Science Buddies: "Ask an Expert"

I have planned a light storage system (LSS) to store solar energy in its raw form i.e. light by circulating it within an optical system without converting it to other forms like electricity. It is comparatively much much cheaper than solar cells and more efficient. It could be the answer to next gen lighting problems. So my question is: if we place two convex lens of different sizes one after another such that their focal points coincide. Now if we send a beam of parallel normally through the bigger one then after the two refractions will we get a smaller beam of parallel rays? Moreover how much energy will be lost by reflection along the interface? I shall be highly obliged if you can help.

Welcome to the Forum, saranyuc3. You asked:

“if we place two convex lens of different sizes one after another such that their focal points coincide. Now if we send a beam of parallel normally through the bigger one then after the two refractions will we get a smaller beam of parallel rays?”

If the two lenses have the same focal length, then they would need to be the same size in order to capture all the light entering the first lens. More generally, the ratio (D/f), where D is the diameter of the lens and f the focal length, must be the same, as is evident if one makes a simple sketch of the two lenses, the rays going straight through the centers of the lenses, and the rays that are maximally refracted at the edge of each lens.

“how much energy will be lost by reflection along the interface?”

For bare glass lenses, several percent reflectance will occur at each surface. However anti-reflective coatings can reduce the reflectance to 1% or less — see the article with that name in Wikipedia for details and further references.

saranyuc3 - I don't believe there is any way of storing light, in the form of light, for any appreciable amount of time. Reflectors are imperfect and photons would be lost (energy depleted) after a few transitions. Transmission through media (glass, fibre-optics etc) would also be very 'lossy' as well. If you do an internet search on your subject "storing light in the form of light" you will find many references on the subject. The time constant for the decay of stored light energy, in very efficient systems, would be likely be in the order of maybe pico or nanoseconds. There wouldn't seem to be practical way to store and release light without transitioning the energy into a storable form like electrical energy, heat or other physical state. The problem, much like alchemy, is very attractive, but probably not resolvable with physics as we currently understand it. Sorry to be a bit negative, but I think you should revise your hypothesis, or demonstrate why it wouldn't work on a practical basis.

Rick Marz

Rick's correct I believe; no one has succeeded in storing light for very long. The record must be the Laser-Interferometric Gravitational Observatory (LIGO) -- it recycles light between mirrors spaced 4 km apart for about 75 round trips!! That's an astonishing 2 milliseconds. This feat required many years of effort by a world-class team of physicists masterminded by Ray Weiss.

Thank you for finding the fault. I guess I'll have to change the topic. Another question: We know that the for a regular polygon, we have value of each interior angle = (n-2).180/n = (1-2/n).180 so as n tends to infinity, we have each angle = (1 - 2.0).180 = 180 i.e. a straight line. Now for a circle, n tends to infinity so is each interior angle of the circle is 180? If yes, then will a light ray sent at grazing angle of incidence on a convex mirror travel along the curve of the mirror?

saranyuc3 - As 'n' approaches infinity, the interior angle approaches 180⁰, but never reaches exactly 180⁰, therefore you don't have a straight line path. There is always an angle, although infinitesimal.

Rick Marz

You asked "...will a light ray sent at grazing angle of incidence on a convex mirror travel along the curve of the mirror?"

It seems the answer is yes: "In the whispering-gallery modes of a transparent microsphere, light orbits inside the sphere, where it is confined by total internal reflection." see

http://www.techbriefs.com/index.php?opt ... 20619.html

Pretty amazing, huh?