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Angle of Incidence

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What is Angle of Incidence? 

In Physics, the angle of incidence can be depicted as the angle formed in between a ray propagated on a surface and the line normal to the point of occurrence on the same surface. When a ray of light falls upon the surface of a mirror, it reflects back in return.

We need to study in detail about the concept of reflection of light to understand the angle of incidence. This article will clearly deliver you the information about the angle of incidence along with some important concepts related to this topic. 

Here are some key points regarding the angle of incidence:

  1. The incident ray is the ray which strikes first upon the smooth surface of the mirror.

  2. The reflected ray is the ray that drives away from the point of an incident of the ray.

  3. The point of incidence is the place where the ray of light is propagated.

  4. A normal is known as a perpendicular line which is drawn from the same point.

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Angle of Incidence Formula

We can find the angle of incidence by using Snell’s Law.

According to this law, 

\[\frac{\text{sin sini}}{\text{sin sinr}}\] = \[\frac{n_{r}}{n_{i}}\]

Here, i = the angle of incidence

r = the angle of refraction

ni = the index in the incident medium

nr = the index in the refracting medium

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Angle of Incidence and Angle of Reflection 

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From the above figure, we can infer following three things, such as:

  1. A ray of light falls on the point P of the smooth surface of a mirror.

  2. The same ray gets reflected from the point of incident P.

  3. After detailed observations, scientists have concluded that the angle of incidence is equal to the angle of reflection. A perpendicular is drawn to point P, which divides both the angles. The normal drawn on the point P of a plane mirror helps to relate the angle of incident ray, and angle of reflected ray.

Which means, i = r


Angle of Incidence and Angle of Refraction

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  • The Angle of Incidence 

It is the angle that covers between the normal and the incident ray. It is made when the ray of light touches the surface of the glass bar.

  • The Angle of Refraction

It is the angle that covers between the normal and the refracted ray. It is formed when the ray of light makes its way out of the glass bar.

As we know, the angle of incidence is equal to the angle of refraction; they remain in a constant relation for this type of behavior. 


Relation Between Angle of Incidence and Angle of Refraction

Scientists have named the refraction of light, when the path of light passes through one medium to another as the refraction of light. There are multiple factors in the refraction process, such as incident ray, refracted ray, normal (perpendicular to the point of the incident), and point of incidence.

There are two mediums that the ray of light makes the contact. The first name is a rarer medium and the second one is a denser medium. 

The speed of light in the rarer medium is more as compared to the speed of light in the denser medium. 

In the angle of incidence and angle of refraction, the medium has a huge impact.

An example of the rarer medium is air or any kind of gas. Glass, diamonds, and kerosene are the denser medium. The speed of light is blocked inside the denser medium whereas there is no opposition from any rarer medium to the speed of light. 


Difference Between Angle of Incidence and Angle of Refraction

Most importantly, the difference between the angle of refraction and the angle of incidence is the sequential order of the two angles. The incident angle and refracted angle are unequal.  

Firstly, it is created by a wave due to the different mediums.

When the beam of light gets refracted from a rarer to a denser medium, the angle of incidence lies between 0 to 900.

Nevertheless, we can’t be sure about the angle of refraction when the light ray comes from the rarer medium.

The above explanation is not applicable to a condition where the ray of light travels from a denser medium.

If we do some modifications when the incident angle is inclined progressively, a change can be seen to the angle of refraction. 

The angle of refraction changes, which means it inclines rapidly when a certain value of the incident angle is not reached.

The refracted ray of light reaches its maximum point (90where the refracted ray drives along with the border) at this critical angle of the incident ray.


Do you know? 

These are the main points that students must know about:

1. We use the unit of the degree to measure the angle of the incidence as well as the angle of refraction.

2. All the rays such as refracted ray, incident ray lie on the same interface along with the normal at the point of incident.

FAQ (Frequently Asked Questions)

Q1. Explain the Reason for the Angle of Incidence Superior to the Angle of Refraction.

Ans: When a ray of light passes through one medium to another (rarer to denser), the angle of incidence changes. The angle of incidence is larger than that of the angle of refraction due to the change of the medium. The light beam drifts away from the normal when it changes the medium. It happens only when the light passes from denser to rarer.

Q2. What Do you Mean By Critical Angle Formula?

Ans: We can find the value of a critical angle by considering the Snell’s Law. We can do so by applying the refraction angle equal to 90 degrees. Some parts of the light will be reflected, and some parts of the incident light will be transmitted to any angle of incidence, which should be below the critical angle.

Q3. How can you Obtain the Smallest Possible Angle of Incidence?

Ans: When a ray of light passes from one medium to another with less refractive index, then it can be completely reflected from the surface between the two mediums. In this way, one can get the smallest possible angle of incidence.

Q4. Does the Refractive Index Depend on the Angle of Incidence?

Ans: The refractive index is a completely independent quantity. It is not related to the angle of the incidence of light. The refractive index is the measurement of the speed of light, which gets slower due to the change in medium.