# Optics/Nature of light
## Introduction to Light

Light is actually a form of radiation. In fact, radiation isn't always a bad thing. Some of it is actually quite useful like light, microwaves and radio waves. Those are some of the types of electromagnetic radiation you encounter everyday. You probably guessed by now that light is a type of wave. Well, it is! But at the same time, it also has particle-like characteristics. In this section, we will examine the nature of light and whether it is a wave, a particle or both.

## The Electromagnetic Spectrum

| High energy radiation | Low energy radiation |
To understand light, we need to understand the electromagnetic spectrum. Basically, this spectrum shows different forms of electromagnetic radiation and where they belong on the spectrum. There are a few trends to the spectrum which will be discussed later.
What is Electromagnetic Radiation?
A lot of people seem to associate radiation with nuclear waste and things that cause cancer. However, in science, it has a much more general definition. Radiation is a way to transfer energy and have it propagate, or spread out, as it travels.
Now that you know light is a form a radiation, you can see evidence of this every time you use a flash light. Notice how the beam's diameter increases the farther it is away from the source? This is a property of a wave (which will be discussed in more detail in section 1.1.1 - What is a wave?).
As for the "electromagnetic" part, that refers to the fluctuating electric and magnetic fields that make up the waves. The Speed Limit of the Universe
The Speed Limit of the Universe
Aside from the fact that all EM (electromagnetic) radiation are composed of varying electric and magnetic fields that spread out as they travel, there is one other very important similarity between all of them; they all travel at the speed of light.
The fact that there is a speed of light means that light does not appear instantaneously, as opposed to what some great minds thought in ancient times. The speed of light, which is usually denoted with the letter $c$, is $299.792.458\ {\text{m/s}}$ or about $186.000\ {\text{mi/s}}$. That is fast enough to past through the center of the Earth and reach the other side in about $1/50^{\text{th}}$ of a second. Just imagine how difficult it was to measure the speed of light back then!
The reason why the speed of light is so important is because it is the fastest speed in the universe. Nothing can surpass it. Which is why sometimes it is considered "the speed limit of the universe". Different Forms of EM Radiation
Different Forms of EM Radiation
Now that you know some of the similarities between all EM radiation, you're probably wondering what the difference is. The difference could be explained in three different ways:
* Wavelength;
* Frequency;
* Energy.
In this section we will only talk about energy. The other two, wavelength and frequency, will be discussed later in section 1.1.1 - What is a wave?
If you take a look at the spectrum, you will see "low energy radiation" and "high energy radiation" labelled on the diagram. Obviously, this indicates that radiation near the top, like radio waves and microwaves have low energies while radiation near the bottom, like x-rays and gamma rays, have high energies. That is the only difference between all EM waves; they differ by their energy levels.
As you can see, light is a very small part of the electromagnetic spectrum. Even within that small region, it could be isolated and referred to as a "sub-spectrum", this one, however, you should be more familiar with.
The Colors of the Rainbow
This "sub-spectrum" or the range of visible light occupies a very small part of the EM spectrum. Compared to the other types of EM radiation, it takes up the least space on the spectrum.
That small region, however, is split up into all the different colors of the rainbow. Starting from red and ending at violet. This range must also be consistent with the EM spectrum since it is within it, therefore, red is the lowest energy color and violet is the highest energy color. All of this is shown in the diagram above.
Have you ever heard that white and black are not considered colors? Well, that's true. White is actually a combination of all the colors and black is the absent of all the colors. You can see the effects of this when a prism splits up white light into its component colors.

## Light as a Particle

So far, we have discussed light in terms of being a wave. But what about its particle-like characteristics?
To begin, you need to know what a photon is. A photon is a massless particle that travels at the speed of light and carries a certain amount of energy, which we call a quantum (pl: quanta). EM Radiation could also be considered a stream of photons, each carrying a certain amount of energy. The more energetic the radiation is, the more energy its photons are carrying. For example, photons of red light carry less energy that photons of violet light.

## Wave-Particle Duality

So you're probably anxious to answer the question "which is it: wave or particle"? Either that, or you're anxious to finish reading this article.
Well, the truth is, the question cannot be easily answered. Light can exhibit both wave and particle characteristics. This concept is known as the "wave-particle duality" and it is the reason why modern physics or quantum physics was invented.
This leads us to the next section of the wikibook, entitled "Light - Wave or stream of particles?" If you think this section is confusing, you haven't seen anything yet. The next section enters the mysterious world of quantum mechanics: a world that has puzzled many great minds in the past, including a brilliant physicist named Albert Einstein.