such substances may perhaps be explained in the following manner: Let us consider, for example, the case of lampblack—a substance which becomes heated by exposure to rays of all refrangibility. I look upon a mass of this substance as a sort of sponge, with its pores filled with air instead of water. When a beam of sunlight falls upon this mass, the particles of lampblack are heated, and consequently expand, causing a contraction of the airspaces or pores among them. > > Under these circumstances a pulse of air should be expelled, just as we would squeeze out water from a sponge. > > The force with which the air is expelled must be greatly increased by the expansion of the air itself, due to contact with the heated particles of lampblack. When the light is cut off, the converse process takes place. The lampblack particles cool and contract, thus enlarging the air spaces among them, and the inclosed air also becomes cool. Under these circumstances a partial vacuum should be formed among the particles, and the outside air would then be absorbed, as water is by a sponge when the pressure of the hand is removed. Seems plausible. What do you think? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [December 14, 2015, 4:56am UTC](https://boards.straightdope.com/t/how-would-this-lampblack-substance-turn-modulated-light-into-comprehensible-sound/740393/3 "2015-12-14T04:56:38Z")
The entire paper you link to is a good read. Right at the bottom you get to what is more likely the answer. Not that the lamp black itself is expanding and contracting, but that the air in the mass of lamp black particles is heated, and thus expands and contracts. Lamp black has a silly surface area per unit volume, and such a huge surface area would be expected to help conduct more heat in a short time to the included air. Being black of course is also a great help, adsorbing as much heat as possible from the light.