Laws of Reflection - Definition, Diagram, Examples, FAQs

What is Reflection?

The phenomenon when a ray of light strikes an opaque surface, it bounces back instead of passing through it is called reflection. We can see objects and images in mirrors or on reflective surfaces with the help of reflection.

Laws of Reflection

There are two laws of reflection that help to describe how light reflects off surfaces. These laws are:

• The Angle of Incidence Equals the Angle of Reflection: It states that the angle at which the ray strikes is equal to the angle at which it reflects. Mathematically it is represented as:

$$\theta_i=\theta_r$$

• The Incident Ray, the Reflected Ray, and the Normal Lie in the Same Plane: The striking ray ( incident ray), the ray that is bouncing off the surface (reflected ray) and the perpendicular to the surface all lie in the same plane.

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Types of Reflection

1. Regular (Specular) Reflection
2. Irregular (Diffuse) reflection

Regular (Specular) Reflection: Regular Reflection occurs when light falls on a smooth surface and as a result, it creates a clear, and sharp image. The reflected rays are parallel to each other.

This smooth surface can be a mirror. A mirror is made of glass coated with a reflective material. This reflective material helps in reflecting light incident on it uniformly.

Example: Reflection in a plane mirror.

Irregular (Diffuse) reflection: When light falls on an uneven surface, it scatters in different directions, and as a result, no clear image is formed. This type of reflection is called Irregular reflection. This uneven surface can occur due to scratches, wear and tear, dirt on the surface, or due to the material of the surface.

The difference between regular and irregular reflection

Application of Reflection

1. Periscopes use reflection to observe advancing enemies on the battlefield from a safe distance.
2. We see objects because of reflections.
3. Mirrors with concave and convex lens surfaces can reflect a variety of different images.
4. Medical diagnostics rely on reflection, as does optical path communication.
5. The law of reflection governs both light and sound, as both are waves.
6. Our ability to measure distances accurately to objects is based on the law of reflection for sound and light.
7. The echoes of sound are the result of reflections.

Solved Examples on Reflection

Q1. An incident ray of light strikes a plane mirror at an angle of $30^{\circ}$ with the mirror surface. Approximately what angle will the reflection be?

Solution:

The angle of incidence is determined by comparing the incident ray with the normal, so it is not 60° in this case

Following the Law of reflection,

$$
\theta \mathrm{i}=\theta \mathrm{r}
$$
Hence,
The angle of reflection $=600$

Q2: Determine the angle * which would be made by the system of the two mirrors shown in the figure below so that A and B are parallel to one another.

Solution:

Here is a diagram in which we fill in the angles of incidence and reflection and also label the rays as they are incident and reflected.

Angles I + r and i' + r’, which represent the incident wave at A and the reflected wave at B, have to be supplementary. (Geometry: cross-section cut between parallel lines).

Therefore,

$$
i+r+i^{\prime}+r^{\prime}=180^{\circ}
$$

As a result of the law of reflection, $\mathrm{i}=\mathrm{r}$ and $\mathrm{r}^{\prime}=\mathrm{i}^{\prime}$
Substitute to obtain

$$
\begin{aligned}
& i+i+i^{\prime}+i^{\prime}=180^{\circ} \\
& i+i^{\prime}=90
\end{aligned}
$$
In triangle $A O B$, we have

$$
\begin{aligned}
& \alpha+(90-r)+\left(90-i^{\prime}\right)=180^{\circ} \\
& \alpha=r+i^{\prime}=i+i^{\prime}=90^{\circ}
\end{aligned}
$$
If $\alpha=90^{\circ}$, the downward ray passes through $A$ and the upward ray passes through $B$.