The nature of light
The light becomes an interesting topic only when we start asking questions about its nature. What is it? What does it consist of? How can we use it? And of course, how fast is it? If you think about it, the light that comes from the Sun is all around us. How can it have speed? How can it be consisting of something? It turns out that the light is even weirder than these simple questions make it look like.
In the past, it was thought that the light travels instantaneously from one point to another. Questions regarding its speed started rising when Ole Rømer saw that the elapsed time between Io’s eclipses on Jupiter is shorter when Jupiter is closer to Earth and longer when Jupiter is farther from the Earth. He concluded that the light comming from Jupiter reaches Earth in a shorter period of time when Jupiter is closer and viceversa, therefore the speed of light must be finite. However, it was Albert Michelson who accurately calculated the exact speed of light in vacuum, which is nearly three hundred thousand kilometers per second. Consequently, a light-year is the distance that the light can travel in one year and it’s now used to express distances between objects in space.
The light emitted by the Sun needs nearly nine minutes to reach Earth. The light emitted by distant stars and galaxies that we see with the naked eye on the sky needs million and even billion years to reach Earth. In other words, when we see a galaxy through a telescope, we see that galaxy the way it looked like million or billion years ago, when the light we see today - its light - began its journey from it to us, to the Earth. If a star ten million light-years away from Earth would explode in this moment, we would still be able to see it on the sky for another ten million years from now, that’s the time needed for the last ray of light emitted by that star to reach Earth. Only after that ray of light reached us, the star would disappear from our sky. Nothing that we see on the sky is actually there, we see how the universe looked like in the past, and that’s truly amazing.
Isaac Newton believed that the straight reflected ray of light is the perfect prove that the light consists of particles – nothing else can travel in such a straight line – confirming thus Pierre Gassendi’s theory (1592–1655). He also explained refraction as a normal effect of light particles entering and moving into a denser environment. Meanwhile, some of his contemporaries (Robert Hooke, Christian Huygens, and Augustin-Jean Fresnel) mathematically concluded that if the environment directly affects the speed of light, then the light actually acts as a wave. These theories were fully contradictory, but they were also both correct. Now everytime science faced such problems, someone had to step up :) This time it was James Clerk Maxwell’s time, who independently discovered four simple equations which described waves of oscillating electric and magnetic fields. When he calculated their propagation speed, he was astonished. Their speed was equal to the speed of light. It quickly became apparent that visible light, ultraviolet light, and infrared light (phenomenon thought previously to be unrelated) were all electromagnetic waves of differing frequency.
The light is a wave-type disturbance in the electromagnetic field, the same way the sound is a wave-type disturbance in the atmosphere (there are no sounds in space, only in sci-fi movies), the same way a sea wave is a disturbance in the calm water. It is also described by a flux of massless subatomic particles called photons. The fact that the light has both particle- and wave-like natures is known as particle-wave duality. This discovery had a tremendous impact in science.
The Doppler effect
When a car which is moving towards you horns, the sound’s intensity increases and reaches its peak when the car is at the closest point away from you. As soon as it starts moving away from you, the horn’s intensity starts decreasing. As I said before, the sound is an oscillating disturbance in the atmosphere, just like the waves made by a rock thrown in the water. If the car isn’t moving, the distance between two sound waves is constant, isn’t changing. But if the car moves away from you, the distance between the waves increases constantly, reaching your ears more and more rarely, causing the sound’s intensity to decrease. The opposite effect happens when the car is moving towards you. This is the exact case of light waves. If a source of light moves towards the observer, its light color shifts to blue, and if the source of light moves away from the observer, its color shifts to red. Christian Doppler was first to see this effect which now bears his name – the Doppler effect. Scientists soon used it to see if the universe is slowing down (as expected) or not by studying the light comming from distant stars and galaxies in the observable universe. Turned out it isn’t slowing down, it’s actually expanding at an accelerating speed, but that’s another story :)