Physics Tutorial: Reflection of Light

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In this Physics tutorial, you will learn:

What is reflection of waves in general and that of light in particular?
How many types of reflection are there?
How do the features of reflecting surfaces affect the reflection of light?
What happens to light when it falls on objects?
Why do we use the concept of normal line in reflection?
What do the Laws of Reflection say?

Introduction to Reflection of Light

How it is possible that you can see the image of yourself on a mirror but you cannot see it on a wall or a door?

Why do we clearly see objects around us during the day despite we are inside a building and we are not directly exposed to sunlight?

Why are we burned at beach even when we stay all the day under the umbrella?

Why it seems like being a water pond at the end of the road in front of us when we are driving on a straight road during a hot summer day?

All these questions are related to a very important phenomenon that occurs in waves in general and in light waves in particular we will discuss in this tutorial. It is known as reflection and it is an integral part of our everyday lives.

What is Reflection?

We have discussed about situations in which a wave encounters a small obstacle and therefore, the wave passes around this obstacle but it still moves in the original direction. Such phenomenon is known as diffraction, as discussed in our physics tutorial on the Diffraction of Waves.

But what happens when a wave encounters a large obstacle on its way? In such cases, the wave cannot pass around the obstacle but it turns back after colliding with the obstacle as shown in the figure. We say the wave reflects.

Physics Tutorials: This image provides visual information for the physics tutorial Reflection of Light

By definition, reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples of waves' reflection include the reflection of sound and water waves and especially of light waves.

Reflection of Light. Types of Reflection

Being a wave (EM wave), light also reflects when it encounters a large obstacle. During this process, the light bounces off the obstacle and changes its original direction.

Things to consider when dealing with reflection of light:

If the reflective surface is ideal, light wave does not loose energy. As a result, the reflected wave will have the same features as the original wave (frequency, wavelength, amplitude, speed etc.) except direction.
The best reflective surfaces are those surfaces that are 100% flat and smooth. This is very difficult to achieve as all surfaces have a number of holes, which distort their smoothness. However, flat surfaces made of glass are very close to be smooth. Therefore, we often consider glass mirrors when discussing about reflection of light.

There are two kinds of reflection based on the properties of reflecting surfaces:

a) Regular reflection

In regular reflection, the reflected light beam has the same shape as the original one. Regular reflection occurs when the reflecting surface is very smooth as shown in the figure below.

b) Diffuse reflection

In this type of reflection, rays forming a regular beam fall on a non-regular (rough) surface. As a result, rays are reflected in different angles (they diffuse) as shown in the figure below.

Diffuse reflection is the reason why we see objects around us despite that they may not be exposed to direct sunlight. Indeed, most of rays coming from the Sun are absorbed by objects on which the sunlight falls. As a result, these objects become hot. Only the rays composed by light waves which have the frequency corresponding to the colours of objects are reflected by objects' surface to our eyes. As a result, we are able to see both the shape and colours of objects illuminated by the sunlight.

But why rough (matt) surfaces are bad reflectors of light? This occurs because a light ray makes a number of collisions on objects surface before moving away from it. As a result, the light loses energy every time it collides, making the reflection more difficult as shown in the figure below in which light energy is indicated by the ray thickness.

What Happens to Light When it Falls on Objects?

There are three possible situations a light ray may experience when it falls on objects.

Reflection. It occurs when light does not penetrate inside the object but is repelled by the objects surface.
Absorption. It occurs when light encounters a rough surface and it is absorbed by the object after making a number of collisions on its surface.
Emission. It occurs when the absorbed light wave is too energetic and therefore, the object releases a part of it into the environment.

Look at the figure:

In general, good absorbers are also good emitters of light. This is because a material that accumulates large amount of sun radiation through absorption is more likely to emit some part of this radiation. Matt (rough) and dark coloured (possibly black) surfaces are good absorbers of sun radiation. This is the reason why we prefer to wear black clothes in winter.

On the other hand, bright (possibly white) coloured and smooth (flat) surfaces are bad absorbers of light as it is mostly reflected by such surfaces. As a result, they cannot accumulate energy through absorption. This is the reason why we prefer to wear white clothes in summer.

Laws of Reflection

First we must introduce the concept of normal line which is very helpful when dealing with the laws of reflection. Thus, normal line is a line that starts at the point in which the light ray touches the surface and is perpendicular to this surface as shown in the figure.

Said this, we will now explain the two laws of reflection.

1. Incident ray, reflected ray and the normal line lie all at the same plane

This means if you observe the situation from aside you will see only one ray.

2. The incident angle is equal to the reflection angle

The above angles are taken from the given ray to the normal line, not from the ray to the reflecting surface. This avoids issues arisen from irregular reflecting surfaces in calculating the angles.

In the figure above, î = ȓ based on the second law of reflection. This law is very important in determining the path direction of light rays after reflection.

Example 1

Two flat mirrors are placed at 1000 to each other and the incident ray forms a 700 angle to the first mirror as shown in the figure.

What is the angle formed by the reflecting ray on the second mirror to the surface of this mirror?

Solution 1

From the 2nd Law of Reflection, it is clear that the reflection angle to the first mirror is 700 as well. Therefore, its complementary angle is 900 - 700 = 200.

In the triangle formed by the two mirrors and the reflected ray to the first mirror, we therefore have two known angles: 200 and 1000. Hence, the third angle in this triangle will be

3rd angle of triangle = 180⁰ - (20⁰ + 100⁰ )
= 180⁰ - 120⁰
= 60⁰

Again, based on the 2nd Law of reflection, it is easy to deduce that this value corresponds to the unknown angle x. Thus, we have x = 600.

Look at the figure in which the values of all angles involved are shown.

Physics Revision: Reflection of Light Summary

Things to consider when dealing with reflection of light:

If the reflective surface is ideal, light wave does not loose energy. As a result, the reflected wave will have the same features as the original wave (frequency, wavelength, amplitude, speed etc.) except direction.
The best reflective surfaces are those surfaces that are 100% flat and smooth. This is very difficult to achieve as all surfaces have a number of holes, which distort their smoothness. However, flat surfaces made of glass are very close to be smooth. Therefore, we often consider glass mirrors when discussing about reflection of light.

There are two kinds of reflection based on the properties of reflecting surfaces:

Regular reflection: In this kind of reflection, the reflected light beam has the same shape as the original one. Regular reflection occurs when the reflecting surface is very smooth.
Diffuse reflection:In this type of reflection, rays forming a regular beam fall on a non-regular (rough) surface. As a result, rays are reflected in different angles (they diffuse). Diffuse reflection is the reason why we see objects around us despite that they may not be exposed to direct sunlight. Indeed, most of rays coming from the Sun are absorbed by objects on which the sunlight falls. As a result, these objects become hot. Only the rays composed by light waves which have the frequency corresponding to the colours of objects are reflected by objects' surface to our eyes. As a result, we are able to see both the shape and colours of objects illuminated by the sunlight.

There are three possible situations a light ray may experience when it falls on objects.

Reflection. It occurs when light does not penetrate inside the object but is repelled by the objects surface.
Absorption. It occurs when light encounters a rough surface and it is absorbed by the object after making a number of collisions on its surface.
Emission. It occurs when the absorbed light wave is too energetic and therefore, the object releases a part of it into the environment.

In general, good absorbers are also good emitters of light. Matt and black surfaces are good absorber (and therefore good emitters) of light while bright and shiny surfaces are bad absorbers (and therefore, good reflectors) of light.

The two laws of reflection are:

Incident ray, reflected ray and the normal line lie all at the same plane.
The incident angle is equal to the reflection angle.

Physics Revision Questions for Reflection of Light

1. A light ray of wavelength 600 nm and frequency 500 THz (1 T = 1012) fall on a reflective surface. Which option below shows a quantity belonging to the reflected wave?

c = 2.5 × 108 m/s
f = 500 THz
λ = 1200 nm
A = 2 mm

Correct Answer: B

2. The angle formed by a ray incident to a plane mirror to the mirror surface is 250. What is the value of the reflection angle?

Correct Answer: C

3. Two plane mirrors in contact form a 1100 angle with each other. A light ray is incident at 530 to the first mirror. What is the angle of the reflected ray to the second mirror?

Correct Answer: A

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