Chapter 6

Introduction

Throughout Part II, we have explored the fundamental nature of light.

We discovered that light behaves as both a wave and a particle, spans a vast electromagnetic spectrum, is emitted by hot objects according to their temperature, and that every photon carries a precise amount of energy determined by its wavelength. We also learned that when light encounters matter, it may be reflected, transmitted or absorbed.

Now it is time to bring all of these ideas together.

The sunlight reaching Earth is not simply a random mixture of colours. It is the result of an extraordinary journey that began deep inside the Sun, continued through its atmosphere, crossed almost 150 million kilometres of space, and finally passed through Earth's atmosphere before arriving at the ground.

Each stage subtly changes the light along the way. The result is the solar spectrum: the true distribution of sunlight that reaches Earth's surface.

What is a Spectrum?

Imagine collecting every photon arriving from the Sun and sorting them according to their wavelength. Some wavelengths would appear in enormous numbers. Others would be comparatively rare.

If we plotted this information on a graph, we would obtain a spectrum.

Rather than asking, "How much sunlight is there?", a spectrum asks a much more revealing question: "How much sunlight exists at each wavelength?"

On a solar spectrum graph, the horizontal axis represents wavelength, while the vertical axis shows how much energy is present at each wavelength. This simple graph allows scientists to see exactly how sunlight is distributed across the electromagnetic spectrum.

The Sun's Fingerprint

If the Sun were a perfect black body, its spectrum would form a perfectly smooth curve.

It doesn't.

As photons travel through the Sun's outer atmosphere, atoms absorb very specific wavelengths before the light escapes into space. These missing wavelengths appear as hundreds of thin dark lines crossing the spectrum.

They are known as Fraunhofer lines.

Each chemical element absorbs its own unique pattern of wavelengths, almost like a fingerprint. By studying these patterns, astronomers can determine what the Sun is made of without ever leaving Earth. The same technique is now used to study distant stars and even the atmospheres of planets orbiting other suns.

The Final Part of the Journey

After leaving the Sun, sunlight travels almost 150 million kilometres through the near-perfect vacuum of space. For almost the entire journey, nothing changes.

The final few hundred kilometres are very different.

As sunlight passes through Earth's atmosphere, photons begin interacting with molecules of oxygen, ozone, water vapour and carbon dioxide. Some wavelengths are absorbed, while others are scattered in different directions. Clouds, dust and pollution can remove even more light before it reaches the ground.

As a result, the sunlight arriving at Earth's surface is no longer identical to the light that originally left the Sun. Fortunately, Earth's atmosphere is largely transparent to visible light, allowing much of the Sun's energy to reach the surface.

Atmospheric Windows

The atmosphere does not absorb every wavelength equally. Instead, there are certain regions where radiation passes through almost unhindered.

These regions are known as atmospheric windows.

The largest atmospheric window lies within the visible part of the electromagnetic spectrum. This is why so much of the Sun's visible light reaches Earth's surface, while many ultraviolet and infrared wavelengths are partially absorbed before arriving at the ground.

One of the most familiar examples of atmospheric scattering is the colour of the sky. Shorter blue wavelengths are scattered much more strongly than longer red wavelengths, causing the sky to appear blue during the day and the Sun to appear redder at sunrise and sunset.

Air Mass

The amount of atmosphere sunlight passes through depends on the position of the Sun in the sky. When the Sun is directly overhead, sunlight follows its shortest possible path through the atmosphere.

As the Sun moves closer to the horizon, the light must travel through a much thicker layer of air before reaching the ground.

This effect is described using Air Mass (AM).

An Air Mass of 1 represents the shortest atmospheric path, while larger Air Mass values represent progressively longer paths through the atmosphere.

The greater the Air Mass, the more sunlight is absorbed and scattered, reducing the intensity of sunlight reaching Earth's surface.

Standard Solar Spectra

Because the atmosphere changes sunlight, scientists use standard solar spectra when comparing measurements.

AM0 represents sunlight measured outside Earth's atmosphere and is used for satellites and spacecraft.

Most solar energy systems, however, operate on Earth's surface. For this reason, engineers commonly use AM1.5, which represents sunlight that has travelled through one and a half times the minimum atmospheric thickness.

These standard spectra allow measurements taken in different places and at different times to be compared under consistent conditions.

Looking Ahead

We have now completed our journey through the world of light.

From its birth inside the Sun to its arrival at Earth's surface, we have discovered what light is, how it is produced, how it travels, how it interacts with matter, and why the sunlight reaching Earth has the spectrum that it does.

But our story is only half complete.

To understand how sunlight can eventually be transformed into electricity, we must now turn our attention away from the photon and towards the material it strikes.

In Part 3, we'll explore the atomic structure of matter and discover why certain materials, such as silicon, are able to harness the energy carried by light.