Deviation is related to angle of incidence. The Snell's formula or refraction law, the formula that shows the relation between the angle of the incident ray, i, and the angle of the refracted ray, r, is:sin i/sir r = n, sin r = sin i/nwhere n is the relative refractive index (greater than 1, if the light enter an denser optical medium).Now, if the i equal zero (i.e. normal to surface) then this equation is valid only if r = 0. So, no deviation occurs.

Introduction:
When a light ray passes from one medium to another, it undergoes refraction, which is the bending of light due to a change in its speed. The amount of bending, or deviation, depends on the angle of incidence and the properties of the two mediums involved. However, when the incidence angle is 0°, the light ray does not deviate during refraction, regardless of the medium change. This phenomenon can be explained by considering the laws of refraction and the behavior of light at the interface between two mediums.

Laws of Refraction:
To understand why a light ray does not deviate when the incidence angle is 0°, we need to consider the laws of refraction, also known as Snell's laws. These laws describe the behavior of light when it passes from one medium to another. The two laws are as follows:

1. The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, known as the refractive index (n) of the two mediums.

Explanation:
When the incidence angle is 0°, the incident ray is perpendicular to the interface between the two mediums. In this case, the light ray does not change its direction because it is already traveling along the normal to the interface. Thus, there is no deviation during refraction.

Since the angle of incidence is 0°, the sine of the incidence angle is also 0. According to Snell's second law, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. Therefore, the sine of the angle of refraction must also be 0, which means the angle of refraction is also 0°.

As a result, the light ray continues to travel in a straight line without any bending or deviation, even though the medium is changing. This is because the light ray is already aligned with the normal, and the refractive index of the two mediums does not affect its direction in this particular scenario.

Conclusion:
When the incidence angle is 0°, the light ray does not deviate during refraction, regardless of the medium change. This is because the incident ray is already aligned with the normal, and the refractive index of the two mediums does not affect its direction. Understanding the laws of refraction helps explain this phenomenon, as the angle of incidence and the behavior of light at the interface play crucial roles in determining the amount of deviation during refraction.

Because angle of incident is zero