Chapter 5

Introduction

Throughout this part, we have discovered that light behaves as both a wave and a particle, explored the electromagnetic spectrum, learned why hot objects emit light, and seen that every photon carries a precise amount of energy determined by its wavelength. But simply carrying energy is not enough.

For anything to happen, the photon must first interact with matter. Whether it strikes a leaf, a window or a solar panel, every encounter begins in one of three ways.

Three Possible Outcomes

Whenever a photon reaches a material, one of three things can happen:

1. It may be reflected, bouncing away from the surface.

2. It may be transmitted, passing through the material.

3. Or it may be absorbed, transferring its energy to the material.

Every interaction between light and matter can be described using these three simple possibilities.

Reflection

Not every photon enters a material.

Some simply bounce off its surface, a process known as reflection.

Reflection is the reason we can see the world around us. Light reflects from objects into our eyes, allowing us to perceive their shape, colour and texture. Without reflected light, even the brightest object would be invisible. Highly polished surfaces, such as mirrors, reflect almost all the light that reaches them, while rougher or darker surfaces reflect much less.

Transmission

Some materials allow light to pass straight through them. 

This process is known as transmission.

Glass, water and air are all largely transparent to visible light, allowing most photons to travel through with very little interaction. This is why sunlight can pass through a window while still producing a faint reflection from its surface. The amount of light transmitted depends on both the material and the wavelength of the light.

Absorption

Sometimes a photon never emerges.

Instead, it transfers all of its energy to the material it has struck. This process is called absorption.

What happens next depends on both the energy carried by the photon and the material itself. The absorbed energy may become heat, trigger a chemical reaction, or be transferred to one of the material's electrons. This will be covered in more detail in Part 3.

Although these outcomes appear very different, they all begin with the same event: a photon giving up its energy.

Looking Ahead

We have now explored the fundamental ways in which light interacts with matter. Whether a photon is reflected, transmitted or absorbed depends on both the energy it carries and the material it encounters.

But before a photon can strike a solar panel, it must first complete an extraordinary journey.

After being created deep inside the Sun, it escapes through the Sun's atmosphere, travels almost 150 million kilometres across space, and finally passes through Earth's atmosphere before reaching the ground.

Each stage subtly changes the sunlight that eventually arrives at Earth's surface.

In the next chapter, we'll bring everything we've learned together as we explore the solar spectrum, the true distribution of sunlight that reaches our planet.