Light

If you have ever spent a day at the beach, you would have noticed waves breaking against the shore. Light doesn’t immediately fit this description of a wave, even though it is a wave. Light waves are periodic, just like the water waves in the ocean. They can also experience properties of wave motion that are usually associated with other types of waves, such as reflection, refraction and diffraction, to state a few examples.

The definition of light

Before we can discuss how light behaves, we must be able to identify what light is. There are many descriptions that can be used to classify waves but to avoid confusion, we will consider light as purely a wave. We can then define it as below.

The nature of light

We have stated that light is a wave, suggesting that every ray of light should have an associated wavelength and travel in a straight line.

Light waves can have different wavelengths, which determine the colour of visible light that is observed. This means that every time you view a beautiful red rose on a sunny day, your eyes perceive a reflection of red light from the rose, which has a wavelength of about 650 nm.

We can be more precise now and say that the nature of light is that of a transverse wave. It carries energy from one point to another, just as all travelling waves do. The word transverse is simply a reference to the fact that electric and magnetic fields are oscillating (waving), hence the word electromagnetic.

White light, like light from the sun, consists of all the wavelengths of visible light together. We know this because light exhibits a property called dispersion when passing through a glass prism, as shown in the figure below.

White light is dispersed through a prism into all the colours in the visible spectrum, Wikimedia Commons CC BY-SA 4.0

White light, once dispersed, gives us all the colours in the visible electromagnetic spectrum, which is what you would see in a rainbow. Red light has a wavelength of about 700 nanometres (700 x 10⁹ metres), while violet light at the opposite end of the electromagnetic spectrum has a wavelength of about 380 nanometres.

The speed of light

All travelling waves can essentially be thought of as energy-carrying oscillations in motion. This means that light waves should be no different, and if light is in motion, it must have a speed. It turns out that light not only has a speed but that its speed is the fastest in the universe. The speed of light in free space, c, is 300 million metres per second (3.00 x 10⁸ m/s), which is a constant and is essentially the speed limit of the universe that cannot be exceeded by any other object.

This statement tells us that irrespective of the colour (or wavelength) of light, its speed is always the same. We know, however, from the topic of waves that the speed of a wave v, or in this case, c, can be written as follows:

$$\begin{gathered} \text{speed = frequency × wavelength} \\ c=f\lambda \end{gathered}$$

Here, we know that the wavelength of the light is λ, and the frequency of the wave is f. This is known as the wave equation. Therefore, to keep c constant, the wavelength and frequency of the wave must be inversely proportional (i.e., an increase in one leads to a decrease in the other and vice versa).

Let us use this equation to find the wavelength of red light in the following example.

The properties of light

There are many properties of light waves, but we will discuss two in detail: reflection and refraction. These are two of the more important properties of light, which can be used to make observations of distant objects in the universe.

Reflection

We have mentioned before that the colour of a rose is due to the reflection of visible light from the rose. Reflection is a property of wave motion that is exhibited by visible light. Reflection occurs when light that is travelling through a certain medium is incident onto a boundary between two media, and the light ‘bounces’ off the boundary or changes direction upon striking this boundary and moves on in the original medium. For an illustration of this, see the diagram below.

A ray of light travelling through the air is incident on a plane mirror. It is reflected, that is, it changes direction and remains in the air, Wikimedia Commons CC BY 4.0

We can see that the angle with respect to the plane mirror is the same for the incoming electromagnetic ray as it is for the reflected ray. Picture yourself standing in the path of the reflected ray and replace the plane mirror with a calm blue lake. The lake reflects the visible light rays from the sky (clouds and all) and sends the reflected rays into your eyes, which is why you can view the sky above whilst looking down.

Refraction

Refraction is another property of wave motion that is displayed by visible light. It occurs when light rays move from one medium to another and, upon doing so, change direction and speed. This is illustrated by the red arrow in the figure below.

A ray of light strikes the boundary between two media. It undergoes refraction by changing direction and moving along the red line, adapted from an image by CNX OpenStax CC BY 4.0

The types of media and the wavelength (colour) of incident light determine the extent to which a light ray is refracted when moving between the media. Refraction is the reason why objects in a deep pool or lake seem closer to the surface than they actually are. This phenomenon is called apparent depth and has tricked many fisherfolks in the past.

Other properties of light

Reflection and refraction are not the only properties of light waves, but they are two of the more important ones. Other properties of light include interference, diffraction, polarisation, scattering, and dispersion. Like apparent depth, a further study of each of these properties will give us insights into phenomena we experience every day but can’t seem to explain.