Like in class… Why not clap? Stand up? Whistle? It’s a good visual to be sure, but how did the practice originate? Is it related to the act of waving?
Two of your alternatives involve sound, which is a less reliable method of locating the exact questioner. The other involves more physical activity.
That is to say, I haven’t the slightest
I’m sure the practice developed from people being smart enough to recognize hand-raising as the best way to signal for attention without being unnecessarily intrusive.
You tell a class of 30 kids to start whistling or clapping whenever they want to ask or answer a question, and tell us how you get on…
Because of this:
Well yeah that would be crazy!
Seems pretty clear to me.
The point of hand raising is to attract attention, but without compelling anyone to give that attention. The raised hand raises part of the body above the level of the surrounding people in a simple fashion, which does not obscure the view of people behind. It can be ignored temporarily or indeed completely. In general, one raised hand does not interfere with others, so an entire class or other group can raise their hands without confilict.
If you stand, many of these benefits also apply, but tall persons will obscure the view of those behind them, who may also be standing to attract attention.
Sound is not a useful way to indicate that you wish to attract attention, unless that attention is needed urgently - as in an emergency. If more than one person makes a noise, the noises conflict. It is also harder to tell which direction a noise has come from - unless the noise continues to be made, which is disruptive of other activity, particularly for a teacher.
It can be annoying to be called by someone in a crowd who does not wave their hand to attract your attention. I can confirm this from one of my activities where I am often called to assist people in a crowded room. It is often impossible to tell where the person is who called me - it never occurs to them that I only know their general direction in the crowd and they should raise their hand.
Standing up to ask a question doesn’t work if you’re already standing. (It’s not unusual for a speaker to be addressing an audience where everyone is standing, or at least where not everyone is seated.) Raising your hand doesn’t present such a problem, since people don’t normally sit or stand with their arms raised.
Raising an arm is simply an intuitive way to get another person’s attention, not just in a classroom, but in any number of other contexts. We wave to find each other at airports. Sound cues are unreliable, and raising an arm is an easy and reliable visual cue. It doubtlessly predates classrooms, and may predate civilization.