# The Naked Scientists Forum

### Author Topic: Lighting a room with one torch?  (Read 2579 times)

• Hero Member
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##### Lighting a room with one torch?
« on: 18/04/2007 00:46:26 »
If you stood in the centre of a dark room with wide beam torch and pointed the beam at a wall and then spin the torch fast, would it light up the walls evenly?

Would it look continuous and at the same brightness as a stationary beam?

Never tried it but wondered, hope that makes sense.

Mmm, maybe this is not general science.

Bee

« Last Edit: 18/04/2007 00:48:36 by that mad man »

#### Seany

• Neilep Level Member
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##### Lighting a room with one torch?
« Reply #1 on: 18/04/2007 00:51:06 »
How long do you think it would take to spin around. If you were quick, maybe 0.5 second.

How fast do you think light is? 299,792,458 m/s. Basically 300,000km/s.

I don't think that the light would "spread over" the area. I think it would concentrate on a specific area, then the area next to it, then the area next to it and so on. I don't think that it will "spread" over the wall, by your spinning movement.

• Hero Member
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##### Lighting a room with one torch?
« Reply #2 on: 18/04/2007 01:03:20 »
Lets say the torch beam was spinning at 1000rpm!

Bee

#### another_someone

• Guest
##### Lighting a room with one torch?
« Reply #3 on: 18/04/2007 01:25:37 »
There are lots of possible paths around the room.

The corners of the room will be further away than the flat region opposite you, and so will receive less light.

Some light paths might have complex routes around the room, while other light paths will just bounce straight back to the torch, and be absorbed by the torch itself.

And then you may have to take into account standing waves, and constructive and destructive interference patterns (depending on the size and shape of the room and smoothness of the walls).

#### lyner

• Guest
##### Lighting a room with one torch?
« Reply #4 on: 18/04/2007 10:11:14 »
A Cathode ray tube (normal TV display) works on this principle. A single spot is scanned over the tube face and your eye sees it as a uniform field of light. Our eyes have a 'lag' in their response, particularly the cones, at the centre of the retina so we just percieve the average brightness of an object.
In the same way, the one torch trick would work if it scanned fast enough. 50 times a second works for TV - just.
Interference wouldn't be a problem because the light from a torch is not coherent. You don't see it with ordinary lighting so you wouldn't see it with your scanning torch. Also, the varying brightness wouldn't necessarily bother you; a single light bulb in the centre of the room would have the same effect. Dark corners have always been a  bit of a problem - particularly when all you had was one candle.
btw, you can actually do the experiment in a rather naff sort of way, just by waving a torch around fast in a dark place. You definitely 'see' more than what is in the torch beam at any one instant.

• Hero Member
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##### Lighting a room with one torch?
« Reply #5 on: 18/04/2007 14:36:01 »
Ahhh, thanks another_someone, i should have thought about the corners and made the room round.

@ sophiecentaur, I forgot that about a tv scanning...

Thanks

Bee

#### The Naked Scientists Forum

##### Lighting a room with one torch?
« Reply #5 on: 18/04/2007 14:36:01 »