Can a Laser Light Be Magnified with a Magnifying Glass?

[ehummel]

Hi all, can a laser light be magnified by means of a magnifying glass?

 

[jtbell]

What do you mean, exactly, by "magnify"?

 

[ehummel]

Hi, i am referring to making the beam brighter by focusing the beam to a smaller beam like when one focuses sunlight thru a magnifying glass.

 

[mgb_phys]

Yes you can - but a laser beam is generally already pretty small and you would need fairly good optics for the focussing down to win over optical errors scattering light out of the beam.
A microscope objective would work, a dollar store magnifying glass wouldn't.

 

[Andy Resnick]

Hi, i am referring to making the beam brighter by focusing the beam to a smaller beam like when one focuses sunlight thru a magnifying glass.

"Brightness" is a conserved property of a source of light. Note that 'brightness' has a specific radiometric definition that is different than the common use of the term.

Focusing a beam with a lens increases the intensity (technically, the radiant incidance) but not the brightness.

Conservation of brightness is why Archimedes' mirror trick doesn't work.

 

[ehummel]

English Please... I have a little handheld green laser that I want to mount on a board with mirrors that will bounce the beam back and forth. This laser is very bright to the eye. You can see the beam very well in a shadowed area or at night. This laser will actually light a match. but I was thinking that by the time it is reflected 4 or 5 times, there would be a lose due to the inefficiency of the mirrors. I don't know much about optics so I may be totally off base with this thought. I was thinking that if I introduce a magnifying glass at the point where the visual brightness of the beam drops off, i could get a few more bounces out of it. If this sounds totally stupid, forgive me.

 

[ehummel]

BTW: what is Archimedes' mirror trick? Sounds interesting...

 

[ehummel]

Yes you can - but a laser beam is generally already pretty small and you would need fairly good optics for the focussing down to win over optical errors scattering light out of the beam.
A microscope objective would work, a dollar store magnifying glass wouldn't.

Sorry, i missed your response. OK, this pretty much answers my question. I have a couple of old microscopes laying around. I think I may try that. Thank you.

 

[mgb_phys]

I don't know much about optics so I may be totally off base with this thought. I was thinking that if I introduce a magnifying glass at the point where the visual brightness of the beam drops off, i could get a few more bounces out of it.

Nope, you can squeeze the beam down to make the point brighter but at the other side of the minimum point the beam spreads out - it doesn't restore any of the light lost by the mirror bounces.
The mirror's are losing light because of reflection losses and because they are not perfectly flat and so some light is being scattered out of the beam.

ps. Be careful a laser that can ignite matches can easily damage your eyesight with even an accidental reflection from a shiny bit of metal like a watch strap or a dropped part.

 

[ehummel]

Nope, you can squeeze the beam down to make the point brighter but at the other side of the minimum point the beam spreads out - it doesn't restore any of the light lost by the mirror bounces.
The mirror's are losing light because of reflection losses and because they are not perfectly flat and so some light is being scattered out of the beam.

ps. Be careful a laser that can ignite matches can easily damage your eyesight with even an accidental reflection from a shiny bit of metal like a watch strap or a dropped part.

So the best thing i can do is have the highest quality mirrors possible and or have a more powerful laser.

BTW: Yes, I caught 1 of those accidental reflections 1 time. It is NOT a toy!

 

[russ_watters]

BTW: what is Archimedes' mirror trick? Sounds interesting...

Archimedes death ray: http://en.wikipedia.org/wiki/Archimedes#The_Archimedes_Heat_Ray_.E2.80.93_myth_or_reality.3F

In any case, I don't agree with that answer: the death ray idea works just fine in theory and in modern reality because the more mirrors you have, the more light you can gather. "Brightness" is not constant, it is directly proportional to the number of mirrors. It didn't work for Archimedes because it is impossible for a bunch of people to simultaneously manually aim mirrors (no one will know which bright spot is theirs).

The exact concept is used for concentrating sunlight for solar power:
http://www.google.com/imgres?imgurl...TI0OII&sa=X&oi=image_result&resnum=3&ct=image

 

[Andy Resnick]

No. Radiometric brightness (sometimes also called the etendue) is a conserved quantity.

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

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

Archimedes' death ray is impossible in principle due to thermodynamic considerations, which I have discussed in previous threads.

 

[russ_watters]

No. Radiometric brightness (sometimes also called the etendue) is a conserved quantity.

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

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

Archimedes' death ray is impossible in principle due to thermodynamic considerations, which I have discussed in previous threads.

Maybe we're just talking past each other here. Are you saying that you can't make a "bright" spot by reflecting some sunlight with a mirror, then make it brighter and hotter by adding more mirrors? [edit]...or a magnifying glass or large telescope, for that matter (except that the mirrors in the solar tower are flat).

How does this idea differ from a solar furnace or solar power tower, for example? I'll do a search and see if I can figure out what you mean from other threads...

 

[mgb_phys]

For a solar tower there is obviously a maximum temperature you can reach = the blackbody temperature of the source.
But I'm not clear there is a power limit based on the area of mirrors or the solid angle of the sun (other than practical limitations).

 

[russ_watters]

Found a thread where you discuss it Andy...so it's a matter of distance. I'll have to think about that...

I started a new thread discussing the issue: https://www.physicsforums.com/showthread.php?t=327585