The angle of incidence is the angle between the normal line orthogonal to the surface of the prism (i.e., coming straight out from the prism) and the direction of the incident ray. For all intents and purposes we can consider the prism to have a straight edge, so that the normal line simply makes a right angle with that straight edge. Show If the angle of incidence is $0^\circ$, the light is exactly along the normal line. If the angle increases, the pie wedge between the normal line and the incident ray gets bigger. When the angle approaches $90^\circ$, the incident ray is almost parallel to the prism - we know this because the angle between the normal line and the prism's edge is exactly $90^\circ$. Once the angle equals $90^\circ$, the incident ray just travels parallel to the prism, but never really gets closer to it! And if the angle is greater than $90^\circ$, that means that the light is incident from inside the prism! So that is a completely different scenario, the scenario of light travelling from inside of a prism to outside of a prism, and in that scenario we typically just reverse the whole problem and define the angle of incidence from a normal line inside of the prism. Any time that the incident light is actually going into the prism, the angle will always be less than $90^\circ$. In optics, angle of incidence can be defined as the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence (called as normal). To understand the angle of incidence, we have to first look into the concept of reflection of light. We all know that when a ray of light hits a polished surface like a mirror, it is reflected back. Here are some key points to make you understand the concept of angle of incidence easily. What is Angle of Incidence?The incident ray and reflected ray form two angles at the point of incidence:
Angle of Incidence FormulaThe angle of incidence is equal to the reflected angle through the law of reflection. The angle of incidence and the angle of reflection is always equal, and they are both on the same plane along with the normal. Examples of Angle of IncidenceExample 1: Consider a light ray which is incident on a flat surface such that it makes an angle of 10° with the surface. 
Solution:
Example 2: The angle at which the light ray strikes the reflective plane surface is 56°. Find
Solution: Consider the below diagram to answer the questions:
What is the Relationship between the Angle of Incidence and Angle of Reflection?When light passes from one medium to another with different densities, its path gets deviated. This phenomenon is called the refraction of light. Like a reflection, there are similar components in refraction too. They are:
The angle formed at the point of incidence between the incident ray and the normal is called the angle of incidence. The angle formed between the refracted ray and the normal is called the angle of refraction. The relationship between the angle of incidence and angle of refraction is explained by Snell’s law, which states that the ratio of the sine of the angle of refraction and the sine of the angle of incidence is always constant and equivalent to the ratio of phase velocities of the two mediums it is passing through.
In total internal reflection, when the angle of incidence is equal to the critical angle, the angle of reflection will be 90°.
The angle of incidence is equal to the angle of reflection because the light ray selects the shortest path to reach the destination. This behaviour of light is known as Fermat’s principle. The same behaviour is shown by the light ray when it gets reflected from the plane surface. Therefore, the angle of incidence and the angle of reflection are equal.
The ray of light is incident normally on a plane mirror, which means that the angle of incidence is 0° and not 90° degrees. Therefore, the angle of reflection is 0°.
The angle of reflection decreases when the angle between the incident ray and the mirror is increased. This is because of the glancing angle of incidence. The glancing angle of incidence is the angle formed between the incident ray and the mirror. Also, the sum of glancing angle of incidence and the angle of incidence angle is 90°. So, an increase in the glancing angle results in the decrease of incidence angle. We know that, angle of incidence is equal to angle of reflection. Hence, the angle of reflection decreases.
The given statement is true. That is, the laws of reflection hold good for all types of mirrors. To learn about other Physics – related topics download BYJU’S – The Learning App.
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