When the ray of light travels from air to glass slab and strikes the surface of separation at 90 degree , then it ..........

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When a ray of light travels from air to a glass slab and strikes the surface of separation (boundary) between the air and glass at a 90-degree angle, it undergoes a phenomenon known as "normal incidence."

To understand what happens in this scenario, we need to apply Snell's law of refraction. Snell's law states that the angle of incidence (the angle between the incident ray and the normal), multiplied by the refractive index of the medium of incidence, is equal to the angle of refraction (the angle between the refracted ray and the normal), multiplied by the refractive index of the medium of refraction.

In this case, since the ray of light strikes the surface at a 90-degree angle (which means it is perpendicular to the surface), the angle of incidence is 0 degrees. Therefore, we can solve Snell's law as follows:

0 * refractive index of air = angle of refraction * refractive index of glass

Since the angle of incidence is 0 degrees, the refracted ray will also be perpendicular to the surface (90 degrees or normal to the surface). So the angle of refraction is 90 degrees.

Substituting these values, we get:

0 * refractive index of air = 90 * refractive index of glass

As any number multiplied by 0 is always 0, we can conclude that the refractive index of air is zero, which is incorrect. Therefore, this scenario of light traveling from air to a glass slab and striking the surface at a 90-degree angle is not possible.

Instead, if light strikes the surface at a 90-degree angle, it will follow a path along the surface, i.e., parallel to the surface of the glass slab. This is known as "total internal reflection."