If there is something difficult to measure, it is thespeed of light. In 1629, the Dutch physicist Isaac Beckman proposed doing this by observing the reflection of a cannon shot in a mirror located one mile (1.6 km) away. Nine years later Galileo proposed to measure it by observing the delay that would be observed at a certain distance when covering a lit lamp. Unfortunately, the speed of light is high enough to be able to perceive this delay, which causedthe famous astronomer Johannes Kepler claimed that the speed of light was infinite.
Despite the failures, the ideas of Beckman and Galileo were correct. They were only wrong in the distance to consider. The first one to do it wasOle Christiensen Rømerin 1676, using the orbit of Jupiter’s moon as a flashlight, observed at two different times: when the Earth was in two diametrically opposite places in its orbit. In this way he verified that the period of the orbit of Io was 22 minutes longer when the Earth was further from the gas giant than when it was closer. It is what traveling to about300,000 km/s…
In this experiment we use to measure the speed of light a result of wave physics that studies, as its name suggests, the properties and characteristics of waves. In particular, a type of waves calledstanding waves.
The concept is the following. Let’s imagine a guitar string. If we press it in the center, the disturbance generated will go towards each of the extremes and there it will be reflected, returning on its way. What we then have are two identical waves (since they have both been generated with the same pulsation) but with opposite directions. They are destined to crash. And this is where the difference appears with what would be a collision between two billiard balls, which we all know. Chen two equal waves meet, what is produced is interference: they do not disappear but something else is produced. In this case, a standing wave.What is it? Simply put, in a standing wave each point on the string moves in a unique characteristic way. It is not a traveling wave, like the waves in a pond or on a beach: if we are on a mat on the beach sometimes we will be still and other times we will begin to sway due to the passage of a wave. If they were standing waves that would not happen. Depending on where we were, we would always move in the same way. That is, there are places that do not move the least while others always oscillate with the same elongation. These are the type of waves that are generated in stringed instruments, such as guitars and pianos, and in organ pipes. And also in themicrowave ovens.
In this case,the electromagnetic waves generated by the furnace are confined between the furnace walls, with what they behave like those of the strings of a guitar. As they are standing waves, there are areas in which the oscillation is maximum and others in which the oscillation is minimum.This is what we see with the chocolate bar(and for that it is very important that it does not rotate inside the oven): the areas of maximum amplitude are where the “holes” in the tablet are produced. Measuring the distance between two successive “holes” or between two alternate ones, we will have the speed of light. How? For that we need a result of wave physics, which relates that distance, called the wavelength, with the frequency of the microwaves and the speed. And it goes like this: speed of a wave = frequency per wavelength. In the case of ovens, the frequency of the microwaves it generates is written on the specification plate located behind the appliance: 2.45 Gigahertz (precisely, in the same range as Wi-Fi or Bluetooth and that is why they can interfere). The prefix giga means one billion, so the microwave frequency is 2,450,000,000 hertz.
Now we have to measure the wavelength, which is given by the separation distance between the centers of two alternate “holes” or twice the distance between two consecutive ones. In our experiment the wavelength value is 12.5 cm or 0.125 meters. Therefore the speed of light is: 0.125 x 2,450,000,000 = 306,250,000 meters / second
To get to kilometers per second, you just have to divide by 1,000, and it is 306,250 km / s. Not bad at all, using just one chocolate bar!
