Chapter 1

Introduction

In Part I, we followed the long chain of events that led to the birth of the Sun. Gravity gathered hydrogen into an enormous cloud. The cloud collapsed into a protostar. Its core grew hotter and denser until nuclear fusion finally began. The Sun was born.

From that moment onward, it began releasing extraordinary amounts of energy into space. Some of that energy eventually reaches Earth as sunlight. We see it every day. We feel it warming our skin. Plants use it to grow, weather systems are driven by it, and solar panels convert it into electricity.

Light feels familiar. But it is one of the strangest things in nature. It can cross empty space without requiring air, water or any other material to carry it. It travels faster than anything else in the Universe. In some experiments it behaves like a wave, while in others it behaves like a stream of particles.

Before we can understand how sunlight becomes electricity, we first need to answer a more fundamental question. What is light?

Electromagnetic Radiation

Light is a form of electromagnetic radiation. Visible light is only the small portion that human eyes can detect. It belongs to a much larger family that also includes radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays and gamma rays.

These forms of radiation can produce very different effects, but they are all expressions of the same underlying phenomenon. They are all made from changing electric and magnetic fields travelling through space. To understand what that means, we first need to understand the idea of a field.

Electric and Magnetic Fields

To understand how light travels through empty space, we first need to understand the idea of a field. 

A field is a region of space in which an object can experience a force without being in direct contact with the source of that force.

Gravity provides a familiar example. The Earth creates a gravitational field around itself, causing any object within that field to experience a pull towards the planet, even though nothing physical connects the two.

Electric charges behave in a similar way. A charged particle, such as an electron or proton, creates an electric field around itself. The strength of which decreases the further away you get from the charged particle (the same way gravity works). If another charged particle enters this field, it experiences an electric force. Opposite charges attract one another, while like charges repel.

But something remarkable happens when the electric field changes for example, when the charged particle moves.

In addition to its electric field, as well as having an electric field a moving charge also creates a magnetic field around itself. Unlike an electric field, which acts directly on electric charges whether they are moving or stationary, a magnetic field mainly exerts forces on moving charges and on magnetic materials such as iron.

A useful way to think about the difference is this: an electric field behaves like someone pushing or pulling a shopping trolley, causing it to speed up or slow down. A magnetic field is more like someone pushing a moving trolley from the side, causing it to change direction instead. Rather than simply attracting or repelling charges, magnetic fields tend to steer moving charges onto a new path.

(Sidenote on why iron is magnetic):

Every electron behaves like a tiny magnet. In most materials these tiny magnetic fields point in random directions, cancelling one another out so that the material has almost no overall magnetism. Iron is different. Inside iron, enormous numbers of electrons naturally align so that many of their tiny magnetic fields point in the same direction. Instead of cancelling one another out, they combine to produce a much stronger magnetic field, allowing the material to behave as a magnet.

Maxwell's Discovery

For centuries, electricity and magnetism were thought to be completely separate phenomena. Electricity explained the behaviour of charged particles, while magnetism seemed to belong only to magnets.

During the nineteenth century, scientists such as Michael Faraday carried out experiments showing that electricity and magnetism were somehow connected. For example, Faraday discovered that moving a magnet through a coil of wire produced an electric current. Although the connection was clear, nobody fully understood why.

Scottish physicist James Clerk Maxwell studied these experimental results and developed a set of mathematical equations that united electricity and magnetism into a single theory called electromagnetism.

His equations revealed something extraordinary. Whenever an electric field changes, it creates a magnetic field. Likewise, a changing magnetic field creates a new electric field.

A useful analogy is a pair of dominoes standing side by side. When one falls, it knocks over the other. That domino then knocks over the next, creating a chain reaction that continues by itself. In a similar way, a changing electric field creates a magnetic field, which creates another electric field, and so on.

Rather than remaining in one place, this chain reaction travels through space as a self-sustaining disturbance known as an electromagnetic wave. Light is one example of an electromagnetic wave.

Light as a Wave

Imagine dropping a stone into a still pond. Ripples spread outward from the point of impact, carrying energy across the surface. The water moves locally as the disturbance passes, but the entire body of water does not travel across the pond. The wave carries the energy. Light behaves in a broadly similar way.

Instead of ripples in water, light consists of oscillating electric and magnetic fields moving through space. There is, however, one crucial difference. Water waves require water. Sound waves require air, liquid or solid matter through which vibrations can pass. Light requires no material medium at all. Electromagnetic waves can travel through the vacuum of space.

This is why sunlight can leave the Sun and cross almost 150 million kilometres of near-empty space before reaching Earth. In a vacuum, light travels at approximately 300,000 kilometres per second. Even at this enormous speed, sunlight still takes around eight minutes and twenty seconds to reach us.

Whenever you look at the Sun, you are therefore seeing it as it was more than eight minutes earlier.

Light as a Particle

The wave model explained many properties of light, including reflection, refraction, diffraction and interference. For a time, it seemed that the mystery had been solved. Then experiments revealed something the wave model alone could not explain. When light interacts with matter, it sometimes behaves as though its energy arrives in individual packets.

These packets are called photons. A photon is a single quantum of electromagnetic radiation.

When sunlight reaches a surface, it does not transfer its energy as one perfectly smooth flow. Instead, enormous numbers of individual photons arrive and interact separately with the atoms and electrons in the material.

This particle-like behaviour is essential to solar technology.

A photovoltaic cell absorbs sunlight through individual interactions between photons and electrons.

The Photoelectric Effect

One of the clearest demonstrations of the particle-like nature of light came from the photoelectric effect. When light shines onto certain materials, electrons can be released from their surface. Scientists initially expected that making the light brighter would always give the electrons more energy. 

That was not what they observed.

Some forms of light could release electrons immediately. Other forms could not release electrons at all, even when the light was made much more intense. 

Albert Einstein explained this by proposing that light transfers energy in individual photons. If a photon carries enough energy, it can transfer that energy to an electron. If it does not, simply adding more low-energy photons does not make each one more powerful.

This idea helped establish the photon as a fundamental part of modern physics. It also revealed the basic principle behind photovoltaic conversion. Light can transfer energy directly to electrons inside a material.

Wave-Particle Duality

We are now left with an apparent contradiction. Light travels like a wave. Light interacts like a particle.

So which description is correct? Well... both are.

Light is not a classical wave like a ripple on water. It is not a classical particle like a tiny solid ball. It is a quantum object that can display wave-like or particle-like behaviour depending on the situation. Its wave nature explains how it travels, spreads, reflects and interferes. Its particle nature explains how it transfers energy to matter.

This behaviour is known as wave–particle duality.

Although it challenges our everyday intuition, it has been confirmed repeatedly through experiments and forms the foundation of modern technologies including lasers, fibre-optic communications, semiconductor electronics and photovoltaic cells.

Why This Matters for Solar Energy

Solar engineering relies on both descriptions of light.

The wave model helps us understand how sunlight travels and how different wavelengths behave. The photon model helps us understand how sunlight transfers energy to electrons inside a semiconductor.

When sunlight reaches a solar cell, some of it is reflected. Some passes through. Some is absorbed. When absorption occurs, the energy of a photon may be transferred to an electron inside the material.

That interaction is the first step in producing electricity.

Before we explore what happens inside the solar cell, however, we need to understand the wider family of radiation to which sunlight belongs.

Looking Ahead

We now know that sunlight is a form of electromagnetic radiation.

It travels through space as changing electric and magnetic fields, but it transfers energy in individual packets called photons.

Light is therefore neither simply a wave nor simply a particle. It behaves as both. The next question is how the different forms of electromagnetic radiation are organised.

What separates a radio wave from visible light?

What makes infrared different from ultraviolet?

And where does sunlight fit within the wider picture?

To answer those questions, we must explore the electromagnetic spectrum.