Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Ruben Razquin on 08/07/2009 12:30:04

Title: How could a camera flash move a piece of plastic above it?
Post by: Ruben Razquin on 08/07/2009 12:30:04: Ruben Razquin asked the Naked Scientists:

Hello,

First of all, thank you for your wonderful programme (http://www.thenakedscientists.com/HTML/podcasts/).

There is an interesting and puzzling experiment I did when I was a young boy (about thirty years ago) that I want to share with you and ask you for an explanation, for I did not see clearly what happened.

I used an electronic flash and a small, thin, flat, light and opaque piece of black plastic. I connected two pieces of electric wire to the contacts of the electronic flash, so that I could trigger the flash without touching it, just connecting the two electric wires. Then I put the flash on a table on its back, so the light would go upwards, and I put the small piece of plastic on top of the flash, covering the area that emits the light. I then short-circuited the flash and it flashed MOVING THE PIECE OF PLASTIC upwards just a little bit. I was astonished because I could not believe light could move objects and I could even hear a click, as if I could hear fotons banging on the plastic. Could you please explain this?

Thank you.

Rubén

What do you think?
Title: How could a camera flash move a piece of plastic above it?
Post by: RD on 08/07/2009 12:49:26: There is a mechanical shock when the electrical discharge occurs in the Xenon flash tube.
This is responsible for the "Pop" or "click" noise when the flash is fired.
Not caused by photons, caused by changes in the Xenon gas in the tube : a mini explosion.
Title: How could a camera flash move a piece of plastic above it?
Post by: Bored chemist on 08/07/2009 13:44:36: If you fire a slash gun at a thin piece of dark plastic you will heat the side of the plastic that faces the flash. That warm plastic will heat the air near it which will expand. That might be enough to move the plastic.
Title: How could a camera flash move a piece of plastic above it?
Post by: LeeE on 08/07/2009 18:41:34: I think RD is correct. The fact that you can hear the flash means that there's been some physical movement of part of the flash.

Many years ago I occasionally used to use a hand-held flash (for bounce lighting), which was powered by lead-acid batteries, and you could just about detect very slight heating if you put your hand over the flash head and fired it. It was quite an old unit even then though, and was probably quite a bit less efficient than modern units; you only want light out of flashguns and any heat produced is just wasting your power (heh - unless you're doing IR photography perhaps).
Title: How could a camera flash move a piece of plastic above it?
Post by: neilep on 08/07/2009 18:56:40: When I flash I make people move !
Title: How could a camera flash move a piece of plastic above it?
Post by: Karen W. on 08/07/2009 19:02:03: LOL.....I bet you do too! LOL...LOL..
Title: How could a camera flash move a piece of plastic above it?
Post by: Soul Surfer on 08/07/2009 23:52:31: Another effect that could move the plastic was the electrostatic effect of unleashing the several hundred volts that powers the flash
Title: How could a camera flash move a piece of plastic above it?
Post by: lyner on 09/07/2009 00:16:27: The energy stored in the capacitor of a flashgun is
E =CV²/2
1mF and 300V are typical, giving
E=45J
total energy.
Not a lot in terms of heat but enough to take the chill off a resistor (I happen to have done that very thing last Sunday whilst mending my flashgun)
Could it also heat up enough air to cause expansion? I'm not sure what sum to use.
Title: How could a camera flash move a piece of plastic above it?
Post by: syhprum on 09/07/2009 05:23:21: 1mF seems very large would not 1μF be more typical ?
Title: How could a camera flash move a piece of plastic above it?
Post by: rhade on 09/07/2009 11:26:08: Quote from: Bored chemist on 08/07/2009 13:44:36
If you fire a slash gun

Slash gun? What's that? High tec Jack the Ripper?
Title: How could a camera flash move a piece of plastic above it?
Post by: RD on 09/07/2009 13:11:52: The capacitors used in the flash in "disposable" film cameras are around 200μF 300V.
A professional camera flash would easily be 5x more powerful.
Title: How could a camera flash move a piece of plastic above it?
Post by: lyner on 09/07/2009 18:51:15: Quote from: syhprum on 09/07/2009 05:23:21
1mF seems very large would not 1μF be more typical ?
No.
The one in my Hi-end consumer flash is 1mF. Why just use 1muF? It wouldn't store enough energy. (0.047J)
Title: How could a camera flash move a piece of plastic above it?
Post by: Bored chemist on 09/07/2009 19:03:51: Quote from: rhade on 09/07/2009 11:26:08
Quote from: Bored chemist on 08/07/2009 13:44:36
If you fire a slash gun

Slash gun? What's that? High tec Jack the Ripper?
Oops!
Title: How could a camera flash move a piece of plastic above it?
Post by: thedoc on 04/08/2009 19:21:25: Listen to the answer to this question on our podcast. (http://www.thenakedscientists.com/HTML/podcasts/show/2009.08.02/)
Title: How could a camera flash move a piece of plastic above it?
Post by: syhprum on 08/08/2009 20:31:13: I admit my error having grownup at a time when 8 μF capacitors were considered large I forget how compact present day capacitors are.