How to measure the speed of light? Alternative applications of microwave ovens and thermal imaging cameras

How to measure the speed of light? Generally speaking, it requires very precise instruments to accurately measure, but today I plan to use a microwave oven and a plastic ruler to measure the speed of light. Since light is also an electromagnetic wave, but the frequency is very high, so the speed of light is the same as the speed of electromagnetic waves. Only then can I determine the speed of light by measuring the speed of electromagnetic waves. Since we are using consumer electronics and plastic rulers as measuring tools, of course there will be some errors in the measurement results. But I didn’t do very precise measurements in the first place. This was just for fun. You can start with this. Guess what the final measured speed of light is.

How to calculate wave speed

Let's do some science first. How to calculate the speed of a wave? This is exactly the same as calculating the speed of a car. Just divide the distance traveled by the object by the time it takes to get the speed. The same goes for calculating wave speed. Electromagnetic waves must have peaks and troughs. The distance between the two vertical line peaks in the figure below is called the wavelength. When the wave is moving, the red dot in the figure below will first go down and then go up. When the red dot When it returns to the original height of the wave crest, the time it takes for the next time is called the Period. At this time, the electromagnetic wave has just moved a distance of one wavelength, so the way to calculate the wave speed is to divide the wavelength by the period, or wavelength. Multiply the frequency.

Wave speed calculation formula

So to measure the speed of light, or to measure the speed of electromagnetic waves, as long as we can get the frequency and wavelength, we can calculate the wave speed, which is the speed of light. The speed of light is currently defined as 299,792,458m/s. This speed only occurs in vacuum. If it is in the air, it will be slightly slower. However, compared with the error of our experiment, this small amount is actually much smaller and cannot be felt at all. Wait for the speed of light measurement Key Element – ​​Frequency

When I use a microwave oven, I use its frequency. The label on the back of the microwave oven will indicate its working frequency. For example, the working frequency of my microwave oven is 2450MHz. Basically, all microwave ovens are at this frequency. I just take it. It is actually possible to calculate the wave speed at this frequency.

Generally, the frequency of microwave ovens is 2450MHz.

But just like the net weight label on snack packaging, there will always be an error between the label and the actual content. I am very curious about how much the frequency will shift when the microwave oven actually works? Out of curiosity, I asked for spectrum analysis. The frequency actually measured by the instrument is as shown below. Let me start by saying this! This is just driven by curiosity. Even if this spectrum analyzer is not used, it can still calculate the speed of light. I use the Wifi antenna on the left to receive the electromagnetic waves leaked from the microwave oven. I set the center frequency to 2450MHz. If it starts running later, a power line appears in the middle of the screen, which means that the working frequency is really exactly 2450MHz, but I don't think it will be so good. The microwave oven is not powered on yet, so the screen of the spectrum in the picture below is empty.

Use spectrum to measure actual frequency

In order to prevent the microwave oven from idling, I must put a cup of water in the microwave oven to heat it during operation. Otherwise, the electromagnetic waves that are not consumed will damage the magnetron inside, which is the heart of the microwave oven. After pressing the heating button, a sharp peak will appear on the spectrum screen as shown below. It is obvious that the peak frequency is not exactly in the middle. According to the Marker, this frequency is located at 2.460625GHz, which is far away from the original nominal frequency of 2450MHz. , offset by about 10MHz. This kind of offset does not matter at the time, because the microwave oven is only used for heating, and it does not matter if it is off by a few dozen MHz. After all, the microwave oven is not a communication device. If it is a communication device like Wifi, the frequency requirements are very strict. If it is off by 20MHz, it will be on another channel, which is simply the wrong frequency. Therefore, the frequency stability requirements for different applications are different.

The actual operating frequency of the microwave oven deviates

Key elements of light speed measurement-wavelength

Next, we need to measure the wavelength. How to measure it? Let’s do some science first. The cabinet of the microwave oven is made of metal. Metal is like a mirror for electromagnetic waves, so electromagnetic waves will be reflected when they encounter metal. This Reflection will produce standing waves, as shown in the figure below.

The reflected wave and the incident wave superpose to form a standing wave

The so-called standing wave is when two waves of equal size and opposite direction are superimposed on each other, the resulting wave is as shown in the red wave. It will form nodes in some places, where no energy will ever appear. But in some places, wave peaks will form, so if there is no turntable rotating in the microwave oven, the food inside will have a local phenomenon of hot and cold coexistence. When eaten in the mouth, the right side will feel hot and the left side will feel cold. I believe Everyone must have had this experience. It does taste very unpleasant, but today I am going to deliberately use this phenomenon to measure the wavelength. Therefore, when doing the experiment, the turntable must be removed to see the wavelength.

In the case of a microwave oven, in my case, there is a square hole in the wall on the right hand side. Electromagnetic waves are emitted from here. Although this hole is covered with a plate, it is basically transparent to electromagnetic waves. Yes, purely to prevent oil from running into the hole. Then the electromagnetic wave hits the metal wall on the left, and after reflection, it superimposes with the incident wave to form a standing wave. So later we only need to confirm the distance between the two high-temperature points to calculate the wavelength.

Electromagnetic waves generate standing waves in the box

Using cheese to measure wavelengths

Since I've eaten sandwiches that were either cold or hot, I thought it would be easy to use the melting degree of the cheese to measure the wavelength, so I got a pack of cheese and spread it on the plate. If it melts after heating, I should be able to see it. Come out as shown below.

Using cheese to measure wavelengths

The reason why we need to prepare a cardboard box is because it needs to be raised so that the plate is just aligned with the electromagnetic wave outlet. Logically speaking, this can make it easier to receive the electromagnetic wave power. I am not sure whether this is actually the case. The whole thing placed in the box is as shown below. .

Put the cheese in the microwave

After 20 seconds of heating, take out the plate and see the situation as shown below. Only the lower right corner of the plate has obvious traces of melting, and the color becomes darker yellow. The other melted places do not seem to be so obvious.

After heating for 20 seconds, only the lower right corner was obviously melted.

By the way, the lower right corner is the place just to the right of the box door. Unexpectedly, the power is the highest at the edge. This phenomenon may be caused by the size of the physical shapes in the box. After all, electromagnetic waves are in three-dimensional shapes. The transmission and reflection are not as easy to understand as the description in the textbook, which is completely straight-line incident wave and reflected wave. If you really want to understand the three-dimensional transmission method of electromagnetic waves, in addition to understanding the formula of electromagnetic waves, you can also understand it through software simulation. You will find that it is really difficult to understand it intuitively with the human brain.

Using cheese to measure wavelength-segmented observation

Due to the cheese melting experiment just now, only the lower right corner melted. This...how should I say it, it is impossible to determine the wavelength. So I plan to do another experiment, but this time I plan to observe it every 10 seconds to see the melting situation while heating.

The second cheese experiment

In the first 10 seconds, mainly the lower right corner begins to melt, just like before, and no changes can be seen in other positions; in the second 10 seconds, the upper cheese junction begins to melt; in the third 10 seconds, the right corner begins to melt. The junction of the cheese on the square begins to melt, as shown in the picture below. After basically observing for more than 30 seconds, although the cheese melted, there was no obvious peak wavelength, because I actually secretly calculated the wavelength of the microwave oven, and it was obviously not the distance between the three melting positions just now.

The second cheese experiment still showed no obvious melting pattern.

It seems that every time the signs of melting appear at the junction of the cheese, then I will simply tear the cheese into small pieces and do an experiment. Maybe it will be easier to observe, but I have very little cheese left, so We can only lay out half a plate as shown below, let’s see what the result will be.

Use small pieces of cheese to observe wavelengths

After tens of seconds of heating, the melted shredded cheese looks like the picture below. Basically, it is still difficult to judge the wavelength. Some cheeses are completely melted, and some are only slightly softened. Basically, I can’t tell where the wavelength is obtained. Oops. ! What should I do? My room is already filled with the smell of cheese.