What is the critical angle in optics?

What is the critical angle in optics?

• A. Angle between the surface and the incoming light.
• B. Angle of incidence at which total reflection always occurs.
• C. Angle between the normal and the minimum angle of refraction.
• D. Angle between the normal and the maximum angle of refraction, which is the largest angle at which rays inside the diamond can escape.

Answer: (D)

The critical angle is the angle of incidence in a denser medium (like diamond) for which the refracted ray in the less dense medium (air) travels along the boundary, i.e., at 90 degrees to the normal. It’s the limiting angle that still allows refraction; any larger angle causes total internal reflection. In the diamond-to-air case, the largest angle at which light can escape is when the refracted ray in air just skim the surface, which means the refracted ray is at 90 degrees to the normal. So the critical angle is the angle between the normal and that limiting refracted ray. That’s why the option describing the angle between the normal and the maximum angle of refraction—being the largest escaping angle—is correct. The other options don’t match this definition: measuring from the surface instead of the normal, or pointing to the moment you get total internal reflection or to a minimum refraction angle, aren’t how the critical angle is defined.

The critical angle is the angle of incidence in a denser medium (like diamond) for which the refracted ray in the less dense medium (air) travels along the boundary, i.e., at 90 degrees to the normal. It’s the limiting angle that still allows refraction; any larger angle causes total internal reflection. In the diamond-to-air case, the largest angle at which light can escape is when the refracted ray in air just skim the surface, which means the refracted ray is at 90 degrees to the normal. So the critical angle is the angle between the normal and that limiting refracted ray. That’s why the option describing the angle between the normal and the maximum angle of refraction—being the largest escaping angle—is correct. The other options don’t match this definition: measuring from the surface instead of the normal, or pointing to the moment you get total internal reflection or to a minimum refraction angle, aren’t how the critical angle is defined.