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# Light

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If you have ever spent a day at the beach, you would have noticed waves breaking against the shore. Light doesnt 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.

Light is defined as an electromagnetic wave with a wavelength in the visible part of the electromagnetic spectrum (380 to 700 nanometres). That is, light is any electromagnetic wave that we can see with our eyes.

## 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.

The wavelength is the distance between any two successive points on a wave that are in phase (e.g., two successive crests or two successive troughs).

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.

Its important to note that light has different speeds in different materials but that speed c is in a vacuum.

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:

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).

The frequency f of a wave is the number of complete oscillations in the wave passing a fixed point every second. Frequency is measured in s-1 or equivalently Hz.

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

What is the wavelength of the red light that is emitted from a 4.6 x 1014 Hz laser?

We know the frequency of the laser light and that the speed of light c is 3.00 x 108 m/s, so we can apply the wave equation:

This wavelength corresponds to the colour red.

## 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.

The illustration below shows a pencil placed in a beaker of water. The bottom end of the pencil lies at point X but actually appears to be located at Y to someone observing from above. We say that the apparent depth of the bottom end of the pencil is the distance from Y to the surface.

An illustration of apparent depth. A pencil with its bottom end at X actually appears to have its bottom end at Y due to refraction, Wikimedia Commons CC BY-SA 3.0

### 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 cant seem to explain.

## Light - Key takeaways

• Light is a wave and falls in the visible part of the electromagnetic spectrum.
• Light waves travel in straight lines.
• The wavelength of a visible light wave determines its colour.
• Violet light has a wavelength of 380 nanometres, while red light at the other end of the visible part of the electromagnetic spectrum has a wavelength of 700 nanometres.
• The speed of light in free space c is 300 million metres per second (3.00 x 108 m/s) and is the fastest speed of any object in the universe.
• The wave equation is given by c = f x λ, where λ is the wavelength of light while f is the frequency.
• Reflection is a property of light whereby a ray of light strikes the boundary between two media and stays in its original medium.

• Refraction is a property of light whereby a ray of light strikes the boundary between two media and moves from the first medium into the second, changing direction and speed in the process.

• The apparent depth of objects submerged in water can be explained by refraction.

• Other properties of light include interference, diffraction, polarisation, scattering, and dispersion.

Light behaves both as a wave and particle. Light has a speed of 300 million metres per second.

The types of light are the different colours that correspond to different wavelengths in the visible region of the electromagnetic spectrum.

Light can act as both a particle and a wave.

Light is an electromagnetic wave that carries energy in the visible spectrum. Humans can perceive light waves with their eyes.

The properties of light are refraction, reflection, diffraction, interference, dispersion, polarisation, and scattering.

## Final Light Quiz

Question

What is the nature of light?

Wave

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Question

Light waves travel along curved paths

False

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Question

What determines the colour of a light wave?

Wavelength/Frequency

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Question

What colour of light has wavelength of about 380 nanometres?

Violet

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Question

What colour of light has a wavelength of about 700 nanometres?

Red

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Question

The frequency of a wave is the time taken for a complete oscillation to occur.

False

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Question

Which of the following is the correct unit of measurement for frequency?

s-1

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Question

What property of light occurs when light is incident on a boundary between two media and moves from the first medium into the second, and changes direction and speed in the process?

Refraction

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Question

What property of light occurs when a light wave strikes the boundary between two media and stays in its original medium?

Reflection

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Question

Which property of light can be used to explain the phenomenon of apparent depth for a submerged object?

Refraction

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Question

There are other properties of light waves other than reflection and refraction.

True

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Question

Which of these is a property of light waves?

All of these

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Question

Being able to view the image of the sky in a clear blue lake is an example of which property of light waves?

Reflection

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Question

Does light always change direction with a reflection?

Yes

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Question

The angle of incidence is the angle between...

... the incoming ray of light and the normal to the reflection surface

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Question

The angle of reflection is the angle between...

... the reflected ray of light and the normal to the reflection surface

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Question

The perpendicular line through the reflection surface is called the

normal

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Reflection off a smooth surface is called

specular

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Reflection off a rough surface is called

diffuse

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Question

Which type of reflection causes virtual images?

Specular reflection

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Question

What kind of reflection do some bats use?

Sound reflection / echolocation

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Question

Which waves do NOT follow the laws of reflection?

None. All waves follow the laws of reflection.

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Question

What type of reflection do animals use to see things?

Diffuse

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Question

What is reflection of light?

Reflection of light is the change in the direction of light once it hits a reflection surface.

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Question

The angle of incidence is ALWAYS equal to the angle of reflection.

True

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Question

Why do we see a virtual image behind the mirror?

The light from the object is coming from the direction of the mirror, so our brain thinks that must be the direction of the object in the real world.

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Question

Why does reflected light get diffused in the case of a rough surface?

The rough surface means that the normal to the reflection surface differs a lot from place to place, so the angles of reflection also differ a lot from place to place.

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Question

What is the name of a flat smooth reflection surface?

Plane mirror

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Question

When light rays fall on a mirror it is ...

Reflected.

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Question

A ray of light reflects from a plane mirror with an angle of reflection of 32 degrees. What was its angle of incidence?

32 degrees.

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Question

Which surface would provide the best reflection?

Shiny metal surface.

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Question

An image produced by a plane mirror is ...

All the options.

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Question

The light ray which falls on the mirror is ...

Incident ray.

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The size of the image formed by a plane mirror is ...

Same as the object.

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Question

What is the law of reflection?

Angle of incidence = Angle of reflection

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Question

The type of reflection when light is reflected in many directions ...

Diffusion reflection

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Question

A plane mirror always produces a virtual image.

False

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Question

The angles of incidence and reflection are measured with the ...

Normal to the surface of the mirror.

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Question

Properties of an image produced by a convex mirror

Virtual and upright

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Question

Which of the following images can be projected onto a screen?

Real image.

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Question

In a plane mirror, the distance of the image and object from the mirror is

Equal

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Question

What type of reflection occurs when the light falls on a rough uneven surface?

Specular

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Question

What is refraction of light?

Refraction of light is the change in the direction of light once it passes the interface of two media.

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Question

What is a qualitative difference between refraction and reflection?

A wave does not enter the other material in the case of a reflection, but it does continue into the other material in the case of a refraction.

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Question

What is the definition of the refractive index n of a material?

The refractive index is given by n=c/v, where c is the speed of light in a vacuum and v is the speed of light in the material.

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Question

What do you know about the refractive index n of diamond and why?

n>1, because diamond is not a vacuum and therefore light travels through it more slowly than through a vacuum.

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Question

If there is an interface of two different materials that have the same refractive index, what do we know about refraction at this interface?

The angle of incidence is equal to the angle of refraction.

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Question

There is only a critical angle if you go from...

... high to low refractive index.

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Question

The angle with the normal is smaller in the material with the...

... higher refractive index.

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Question

Name an invention that relies heavily on refraction.

Own answer. Possible correct answers are lenses (in glasses or eyes), microscopes, telescopes, prisms, magnifying glasses.

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