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Reflection

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Reflection

Looking at yourself in the mirror, being able to see things in general, and hearing echoes in tunnels are all only possible because of reflections. This article is about the reflection of light specifically. We will define reflection, introduce the types and causes of light reflection, look at what laws apply to reflection, and towards the end, we will go through some examples and practical aspects.

The definition of reflection

In principle, light travels in a straight line as long as there is no event to stop it from doing so. A change of materials through which the light is travelling is such an event. Because light is a wave, it may be absorbed, transmitted, reflected, or a combination thereof. A reflection can take place at the boundary between two materials, also called media, and its definition can be stated as follows.

Reflection of light is the change in the direction of light once it hits the boundary between two media and travels back into the original medium. This boundary is called the reflection surface.

Laws of reflection

To discuss the laws of reflection, we need a set-up (see the figure below). For a reflection, we need a reflection surface and an incoming ray of light, and we will automatically have a reflected ray of light that has a different direction than the incoming ray. The reflection surface has a perpendicular line through it called the normal, the incoming (or incident) ray makes an angle of incidenceθiwith the normal, and the reflected ray makes an angle of reflectionθrwith the normal. The laws of reflection are:

  1. The incoming ray, the reflected ray, and the normal to the reflection surface are all in the same plane;
  2. The angle of incidence is the same as the angle of reflection:θi=θr;
  3. The reflected ray is on the other side of the normal than the incoming ray.

Laws 1 and 3 are pretty intuitive and you may not need to remember them explicitly, but law 2 is important to remember. The laws are nicely illustrated in the figure below.

Reflection Diagram of laws of reflection StudySmarterThis 2-dimensional (because of the first law) diagram of reflection illustrates the second and third laws of reflection. A ray of light starts at P, is reflected at O and travels towards Q, Wikimedia Commons CC BY-SA 3.0.

Suppose we have a ray of light and it reflects off a reflection surface to produce a reflected ray of light. The incoming ray makes an angle of20°with the reflection surface. What is the angle of reflection?

Answer: We have to read carefully here: the angle of incidence is the angle with the normal to the reflection surface, so the angle of incidence is90°-20°=70°. The laws of reflection now state that the angle of reflection is also70°.

Being able to draw rays of light that reflect off of surfaces is important. These drawings are called ray diagrams. The figure above is an example of a ray diagram.

Types and causes of light reflection

There are two main types of light reflection, and their causes have to do with the reflection surface being smooth or rough.

Specular reflection

Specular reflection is the reflection that is the result of the reflection surface being smooth.

A mirror and still water are good examples of smooth reflection surfaces. The smoothness of the reflection surface means that the normal to the reflection surface stays the same (in the case of flat reflection surfaces, or plane mirrors) or changes slowly and gradually (in the case of curved surfaces, which we will not go into further in this article) from place to place on the reflection surface. The laws of reflection now say that, because the normal is the same everywhere on the reflection surface, the reflected light from one ray of incoming light will all have the same direction.

This allows for virtual images, which are images of (real) objects that appear to be somewhere that they are not. The reason for this false appearance is that the light that reaches our eyes comes from a direction that is not the direction of the object. See the figure for an illustration. The information that the eye receives will tell it that the object at point A, say an apple, is somewhere behind the mirror (because our brains assume that the light has always travelled in a straight line), while in reality, the apple is in front of the mirror, next to the eye.

Reflection Diagram showing the concept of Virtual image in optics StudySmarterThe eye sees the apple that is at place A through a plane mirror. The direction of the reflected light makes the eye think that the apple is at place A', so it sees a virtual image, Wikimedia Commons.

Diffuse reflection

Diffuse reflection is the reflection that is the result of the reflection surface being rough.

Most regular things have a rough surface: bread, walls, trees, et cetera. The rough surface means that the normal to the reflection surface changes wildly between different places on the surface. The laws of reflection now say that, because the normal is so different, the direction of the reflected ray of light is also different from place to place: the reflected light gets diffused into different directions. See the figure below for an illustration of this effect. This also means that there is no such thing as a mirror image in the case of rough reflection surfaces.

Reflection Diagram of Diffuse and specular reflection StudySmarterIn (a), we see diffuse reflection: the reflected light has different directions. In (b), we see specular reflection: the reflected light has one direction and travels as one ray, Wikimedia Commons CC BY-SA 4.0.

Examples of reflections

Seeing yourself in your bathroom mirror (which is a plane mirror that causes specular reflection) is an everyday example of a reflection of light. Another example of reflections is the diffuse reflection most objects produce, and how these determine what colour we perceive an object to be.

A green plant has a rough surface, so it reflects sunlight diffusely. However, some frequencies of the sunlight are absorbed more than other frequencies, making the reflected light waves have a different frequency spectrum than the incoming sunlight. It is the reflected light wave that hits our eyes, so their spectrum determines which colour we perceive the plant to be, which is green. Apparently, the non-green frequencies of light get absorbed more by the plant than the green frequencies do.

Reflection diagram showing that plants reflects green light StudySmarterThe green leaves of a tree reflect mostly the green frequencies of the incoming light, StudySmarter Originals.

This way, the reflection of light largely determines what colours objects are: red objects will absorb less of the red frequencies than of the other frequencies, etc. However, not only light but all waves can be reflected.

One example is sound waves: they can reflect off tunnel or cave walls and come back to you, making you hear a reverb or echo. Another sound reflection effect most of us have experienced is that sound sometimes appears to come from the side of you where there is a wall, while the sound producer is clearly to your opposite side. The last example of sound reflection is bats using echolocation: they produce a sound themselves and then listen to the echoes produced by the reflection off of walls to figure out where those walls are.

Water waves can also be reflected. When water waves hit a wall, a part of them will be absorbed and the other part will be reflected. The reflections of waves other than light are determined by the same laws of reflection that we talked about earlier!

Importance of light reflection

The reflection of light has been vital for a big part of life on Earth. Most things do not naturally emit visible light, so we can only see them because they reflect the light of the Sun towards our eyes (remember that most things are rough, so there will always be a part of the reflected rays that reaches our eyes). Thus, without light reflection, animals would be blind to everything except the Sun and other things that produce visible light on their own.

Additionally, different materials reflect, transmit, and absorb different colours of light in different quantities. This makes the reflected sunlight coloured. This colour gives animals information about the possible materials that an object is made of.

Reflection - Key takeaways

  • Reflection of light is the change in the direction of light once it hits a reflection surface.
  • There are 3 laws of reflection.
  • Specular reflection is reflection off a smooth surface. This allows for virtual images: images of objects that appear to be somewhere that they are not.
  • Diffuse reflection is reflected off of a rough surface. Most objects have rough surfaces.
  • Not only light can be reflected, but other types of waves as well. The reflection of sound waves can produce echoes and reverbs, and water waves can be reflected off walls. The 3 laws of reflection still apply to these other types of waves.
  • Reflection of light is essential to animals being able to see the world around them: light from the Sun reflects off objects and some of the reflected light reaches the eye of the animal. Without reflection, only light from light-producing objects would reach our eyes.

Frequently Asked Questions about Reflection

The two main types of reflection of light are specular and diffuse reflection. Specular reflection happens when the reflection surface is smooth, so the reflected light travels in one direction. Diffuse reflection happens when the reflection surface is rough. Still water and mirrors cause specular reflection, while most everyday objects cause diffuse reflection.

Reflection of light is the change in the direction of light once it hits the boundary between two media and travels back into the original medium.

An example of reflection is light from the Sun that hits grass, of which a part is reflected towards your eye. This is how you see the grass.

The laws of reflection state that the angle of reflection is equal to the angle of incidence.

The importance of reflection of light is that it enables animals to see. Most objects do not emit a lot of visible light themselves, so animals rely on the light from the Sun that reflects off objects towards their eyes to see those objects.

Final Reflection Quiz

Question

Does light always change direction with a reflection?

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Answer

Yes

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Question

The angle of incidence is the angle between...

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

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The angle of reflection is the angle between...

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... 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

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Answer

normal

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Question

Reflection off a smooth surface is called

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specular

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Reflection off a rough surface is called

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diffuse

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Question

Which type of reflection causes virtual images?

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Answer

Specular reflection

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What kind of reflection do some bats use?

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Answer

Sound reflection / echolocation

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Which waves do NOT follow the laws of reflection?

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Answer

None. All waves follow the laws of reflection.

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What type of reflection do animals use to see things?

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Diffuse

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What is reflection of light?

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

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

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Answer

True

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Question

Why do we see a virtual image behind the mirror?

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Answer

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

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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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What is the name of a flat smooth reflection surface?

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Answer

Plane mirror

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Question

If an object is red, what can be said about the reflection of light by that object?

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Answer

The object absorbs and/or transmits non-red light more than red light. Equivalently, the object reflects more red light than non-red light.

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