Like the first day of many college classes, we decided to start with some review.
In physics, the law of conservation of energy is very important. It states that energy can neither be created nor not destroyed. However, one form of energy can be converted into another form. What are the different types of energy? There is light, heat, electric energy, chemical energy (one example is the electrolysis of water), potential energy, nuclear energy, and more.
The prefix “infra-” as in infrared means “below”. We can translate infrared to “below-red”.
The prefix “ultra-” means “beyond”. Ultraviolet means “beyond-violet”. (It could have been “supra-violet” or “hyper-violet” because those prefixes mean “above” in Latin.) From red to violet, lies the electromagnetic spectrum of visible light. Violet has a wavelength of about 400 nanometers and red has a wavelength of 700 nanometers. Beyond violet are ultraviolet rays, x-rays, and gamma rays. Below red are the infrared light, microwave radiation, and radio waves. The wavelengths of radio waves range from a few millimeters to hundreds of kilometers. As far as we are concerned, the difference between a radio wave and a red light wave is that we can see one type of wave but we can’t see the other.
Waves have two properties: frequency and wavelength. Only a small portion of the electromagnetic spectrum excites our neurons.
In an optical telescope, the visible light source goes through the telescope’s objective lens, continues to the eye piece, and then enters our eyes. In this process, the wavelength of light doesn’t change. A radio telescope is hit by waves that are in the range of radio waves (way beyond visible light rays). Visible light is to an optical telescope as radio waves are to a radio telescope.
Longer wavelength means lower frequency and shorter wavelength means higher frequency. Here’s an interesting question: What does a 400-nanometer wavelength correspond to in terms of frequency? Here is a picture of a wave:
The distance between the two red dots is called the wavelength (λ). That part of the wave is one cycle of the wave. The number of cycles of the wave that happens in one second is called the frequency (f) of the wave. If 1 cycle happens every 1 second, the frequency of that wave is 1.
The scientist Heinrich Hertz created electromagnetic waves for the first time in a laboratory.
Waves are crucial in all of physics. We can go to a pond, drop a pebble into the pond, and see the ripples it creates. When the pebble is dropped, the water molecules are shaken, but they are not willing to go anywhere, so they pretend to go somewhere and then come back to their place. When the pebble is at a height, it has a lot of potential energy. When it drops and hits the water, its potential energy decreases. For example, if there is a boulder at the top of a cliff, and you are at the bottom of that cliff when that boulder falls, it might crush you because its potential energy has reduced (now that its height, in relation to the great earth, has reduced), but since we just learned that energy can not be created or destroyed, the lost potential energy is converted into another form: the kinetic energy; in other words, the boulder gains momentum. When the pebble hits the water, it creates two kinds of waves: sound waves (a faint noise that sounds like “kerplop”) and the ripples.
Think of a few notes in music ( “do, re, mi, fa, so, la, ti, do”) that you can sing. Why does the first note sound different than the last note? The frequency is different. What is frequency in the case of sound waves? The number of times compressions and rarefactions of that medium in 1 second. The higher note which has a higher frequency will have a greater number of compressions and rarefactions happening in 1 second than the lower note.
In the case of light, pure red light will have a higher wavelength and lower frequency.
(The purple curve is the wave of a violet light. It is inaccurate but enough to explain the point.)
The wavelength of violet light is shorter than that of red light. Let’s say that the time between the two red points is 1 second. The number of cycles that happen in one second –the frequency– of the violet light is greater than the red light (again, the picture is inaccurate, but one can get the idea). From the first red dot to the second red dot is λ, from the first to the third red dot is two λ, from the first to the fourth red dot will be three λ, and so on.
Going back to the pebble, the amount of potential energy it will have at 1 cm, 1 meter, and 1 km will be different. If there is a leaf somewhere close in that pool, it may not move when the pebble is dropped from a 1 cm height but it is likely to move when it is dropped from a 1 km height because its energy is much greater. The same pebble at a higher height will have a higher potential.
Now comes the ultimate question: how are wavelength and frequency related? A wave that makes 100 cycles in 1 second has a frequency of 100. That particular wave had a frequency of 100 Hz. From this alone, we will not be able to find the wavelength. There is a crucial piece of information we need to find the wavelength. We know the time it takes (1 second), and we know how many cycles were created, what else do we need to know? We need to know how far the wave went! Let’s say that that particular wave went 10 meters. That means that the wave made 100 cycles in 10 meters.
λ=(10/100)
=(1/10) meters
Let us generalize this result. The number of cycles is f and the length of each of those cycles is λ. Therefore, f*λ is the distance traversed by the wave in 1 second. The distance traversed by the wave in 1 second is its speed, or, its velocity.
Therefore, we find this equation:
f*λ=v
Using the velocity of light (300,000 km/s) and its wavelength (700 nm), we can find the frequency of red light after some calculations. The frequency of red light comes out to be approximately 430 THz (terahertz).
Galileo attempted to find the speed of light but was very far from the accurate answer that we have today. Very few sources have data about the experiments he did.
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