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J. Perelman
"Entertaining physics". Book 2.
Chapter 10. Sound. Wave motion


When the Doppler for the first time (in 1842) came to the conclusion that mutual rapprochement or removal of the observer and of the source of sound or light shall be accompanied by a change in the length of the perceived sound or light waves, he made the bold assumption that this is the cause of the color of the stars. All the stars, he thought, by themselves white; seem, many of them painted because they move quickly towards us. Quickly approaching white stars send earthly observer shorter light waves that generate the sensation of green, blue or purple flowers; on the contrary, quickly receding white stars seem yellow or red.

It was original, but certainly a wrong thought. In order that the eye may have noticed the change in color of the stars, caused by the movement, it would be necessary first of all to give the stars a huge velocities of the order of tens of thousands of kilometers per second. But it would have been inadequate: the fact that simultaneously with the conversion, for example, blue rays coming of the white star violet rays green into blue, spot UV spade purple, red - infrared; in short, an integral part of the white light remain in the cash, so that, despite the General shift of all colors of the spectrum, the eye is not supposed to notice any changes in General coloration.

Another thing - shift dark lines in the spectrum of stars moving towards the observer: these movements are well recovered exact tools and allow us to determine the speed of the stars along the line of sight. (Good spectroscope sets the speed star is even 1 km per second.)

The error of the Doppler remembered the famous physicist Robert Voodoo when the policeman was preparing once fined him for what he did not stop his speeding car, despite the red signal. Wood, as the story began to assure the United States that when driving fast towards the signal red is perceived as green. Whether the policeman versed in physics, he could calculate that to justify the words of the scientist, the car had to race with incredible speed 135 million km per hour.

This calculation. If through l to denote the wavelengths of light emitted by the source (in this case, the signal light), l' is the wavelength perceived by the observer (Professor in the vehicle), through v - vehicle speed, and through C is the speed of light, the relationship between these values, set theory, is as follows:

l / l' = 1 + v / c.

Knowing that the shortest of waves responsible for the red color, equal 0,0063 mm, and the longest wave green equal 0,0056 mm, substitute these values into the formula; the speed of light we also know:

300 000 km/sec. Have:

0,0063 / 0,0056 = 1+ v / 300 000, where the vehicle speed

v = 300 000 / 8= 7500 km per second, or 135 000 000 km per hour. With such rapidity wood during the hours moved away from the COP further than to the Sun. They say that it still was fined for exceeding the permitted speed".

Entertaining physics J. Perelman


System Orphus


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