Student/novice opticians only

[Dave Nelson]

This category has been inactive for so long, I thought I'd try to revive it by hopefully getting some novice opticians to think a bit to solve the following:


1) Ciba claims their tear gel has an index of refraction of 1.33. Can you prove this? How?

2) A book on "survival" tells people that they may use a pair of spectacles to use sunlight to start a fire. Can you? How? Why? under what circumstances?

[harry a saake]

i can to the second one as i have done it, it is simply a matter of focusing the sunlight through the OC on the object you want to start afire, why would you do it, you need a fire for warmth, or to cook etc, in fact some forest fires have been known to start from a piece of glass that was just shaped the right way and at the time the sunlight rays hit the glass a fire was started:angry:

[chip anderson]

Harry:

Must be a plus lens to concentrate the radient energy (heat) into a much smaller area than would be without focus. Optical center is not required but it helps if lens is complete (haveing a center) to achive proper concentration.
Don't remember the detains but Universal Contact lens of Houston (with which I was employeed) used to have a small product (dish attachment) for which it was said: Place any liquid in this, place this on the lensometer and the reading obtained will be the index of refraction.

Chip

[Dave Nelson]

No takers from the novices. OK, yeah, obviously the specs would have to be +. They would also have to be spherical, or close to it, since attempting to focus the infrared component of the suns rays into a cone would produce a very large focal area. There is a concern with both spherical and chromatic abberations as well, since both will also contribute to a larger focal area, and a small, concentrated focal point may be needed to reach the combustion point. Lens size is important as well, since the greater the lens area, the more infrared energy is concentrated into a small area.
As for the index of the tear gel, correct again. Since we can measure the refractive power of a lens where the index and power are known, such as water=1.3375, we can place a little water into a plano lens, and measure its power at, for sake of argument, +4.00. If we replace the water with tear gel, and put a flat lens cover on it to insure a flat surface on the top side, then re-measure the power. If indeed we measure +4.00, we can conclude the index is 1.33, as claimed. (it actually is.)

[HarryChiling]

no longer playing in this sand box

[Bezza]

Actually you could use a sphero cylindrical lens if tilted at the correct angle to form a point.

Hrrrmm, unless there was very little cyl power this wouldnt really be possible as sphero-cylindrical lenses do not produce point foci like spherical lenses do. Although it may be possible to concentrate the light enough to start a fire it would not be a true point focus. If there was a high cylindrical component the line foci produced by each meridian would be too far apart that tilting the lens would not be sufficient to bring them close to a point focus. For example a +2.00x180 produces a horizontal line image at 50cm and a +5.00x90 produces a vertical line image at 20cm.....thats 30cm apart, only an astigmatic lens that produced a very small disc of least confusion would be sufficient to produce enough concentration of light in this scenario.

[chip anderson]

Bezza:

There is no rule that a fire must start in a round spot. Either axis could be brought to a focus and start a fire in a straight line. Possibly even could concentrate enough heat (if the sun is bright enough, the fuel and atmosphere dry enough and the kendling temp of the fuel low enough to start a fire at the mid-point of the interval of Sterm.

Chip

[Bezza]

I wasn't saying that it wasn't possible to start a fire with a sphero-cylindrical lens, just that it wouldnt produce a point focus even if tilted especially if there was a high cyl.
It is however much more difficult to concentrate the light especially in cases where the disc of least confusion is quite large since even at the mid point there will be much less concentration of light.
If you were stranded on a desert island and stuck with only a pair of highly astigmatic glasses to start a fire with i reckon your best bet would be to pop out the lenses and try and best neutralise one of the meridians to produce a more spherical lens system.

[HarryChiling]

no longer playing in this sand box

[HarryChiling]

no longer playing in this sand box

[Dave Nelson]

Matches?

[cocoisland58]

Forgot about this. We used to do this as kids with a magnifying glass. Not in the house mom.

[EyeFitWell]

Couldn't you compare the Ciba Tear Gel to water to proove 1.33? That's water's index. Put both liquids into identical globes and measure the focal length. Voila!

[EyeFitWell]

2) A book on "survival" tells people that they may use a pair of spectacles to use sunlight to start a fire. Can you? How? Why? under what circumstances?

You can do it with a plus lens by holding the lens over something flammable and moving it closer/away from the potential fire until you get the smallest, most concentrated spot you can. Why? because the lens is focusing a beam of light into one tiny point. The light is converted to heat and becomes hot enough to start a fire. Under what circumstances? 1. proper vertex distance 2. strong enough lens 3. During a bright sunny day with dry materials to ignite.
I guess you can do the same with a spherocyl lens if you are using the circle of least confusion as your vertex!

Dave, how'd I do?

[Chris Ryser]

Excert from Wickipedia:


The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a convex lens used to focus the sun's rays to produce fire). The writings of Pliny the Elder (23–79) also show that burning-glasses were known to the Roman Empire[1], and mentions what is possibly the first use of a corrective lens: Nero was said to watch the gladiatorial games using an emerald[2] (presumably concave to correct for myopia, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water. The Arabian mathematician Ibn Sahl (c.940–c.1000) used what is now known as Snell's law to calculate the shape of lenses[3], and Ibn al-Haitham (965–1038) wrote the first major optical treatise which described how the lens in the human eye formed an image on the retina. The oldest lens artifact is dated to 640s BC; a rock crystal lens found at excavations in Ninive.
Recent excavations at the Viking harbor town of Fröjel, Gotland in Sweden have revealed rock crystal lenses produced at Fröjel in the 11th to 12th century via turning on pole-lathes that have an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses concentrate sunlight enough to ignite fires.
Widespread use of lenses did not occur until the invention of spectacles, probably in Italy in the 1280s. Nicholas of Cusa is believed to have been the first to discover the benefits of concave lenses for the treatment of myopia in 1451.'
'
'And more at : http://en.wikipedia.org/wiki/Lens_%28optics%29#History

[Dave Nelson]

You can do it with a plus lens by holding the lens over something flammable and moving it closer/away from the potential fire until you get the smallest, most concentrated spot you can. Why? because the lens is focusing a beam of light into one tiny point. The light is converted to heat and becomes hot enough to start a fire. Under what circumstances? 1. proper vertex distance 2. strong enough lens 3. During a bright sunny day with dry materials to ignite.
I guess you can do the same with a spherocyl lens if you are using the circle of least confusion as your vertex!

Dave, how'd I do?

Very well, except one teensy tiny thing. Number 2, the lens must be strong enough. Lens strength is measured as a focal length, and it makes no difference if the focal point is at 1 meter, or .25 meters, the result is the same. The size of the lens is more important, since a greater area of light is then brought to a focal point. All else being equal, a weaker, larger lens will be more effective than a smaller stronger lens. Or to put it another way, size matters.:D

[EyeFitWell]

Dave, what if you have a lens that needs to be 12 meters from the earth's surface in order to focus on your fire pit? There's a practical limitation involved, no?

[HarryChiling]

no longer playing in this sand box