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J. Perelman
"Entertaining physics". Book 1.
Chapter 8. Reflection and refraction of light

NEW ROBINSONS

Without a doubt, you know how the characters in the novel of Jules Verne's “Mysterious island”, abandoned on an uninhabited earth, produced a fire without matches and flint. Robinson came to the aid of lightning ignited the tree, new Robinsons Jules Verne helped not a coincidence, but the resourcefulness of an experienced engineer and a solid knowledge of the laws of physics. Remember how surprised naive sailor the Sailor, when, returning from hunting, found the engineer and the reporter in front of a blazing fire.

“ - But who lit the fire? "asked the sailor.

The sun, " replied Spilett.

The journalist was not joking. Indeed, the Sun brought fire, which was so delighted sailor. He couldn't believe his eyes and was so amazed that I could not even ask the engineer.

- So, you had a magnifying glass? - asked the engineer Herbert.

"No, but I produced it.

And he showed him. These were just two glasses, shot by the engineer with their hours and hours Spilett. He combined them with the edge of the clay, pre-filled with water, and thus it was a real fiery lentils, which, by focusing the sun's rays to dry moss, the engineer got the fire”.

The reader will, I think, to know why they should be filled space between the watch glass: is filled with air biconvex lens not focusing rays?

It is not. The hour glass limited by two parallel (concentric) surfaces - external and internal; and it is known from physics that, passing through a medium bounded by such surfaces, rays almost does not change direction. Passing then through the second the same glass, and here they do not deviate, and therefore, not going in focus. To focus the rays in one point, you need to fill the space between the panes of some transparent substance that broke would the rays stronger than the air. So did the engineer in the novel by Jules Verne.

Ordinary carafe of water, if it has a spherical shape, can also serve as incendiary lentils. It was known already to the ancient, who noticed that the water remains cold. It even happened so that standing at the open window carafe with water, lit the curtains, tablecloth, obeglywol table. The huge ball bottles with colored water, which, according to tradition, was decorated with earlier Windows pharmacies, can sometimes be the cause of these disasters, causing ignition of flammable substances in the vicinity.

Small round flask filled with water, even at small sizes flask, bring to boil water, particular object sticking its bloated on a watch glass: sufficient bulb 12 inches in diameter. At 15 cm in focus [Focus is placed very close to the bulb] is the temperature of 120°. To light a cigarette using a flask with water as easily as a glass lens, which Lomonosov in his poem “About the benefits of glass” wrote:

We flame solar glass get here
And Prometheus so easy to imitate.
Cursing Murphy awkward thereof of baloney,
Heaven without sin fire smoke tobacco.

It should be noted, however, that the stirring action of the water lens is significantly weaker than glass. This is due, firstly, to the fact that the refraction of light in water is much less than in the glass, secondly, the water strongly absorbs infrared rays, which play an important role in the heating of the phone

Interestingly, incendiary effect glass rosefinches was known even to the ancient Greeks, more than a Millennium before the invention of spectacles and eye tubes. About it Aristophanes mentions in the famous Comedy “the Clouds”. Socrates offers Stratego task:

“If someone wrote an obligation on you in five talents, how would you destroy it?

Stratied. I have found how to eliminate the obligation, Yes such a way that you recognize his cunningly! Have you noticed, of course, in pharmacies stone beautiful, transparent, who lit?

Socrates. A magnifying glass?

Stratied. Exactly.

Socrates. What is next?

Stratied. While the notary writes, I, standing behind him, will direct rays of the Sun on the obligation, Yes words it will melt...”

Let me remind you for clarifying that the Greeks of the time of Aristophanes was writing on waxed tablets, from which heat could easily melt.

Entertaining physics J. Perelman

 




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