1 - Physics of Light

When we have a Wi-Fi device, it's going to send out signals that we often refer to as RF signals, where RF stands for Radio Frequency. If we didn't know any better, we might think of this as something like a sound or a pressure wave.

As mentioned, the fact that it stands for Radio Frequency, the word 'radio' means something related to audio for many of us. However, that's not the reality. It's not sound waves or any kind of pressure waves. Instead, it involves light waves.

What's interesting about this is that when we look at it from a human perspective, I'm sitting here, looking at this wireless device, and I don't see any of these light waves coming out. That's just one more reason why we might not recognize them for what they are. In fact, this light cannot be seen by the human eye in the same way that, perhaps in a science class, you conducted an experiment where you listened to different frequencies, spanning a range of sound frequencies. They tuned it really low, and suddenly you couldn't hear anything. Then they turned it up, and you could start to hear this faint beep, which increased in pitch as they raised the frequency. Eventually, it also disappeared. This occurs because there are sound waves with very high frequencies that we can't hear and sound waves with low frequencies that we can't hear, and light behaves in a similar manner.

The audio spectrum, also known as the audio bandwidth, encompasses the audible frequencies of human hearing, which typically range between 20 Hz and 20,000 Hz (20 kHz). The audio spectrum is sometimes divided into the following ranges: bass (20 Hz to 140 Hz), upper bass (140 Hz to 350 Hz), midrange (350 Hz to 2.5 kHz), upper midrange (2.5 kHz to 5 kHz), and high end (5 kHz to 20 kHz). These frequency ranges are not exact and are occasionally divided into 150 Hz, 400 Hz, 2 kHz, and 6 kHz.

We have the visible spectrum in the middle, which we can see, and frequencies that are both lower and higher than the visible spectrum, which we cannot see. For now, just understand that we are discussing light waves that are invisible to the naked eye. Effectively, these wireless devices are transmitting signals to each other. It may seem like an unusual way of thinking about it, but they communicate using different frequencies that they can understand.

To illustrate this, think about how when we're across a lake from each other on a dark night, we can't see each other, but I could use a red or green light to signal you. In fact, this is how boats communicate with each other, using green lights on the left and red lights on the right to indicate direction. Similarly, in the realm of access points and wireless devices, they use various frequencies and waveforms to transmit digital information, such as bits and bytes. Although we often describe it as a wave, it possesses a dual nature.

You may have come across the concept of wave-particle duality. We must delve into the nature of light as both a wave and a particle, as it significantly impacts how light behaves depending on the experimental context.

And so, what this means is that sometimes light behaves as a wave, and at other times, it behaves as a particle. Back in the late 1800s and early 1900s, some were working on understanding this phenomenon. Names like Einstein, Maxwell, Planck, and Niels Bohr are well-known in this context. These scientists conducted numerous experiments to unravel the nature of light. Two of the most famous experiments conducted during this period was the double-slit experiment.

If you recall from your middle or high school science classes, there's a concept of a single slit. With a single slit, light can pass through without any issues. However, if light were composed of particles, it should still be able to pass through. The interesting part comes when you introduce a second slit into the wall, creating two separate openings. When you shine light through these slits, a fascinating phenomenon occurs. It results in interference patterns where light waves interact with each other, leading to bright and dark regions. If light were purely composed of particles, it wouldn't exhibit this behavior. This experiment strongly supports the idea that light behaves like a wave.

To provide a simple analogy, imagine water waves hitting a wall with slits while looking down on a lake. Just as the water waves produce patterns, this illustrates how light behaves as well.

Now, while this suggests that light predominantly behaves as a wave, another famous experiment, known as the photoelectric effect, introduced a different perspective. The photoelectric effect essentially demonstrates that light is absorbed in discrete quantities.

So, when we shine light onto an electric, or more precisely, a metallic surface while studying the electric currents it generates, we discover something intriguing. If light were purely behaving as a wave, it would be an analog concept, implying that we should be able to absorb any amount of energy we project onto this photoelectric plate. However, what was observed is that energy absorption behaves more like a digital process. In simpler terms, we have light behaving as particles here, with light being absorbed one particle at a time. Einstein initially termed these particles 'quanta,' but he later renamed them 'photons.' While the term 'photon' occasionally appears in science fiction, it's also a fundamental concept in real science.

However, this aspect of light behaving as a particle doesn't significantly impact us as wireless engineers. Our primary concern revolves around the fact that light behaves like waves.

Now, let's consider another crucial factor in all of this—the speed of light. Science has firmly established the speed of light as a constant, typically represented by the lowercase letter 'c' has been measured to be approximately 2.99 times 10 to the 8 meters per second.

Speed of light ‘c’ = 299,792,458 m/s (meters per second) or (approximately 300,000 kilometers per second; 186,000 miles per second; 670,616,629 million miles per hour)

We rely on the speed of light in various physics and mathematical equations. You might be familiar with a famous equation, E = mc², which was formulated by Einstein. This equation is known as the theory of relativity, and it essentially states that energy equals mass multiplied by the speed of light squared.

In this equation, 'c' represents the speed of light. However, it's important to note that we typically use the speed of light in a vacuum, meaning there are no obstacles between the source of light and where we measure it. As soon as we start passing light through substances like water or even air, its speed begins to decrease.

For practical purposes, especially in the realm of Wi-Fi, where distances are generally not substantial, we can assume a consistent speed of light. We often use the value of 2.99 times 10 to the power of 8 meters per second in our calculations.

To put this speed into perspective, consider that light from the sun takes approximately 8 minutes to reach us here on Earth.

In an unusual way, when we observe the sun, we're looking back in time, although we should never observe it directly with our eyes. When we study the sun, we see what it looked like minutes ago.

The sun is located about 93 million miles away (or 150 million kilometers), and it's a star known as Alpha Centauri, technically a triple star system. The time it takes for light from Alpha Centauri to reach Earth and become visible to us is over 4 years, approximately 4.35 years to be precise. This concept is what we call a 'light year.' When we refer to a light year, we're not measuring time; we're measuring distance. It represents the distance light can travel in one year. So, if it takes 4.35 years for light to travel from Alpha Centauri to Earth, we say Alpha Centauri is 4.35 light years away.

A crucial point to grasp is that Wi-Fi communication relies on radio frequency waves. While light maintains a constant speed in a vacuum, nearly 2.99 times 10 to the 8 meters per second, it's important to understand that communication signals are not instantaneous.

Signals simply don’t appear instantaneously on the other side of a room or wherever we are transmitting them to. When I flip a light switch, it seems like the light comes on immediately because I can't perceive how fast light travels, especially within an enclosed space like this. However, when we slow everything down and consider factors like multi-pathing and the way wireless signals traverse space, possibly bouncing off different surfaces, it becomes evident that they travel at nearly the speed of light.

When we delve into the workings of Wi-Fi signals, it's more akin to the double-slit experiment than exciting or absorbing particles like photons. Instead, it's about understanding how waveforms behave and how we can manipulate them to convey more information and potentially create directional paths through techniques like beamforming. By combining signals, we can steer them in specific directions. The deeper our comprehension of RF signals operating like light waves, the more effectively we can harness this knowledge to advance our technology and enhance our ability to deploy it.

Happy Learning!!