View Full Version : Is it possible to creat darkness by cancelling lightwaves?


plakhapate
06-25-08, 01:22 AM
Suppose we use flat mirror for reflecting the light waves exactly in opposite direction. Then will that reflected beam can cancel the incoming light beam.

P.J.LAKHAPATE
plakhapate@rediffmail.com

Steve100
06-25-08, 02:24 AM
Yes, it is a case of destructive interference.

http://en.wikipedia.org/wiki/Interference

Check this out also.

http://en.wikipedia.org/wiki/Double-slit_experiment

EntropyAlwaysWins
06-25-08, 02:54 AM
Just nitpicking here;
Darkness is not the opposite of light, it's the absence of light, so technically you can't create darkness.

Read-Only
06-25-08, 03:36 AM
Suppose we use flat mirror for reflecting the light waves exactly in opposite direction. Then will that reflected beam can cancel the incoming light beam.

P.J.LAKHAPATE
plakhapate@rediffmail.com

Absolutely not, in the case of ordinary sunlight, which is a mixture of different wavelengths of light. Because in order to do so, you would have to place the mirror at precisely the half-wave point of the incoming beam.

Theoretically, you could do it with a laser beam which consists of only a single frequency, but there are two technical difficulties there. Since we're talking about micro-micrometer precision in placement, the entire apparatus would have to be placed in an environment in which the temperature could NOT vary one bit. Because if it does, the placement will be disturbed and the experiment will totally fail.

The second one is just as bad. Since the incoming bean cannot be canceled by the reflected beam until AFTER it has been reflected, the results would probably look like "bursts" of light and dark. They would occur much, much faster than you could possibly see the infinitesimal periods of darkness.

Zero121
07-06-08, 10:35 AM
Absolutely not, in the case of ordinary sunlight, which is a mixture of different wavelengths of light. Because in order to do so, you would have to place the mirror at precisely the half-wave point of the incoming beam.

Theoretically, you could do it with a laser beam which consists of only a single frequency, but there are two technical difficulties there. Since we're talking about micro-micrometer precision in placement, the entire apparatus would have to be placed in an environment in which the temperature could NOT vary one bit. Because if it does, the placement will be disturbed and the experiment will totally fail.

The second one is just as bad. Since the incoming bean cannot be canceled by the reflected beam until AFTER it has been reflected, the results would probably look like "bursts" of light and dark. They would occur much, much faster than you could possibly see the infinitesimal periods of darkness.



what about just cancelling the visible light (i.e. turning off the source) (if u would count that as "cancelling" light)

Read-Only
07-06-08, 08:32 PM
what about just cancelling the visible light (i.e. turning off the source) (if u would count that as "cancelling" light)

I don't think he'd accept that as an answer. :D

Blue_UK
07-10-08, 06:12 PM
Where does the 'cancelled' energy go?

thecollage
07-10-08, 09:14 PM
Suppose we use flat mirror for reflecting the light waves exactly in opposite direction. Then will that reflected beam can cancel the incoming light beam.

P.J.LAKHAPATE
plakhapate@rediffmail.com

Yes.

James R
07-10-08, 09:35 PM
Where does the 'cancelled' energy go?

It must be redistributed to a different point in space.

For example, if you have two-slit interference, the bright fringes are four times the intensity you would get with two single sources not interfering. To compensate, the dark fringes have zero intensity.

Similarly, if you take two laser beams and combine them using a beam splitter, you can arrange things so that you get no light out of one port of the beam splitter. But, out of the other open port you'll get brighter light.

Tristan
07-10-08, 09:49 PM
Theoretically, you could do it with a laser beam which consists of only a single frequency, but there are two technical difficulties there. Since we're talking about micro-micrometer precision in placement, the entire apparatus would have to be placed in an environment in which the temperature could NOT vary one bit. Because if it does, the placement will be disturbed and the experiment will totally fail.

This actually isn't theoretical. The Large Binocular Telescope is designed to use these principles.


From http://lbti.as.arizona.edu/

The technique takes advantage of the wave-nature light to create destructive interference of the starlight coming from two telescopes loooking at the star. The light is manipulated so that the crest of a lightwave from one telescope lines up with the trough of the lightwave from the other telescope. In the final image, this technqiue makes the star effectively disappear.


Tristan