Introduction:

When light travels from one medium to another, it undergoes a phenomenon known as refraction. Refraction is the bending of light as it passes from one medium to another with a different refractive index. The angle of incidence is the angle between the incident ray and the normal, while the angle of refraction is the angle between the refracted ray and the normal.

Explanation:

When the incident angle is 90°, also known as the angle of incidence of 90°, the light does not deviate during refraction. This can be explained using the laws of refraction and the concept of critical angle.

Law of Refraction:

The law of refraction, also known as Snell's law, states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the refractive indices of the two media. Mathematically, it can be expressed as:

n1 * sin(i) = n2 * sin(r)

where n1 and n2 are the refractive indices of the initial and final media, i is the angle of incidence, and r is the angle of refraction.

Critical Angle:

The critical angle is the angle of incidence at which the angle of refraction becomes 90°. It occurs when light travels from a denser medium to a less dense medium. If the angle of incidence exceeds the critical angle, total internal reflection occurs, and no refraction takes place. The critical angle can be calculated using the formula:

critical angle = arcsin(n2/n1)

where n1 is the refractive index of the initial medium and n2 is the refractive index of the final medium.

Explanation of Light not Deviating:

When the incident angle is 90°, it means the light is incident perpendicular to the interface between the two media. In this case, the angle of refraction can be calculated using Snell's law:

n1 * sin(90°) = n2 * sin(r)

Since sin(90°) = 1, the equation simplifies to:

n1 = n2 * sin(r)

To satisfy this equation, the refractive indices of both media must be equal (n1 = n2). This means that the light is incident at the critical angle, and the angle of refraction becomes 90°. As a result, the light does not deviate but rather follows the interface between the two media.

Conclusion:

In conclusion, when the incident angle is 90° during refraction, the light does not deviate. This occurs when the light is incident at the critical angle, and the refractive indices of the two media are equal. Understanding the laws of refraction and the concept of critical angle helps explain why light behaves in this way.