Light: All About it
Light is a form of energy. It travels in waves, similar to waves of water in oceans. Except that with light, the waves are made up of electric and magnetic fields. Light is basically a wave of electric and magnetic fields, intertwined. Hence, light is also known as Electromagnetic Radiation.
When you’re floating in the ocean, you’ll move up as a wave passes you and then back down and then back up again when the next wave passes you. The distance between the crests of these waves is called the wavelength. Since light is a wave, it also has a wavelength. The energy of light is dependent on its wavelength. Light with a shorter wavelength has more energy and light with a longer wavelength has less energy. Our eyes have a way of detecting the different energies of light — Color! What we see as the colour violet is light with a short wavelength and what we see as the colour red is light with a long wavelength. Colours such as orange, green and yellow have intermediate wavelengths. This range of colours and wavelengths is known as a spectrum. Our eyes can only detect a small range of all the different wavelengths light can have. That range is known as Visible Light.
If light has a slightly shorter wavelength than what our eyes can detect, it becomes invisible to us but it is still light. We call that Ultraviolet Light. Light waves that have a wavelength shorter than Ultraviolet Light are called X-Rays and light with the shortest wavelength of all are called Gamma Rays. On the other side, light that has a longer wavelength than the reddest light our eyes can see is called Infrared Light. Light waves with a longer wavelength than Infrared Light are called Microwaves and light waves with the longest wavelength of all are called Radio Waves. The energy of the light is dependent on its wavelength. The shorter the wavelength of the light, the higher its energy and vice versa. So Ultraviolet Light has a higher energy than violet, X-Rays have a higher energy than Ultraviolet Light and Gamma Rays have the highest energy of all wavelengths. On the other side, Infrared Light has a lower energy than red, Microwaves have a lower energy than Infrared Light and Radio Waves have the lowest energy of all wavelengths. We can also say that Ultraviolet Light is “bluer” than violet light and X-Rays are “bluer” than Ultraviolet Light. Infrared Light is “redder” than red light and Microwaves are “redder” than Infrared Light. So, light with a shorter wavelength is bluer and light with a longer wavelength is redder. By this, it does not mean more blue or red, just that the wavelengths are getting shorter and longer. Together, we call all of these different kinds of light, the Electromagnetic Spectrum or the EM Spectrum.
How is light made?
When matter is heated, it gains energy and then it has to get rid of that energy. Since light is a form of energy, one way to get rid of that energy is to release it in the form of light. The energy of the light emitted by an object is dependent on the temperature. An object that is hotter will emit light with a higher energy, i.e, with a shorter wavelength. An object which is colder will emit light with a lower energy, i.e, with a longer wavelength.
To understand how light is emitted, we zoom in on individual atoms. In general, atoms are made up of three subatomic particles — Protons, Neutrons and Electrons. Protons have a positive electric charge, Electrons have a negative electric charge and Neutrons are neutral. Protons and Neutrons occupy the central part of the atom, in what’s called the nucleus of the atom. Electrons orbit the nucleus and their negative electric charge attract them towards the positive charges of Protons. You might think of the electron orbiting the nucleus like planets orbiting the Sun, but that’s really not the case here. The real situation involves pretty complex Quantum Mechanics. But in the end, electrons are allowed to occupy very specific volumes of space around the nucleus and that depends on the electron’s energy. Electrons whiz around the nucleus with a very precise amount of energy. If you give them an additional precise amount of energy, they will move up to the next energy level but if you give the wrong amount of energy, they’ll just sit in the original energy level. The opposite is also true, electrons can be in a higher energy level and then they can give off energy when they jump down. The amount of energy they release is exactly the same amount needed to get them to jump up to the next energy level. They get this energy through Light. When light hitting the atom has just the right amount of energy, the electron will absorb it and jump to the next energy level. It can also jump back down and emit light at that same energy. An electron can jump up by two, three or four energy levels if you give it exactly the right amount of energy. So when an electron jumps up or down, it absorbs or emits a very specific wavelength of light. This does not end at that. Different electrons of different atoms require different amounts of energy to jump up or down by one or more energy levels. So when an electron of a Hydrogen atom jumps down, it emits light at a different energy, than an electron jumping down in a Nitrogen or a Helium atom.
This rule of thumb is the key to the Universe. Since different atoms emit light at different energies, if we can measure the energy of that light, we can get to know what an object is made of, even though we can’t touch it. But how do we measure the energy of the light? We can measure the energy of the light by using a Spectrometer. A Spectrometer is a device that can precisely measure the wavelength of light, by which we can determine the energy of light. It can therefore differentiate the light emitted by a Hydrogen atom from light emitted by Helium or Lithium. If you can use a spectrometer with a telescope, you can figure out what objects are made of, especially astronomical objects. The atoms in a gas cloud in space are floating free, rarely even touching each other. The atoms emit those individual wavelengths of light, allowing us to determine them. This is how we got to know the Universe is made up of. Stars and gas clouds in space are mostly made up of Hydrogen, with some helium and trace amounts of heavier elements. Venus is made up mostly of Carbon Dioxide, Saturn is mostly made of Hydrogen. Everything in the Universe is made up of its mix of elements. With spectroscopy, we can determine them.
You probably know about the Doppler effect, i.e, the change in the frequency of a wave in relation to the observer. Let us take the example of a bike passing by you. When the bike is headed towards you, the sound waves get compressed, causing the pitch of the sound to rise. After it passes you, the pitch drops because the wavelength is getting longer and longer. The same thing happens with light. If an object is headed towards you, the wavelength of the light from the object gets compressed. We say that the light is “Blue-Shifted”. If it passes you, the wavelength gets longer and so we say that it is “Red-Shifted”. We can apply this to spectroscopy and by measuring the shift of the wavelength we can tell if an object is moving toward or away from us. From this method, astronomers found out that the Universe is actually expanding and it is this redshift that helped us to determine this.
Using other spectroscopic methods, we can also tell if an object is spinning and how fast, the mass and the density of an object and we can get to know if an object has a magnetic field. Many properties of astronomical objects can be found through spectroscopy. Almost everything we know about the Universe is because of the light that comes from objects in it.