Introduction

If you’re anything like me, then at some point in your life you will have had one of those moments when you have a deep, philosophical thought about what light is, or what a shadow is, or how colours are produced, or why things look distorted in water. Alternatively, if you’ve never thought in too much detail about this – have a think about it now.

What even is ‘light’ anyway?

This chapter will seek to answer these questions by reviewing some key physical principles and defining some very important terms. To this end, this chapter will lay the foundation for the rest of the book, so please make sure to obtain a solid understanding of this content before moving on.

What is light?

When we talk about ‘light’ as something that allows us to see objects that exist in the real world, we are actually talking specifically about something referred to as ‘visible light’. The special feature of visible light is that it’s detectable by the sensory receptors in the human eye, which is how we use light to help us to see. It can originate from natural (e.g. from the sun), or artificial (e.g. from a lamp) sources, but in all cases, visible light will illuminate (light up) objects, allowing us to perceive their existence.

However, when we start to think more deeply about what light is, or what it’s composed of, things start to become a little more complicated. For example, research into whether light could be classed as ‘particles’ or as ‘waves’ went on for centuries before scientists finally agreed that light must possess something called wave-particle duality, meaning it exhibits properties of both particles and waves ( Box 1.1 ), and this is particularly relevant when we start to think about the energy associated with the electromagnetic spectrum .

• BOX 1.1

What Is the Difference Between a Particle and a Wave?

Let’s start by using a football as an example particle. If you have a football sitting stationary on the ground in front of you, then the particle is ‘at rest’, but if you approach the football at speed and kick it like Harry Kane, then it will (hopefully) go flying off into the distance. This happens because you have transferred energy to the football – in this case, kinetic (movement-related) energy. This energy allows the football to travel the distance required to land safely between two goalposts (or a neighbour’s garden).

However, waves are different. For this example, let’s think of dropping a stone into a still pond. We would expect the kinetic energy from the stone to move the water and produce a ripple, emanating outwards and taking the energy with it. However, even in a single ripple, the energy is contained within the full ‘wave’, meaning that with waves the energy is much more spread out ( Fig. B1.1 ).

Diagram showing the difference in how energy transfers in a particle relative to a wave. Blue arrows indicate the direction of the energy.

The key difference, then, is that particles will collide with each other and change direction, whereas waves will pass through one another (imagine two footballs colliding relative to two ripples in a pond). This idea will be covered in more detail in later chapters of the book.

It can be slightly bizarre to think of ‘light’ as a type of energy, so let’s discuss that in more detail. The energy packets (quanta) associated with the electromagnetic spectrum are called photons , and these are considered the basic unit of all light. The amount of energy emitted per photon is dictated by its wavelength.

Wavelength is a term to describe the distance between two equal points on a wave. In Fig. 1.1 , you can see that on the shorter wavelength, if I take the distance between two peaks (the very top of a wave), then it should be identical to taking the distance between two troughs (the very bottom of a wave). Typically we measure distances from peak to peak because it is easier to identify the peak of a wave in a diagram than somewhere halfway up. Also, because it is a measure of distance, we need to use distance-related units, for example, nanometres, millimetres, centimetres, or metres.

Two example waves, showing identification of wavelength.

Importantly, the wavelength of a light source is also related to its frequency . Frequency is defined as the number of complete cycles of the wave that pass any given point in 1 second. So, for example, if we shine a laser light at a wall and measure how many full cycles pass a point halfway along the beam within 1 second, we can calculate the wavelength (λ; lambda) by using Equation 1.1 to divide speed of light (c) by frequency (ν; nu) ( Box 1.2 ). This equation also highlights that as wavelength decreases , frequency should increase , which makes sense because if the distance between two corresponding points of the wave is small, more complete cycles should pass through a particular point within a second, specifying a higher frequency.

• BOX 1.2

The Speed of Light

The speed of light, denoted in equations by the letter c (which stands for ‘constant’), is a known speed, recorded to be: 299,792,458 m/s. However, it is important to note that this is the speed that light is recorded to travel in a vacuum, such as that found in outer space. When light is on Earth (in the atmosphere), it travels almost as fast as it would in a vacuum, but if the light comes into contact with any object/material, then it can be slowed down if it is made to change direction slightly. One easy way to think about this is that when light is in a vacuum, there are no electrons or particles to get in the way – it’s just smooth sailing. However, in the Earth’s atmosphere, or in water, the light will need to take very minuscule detours every time it comes into contact with an atom – which inevitably slows it down ( Fig. B1.2 ).

Diagram showing difference in how light (blue arrow) travels unimpeded in a vacuum (left), relative to small detours in the Earth’s atmosphere (right).

For example, if light passes through a diamond (a high-density object), it will be slowed down to 124,000,000 m/s, which is still a great deal faster than you or I could ever hope to run, but it’s been reduced to less than half of the speed of light in a vacuum!

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