Would my ray gun work?

[Tom Mazanec]

When I was in grade school (St. Wenceslaus) I designed a ray gun and asked my science teacher (Mr. Belsito) if it would work.
Put a large, mildly convex lens in sunlight. Just before the focus, put a small, strongly concave lens to cancel out the converging and make a beam. Aim with a mirror.
Mr. Belsito said that of course it would work, that's how lasers work.
Was he right (about it working, not the laser part)?

[glappkaeft]

Since the sun is not a point source there is a hard limit on how well it can be focused, its angular size. That size is half a degree so it won't be very ray-like. There is also the problem of not overheating and damaging the concave lens which also needs to be a pretty advanced apo-chromatic design to minimize chromatic aberration since the sun is a broad band source. Then the second law of thermodynamics limits how hot you can make something using a passive system to the temperature of the source (in this case 5700 K at best).

[profloater]

i may not understand your ray gun, are the convex and concave mirrors reversed?

[grant hutchison]

i may not understand your ray gun, are the convex and concave mirrors reversed?

Lenses.

Grant Hutchison

[PetersCreek]

When I was in grade school (St. Wenceslaus) I designed a ray gun and asked my science teacher (Mr. Belsito) if it would work.
Put a large, mildly convex lens in sunlight. Just before the focus, put a small, strongly concave lens to cancel out the converging and make a beam. Aim with a mirror.
Mr. Belsito said that of course it would work, that's how lasers work.
Was he right (about it working, not the laser part)?

Search of online optical simulators and try it for yourself.

[Tom Mazanec]

Search of online optical simulators and try it for yourself.

I tried and got some hits that don't "fit" what I want.
Could you give a link or two?

[PetersCreek]

I couldn't guess at what would "fit", so no, I don't have any suggestions.

[cjameshuff]

Since the sun is not a point source there is a hard limit on how well it can be focused, its angular size. That size is half a degree so it won't be very ray-like. There is also the problem of not overheating and damaging the concave lens which also needs to be a pretty advanced apo-chromatic design to minimize chromatic aberration since the sun is a broad band source. Then the second law of thermodynamics limits how hot you can make something using a passive system to the temperature of the source (in this case 5700 K at best).

You could eliminate the heat and chromatic aberration issues by using mirrors to make it clear that the fundamental issue is just the geometry. There's just nothing you can do to take two rays of white light hitting a lens or mirror at the same point from different angles and make them parallel.

More detail: https://en.wikipedia.org/wiki/Etendu...ion_of_etendue

[DaveC426913]

At best, what you're doing is concentrating the light with the first lens just like a magnifying glass, and then using the second lens to aim it elsewhere.

But the light that impinges on a target a few feet away will be no more concentrated than that which terrorizes ants. It'll just be beamable over a short distance.

It'll work, but maybe not as spectacularly as you imagined.

(Of course, it depends very much on the size of the primary, which could be arbitrarily large. Doubling the diameter will square the energy.)

[profloater]

Lenses.

Grant Hutchison

Ah yes i am sure I read mirrors and assume it was the convex word that caused my association, i am guilty of speed reading.
Also mirrors would make more practical sense for heat loss and range of frequencies but thanks for the correction.

[George]

Even with the best lenses the beam divergence from non-parallel sunlight will greatly diminish the gun's power. Any intense sunlight concentrated beam at, say, 6 inches will be reduced by a factor of 400 at 10 feet (inverse square) for any given point.

Even lasers diverge more than you might expect. When we used a laser to signal the ISS ("Flash the Station"), the "1 watt" (likely 1/2 watt) laser's ~ 3 or 4mm beam became about 1 km wide at the distance of the ISS. This helped us greatly since hitting the ISS traveling at 18,000 mph would be nigh impossible with a 4 mm beam. Hitting an eyeball of either astronauts Don Pettit or Dan Burbank would be nigher impossible. [Is "nigher" ok? ]

[Tom Mazanec]

I couldn't guess at what would "fit", so no, I don't have any suggestions.

I found a lot of simulations of looking out through glasses.
A couple that looked promising, but I couldn't get them to work on my Mac.
And a few that seemed to be adds for schools.

[Tom Mazanec]

At best, what you're doing is concentrating the light with the first lens just like a magnifying glass, and then using the second lens to aim it elsewhere.

But the light that impinges on a target a few feet away will be no more concentrated than that which terrorizes ants. It'll just be beamable over a short distance.

It'll work, but maybe not as spectacularly as you imagined.

(Of course, it depends very much on the size of the primary, which could be arbitrarily large. Doubling the diameter will square the energy.)

Actually, what I think I am doing (I may be wrong) is using the first lens to concentrate the beam into a small spot, using the second lens to cancel out the converging so it doesn't diverge after the focal point, and then using the mirror to aim.

[George]

Actually, what I think I am doing (I may be wrong) is using the first lens to concentrate the beam into a small spot, using the second lens to cancel out the converging so it doesn't diverge after the focal point, and then using the mirror to aim.

Yes. Though I'm a bit weak in optical design, the following may help. And don't forget Snel's Law, or even Snell's Law -- one of those is right --, that gives you more smearing due to color aberrations.

Rays lenses 2.jpg

[A bit rushed so only the input to the lens is shown for sunlight, though you can guess the output.]

Note: The concave lens was moved to where it should be -- prior to the focal point.

[Hornblower]

Upon doing some relatively simple thought-exercise ray tracing, I have concluded that it will not work. Suppose we wish to concentrate the incident light into a beam 1/10 the diameter of the primary lens. A computer is not needed to analyze it in approximations using thin lenses and small angles. Pencil-and-paper sketches are sufficient. We did it in entry level optics in college.

For the sake of nice round numbers let the Sun's angular diameter be 1/100 radian, which is close to the actual size. A 10 inch f/10 lens will focus the beam from a point source on a prime focal point 100 inches behind the lens. To make a parallel 1-inch beam, place a negative lens with a focal length of 10 inches at a point 10 inches in front of the prime focus. A beam 1 inch in diameter will come out of that secondary lens, and in an accurate thought exercise position it will project that beam any desired distance. So far, so good. Let's say this beam came from a point at the center of the Sun's apparent disk. Things start going sour when we trace the corresponding rays from the edges of the solar disk. They will form similar 1-inch beams, but the back lens will cause those beams to diverge at 10 times the angle at which they entered the front lens. That will cause a rapid drop-off of the brightness as we go farther back from the apparatus.

What we have done here is to independently re-invent a 10x version of Galileo's telescope.

The reason for the divergence of the beams coming out of the back lens is not intuitively obvious for some of us. If you wish, I can upload some sketches to illustrate it. Drawing, scanning and uploading them may take awhile, so please be patient.

[ShinAce]

If the sun was a point, it would work. Since the sun is an extended source, the collimation won't be perfect. It would certainly work well enough to create a ray gun to shoot around a room.

BTW, you don't need a concave lens. You can use two convex lenses. Simply place the second lens after the focal plane, where the rays are diverging again. It's called a beam expander. We use them all the time with lasers to change the beam size.

[Hornblower]

If the sun was a point, it would work. Since the sun is an extended source, the collimation won't be perfect. It would certainly work well enough to create a ray gun to shoot around a room.

BTW, you don't need a concave lens. You can use two convex lenses. Simply place the second lens after the focal plane, where the rays are diverging again. It's called a beam expander. We use them all the time with lasers to change the beam size.

My bold. That depends on what you mean by "well enough." My hypothetical example, or one with a 10-inch focal length positive lens 10 inches outside the prime focus, would make a 10-inch spot on a wall 100 inches beyond the back lens. It will be proportionately larger as you increase the range. It will not heat the surface the way a 1-inch beam would.

I think it is clear that Mr. Belsito missed the consequences of an extended source as compared with a point source.

[George]

The reason for the divergence of the beams coming out of the back lens is not intuitively obvious for some of us. If you wish, I can upload some sketches to illustrate it. Drawing, scanning and uploading them may take awhile, so please be patient.

Thanks to ShinAce, I did need to make one correction to my drawing. Is it that bad otherwise?

[DaveC426913]

Actually, what I think I am doing (I may be wrong) is using the first lens to concentrate the beam into a small spot, using the second lens to cancel out the converging so it doesn't diverge after the focal point, and then using the mirror to aim.

Yes. I'm simply saying that the beam will not get smaller than when it emerges from the primary lens. So, at best, it's simply beaming that spot elsewhere.

You've made a ranged ant-cooker.

[publiusr]

The big problem is trying to aim the thing. Laser conducting fibers maybe?

Case in point:
https://www.youtube.com/watch?v=E3YCACZQ72Q

I don't think the "rifle" you see here is the actual laser device. Note two cords.

I can imagine your system atop a modified solar power plants with the central tower, if the fibers ran too hot. Maybe power generated from the tower could help cool them and the optics?

[Hornblower]

I will take up where George left off on the sketches.
Sketch A:Ray tracing a.jpg
The vertical line between the lenses is their mutual focal plane. The source is a point on the optical axis, and could be the center of the Sun. All rays are parallel to the right of the secondary lens.
Sketch B:Ray tracing b.jpg
The rays are from halfway from the center to the upper limb of the Sun and are focused just below the optical axis on the focal plane. A key point is that the ray that passes through the center of the secondary lens is not deviated. The other rays passing through the focal point emerge from the secondary lens parallel to the undeviated ray. Note that this bundle is more steeply inclined than the center ray in the incident cone from the primary lens.
Sketch C:Ray tracing c.jpg
Here I have stopped down the primary to cut off the outermost rays, including the undeviated one, and I put in a dashed ray to show where the undeviated ray was before stopping down. I will call this a reference ray, and by definition it passes through the focal point and the secondary center. The rays that get through are refracted exactly as in sketch B, and are parallel to the reference ray.
Sketch D:Ray tracing d.jpg
The rays are from the upper limb of the Sun, and all of them miss the center of the secondary. Nevertheless I created a reference ray through the focal point and the secondary center, and set the emerging rays parallel to it. A primary of twice the aperture would have put a real ray where the reference ray is.
Sketch E:Ray tracing e.jpg
I combined A, B and D to make half the beam we would get from the whole Sun. The other half would come from the lower half of the disk and emerge below the optical axis, symmetrical with the ones shown.

[Hornblower]

Addendum to previous post: The refractive characteristics I illustrated are approximations that are excellent for thin lenses, long focal ratios and small angles where sine ~ tangent ~ angle in radians. They are consequences of exact trigonometric calculations from Snell's law. The beam inevitably diverges rapidly from the desired tight beam, and the aberrations we would get from lenses as thick as in these sketches would further degrade it.

[a1call]

Here is my proverbial 2 cents:

• The fact that the sun's ray are not perfectly parallel is irreverent
  
  • They are nearly parallel and the concept is valid in principal/theory given:
    
    • The concave (secondary) lens has a (much) shorter focal length than the larger collector/primary convex lens
    • The lenses are highly aligned and the focal points of the 2 lenses are are precisely coincided
    • This will produce a nearly parallel ray which is more intense than the incident sun rays since the rays will will be concentrated in a smaller diameter than the incident ray
    • One caveat is that the 2 lens' alignment and foci coincidence (Occupying same point in space) requires Optical-Laboratory precision and can not be achieved by homemade apparatus fabrication.
    
    
  
  

BTW, this reminds me of Archimedes' Heat-Ray. A Mirror equivalent with a hole in the middle of the concave mirror is as valid. Forgo the hole and you will end up with a light trap with (theoretically) continuously converging rays (precision, non-parallelism and scattering notwithstanding).
https://en.wikipedia.org/wiki/Archimedes#Heat_ray

[profloater]

If the rays were really parallel and the lenses were perfect, the intensity at the focal point becomes infinite, so this is the clue to the dispersed light output as Hornblower has demonstrated. Also a pinhole experiment will image the sun which would not work with parallel light. I tried to do this arrangement with mirrors when I wanted a device to see the colours of stars by catching more light and without magnification of size, but I guess my mirrors were not good enough because it did not work. It's hard to find binoculars with large objectives and small mag. to achieve what I wanted.

[a1call]

The intensity of focused light is not infinite but is greater than the intensity of the unfocused light. In the same way the intensity of parallel rays occupying a diameter less than the incident light will be greater.
The same principle makes objects seen by telescopes brighter than unaided view (magnification dimming notwithstanding).

[Hornblower]

The intensity of focused light is not infinite but is greater than the intensity of the unfocused light. In the same way the intensity of parallel rays occupying a diameter less than the incident light will be greater.
The same principle makes objects seen by telescopes brighter than unaided view (magnification dimming notwithstanding).

My bold. Not necessarily. If the eyepiece is such that the "magnification" is 1x, the view through the scope will be no brighter than with the naked eye. The exit pupil will be the same diameter as the objective, so your eye pupil effectively stops it down to no gain.

[a1call]

Image search Google for: Laser beam expander
To see images related to the reverse process.
It is common in laser optics.
The literature on the web describes the narrowing process difficult to achive due to worsening of divergence.
FWIW.

[a1call]

This thread got me thinking. If I am not mistaking:

• Laser beam divergence is for the most part due to scattering
• The ratio of scattering over beam diameter will be less the wider the beam is
• The larger the primary/collector convex lens (or equivalent concave mirror) is the "easier" it will be to narrow parallel beams to a given diameter
  
  • It is the same principle that produces sharper images/view in a telescope, the larger the area of the collector is.

[Hornblower]

This thread got me thinking. If I am not mistaking:

• Laser beam divergence is for the most part due to scattering
• The ratio of scattering over beam diameter will be less the wider the beam is
• The larger the primary/collector convex lens (or equivalent concave mirror) is the "easier" it will be to narrow parallel beams to a given diameter
  
  • It is the same principle that produces sharper images/view in a telescope, the larger the area of the collector is.

I don't see any reason for making the primary lens larger than the diameter of the beam as it emerges from the laser.

[a1call]

Scattering occurs in the edge of a lens/mirror/mask(open hole).
The larger the area of a lens is, the less distortion will be introduced in an image, resulting in a sharper image. In the same way the amplified scattering in a beam-narrower will be reduced since the distortion ratio of the input beam will be reduced.