Critical Angle — Definition

Definition

Imagine you're looking at an object underwater while you're standing on the bank of a pond. When light travels from the water (a denser medium) into the air (a rarer medium), it bends, or refracts. This bending is governed by Snell's Law.

Now, consider a light ray originating from an object deep inside the water and moving towards the surface. As this light ray approaches the surface, its angle of incidence (the angle it makes with the normal, an imaginary line perpendicular to the surface) can vary.

When the angle of incidence is small, the light refracts out into the air, bending away from the normal. As you gradually increase the angle of incidence, the angle of refraction in the air also increases, becoming larger than the angle of incidence.

There comes a point, a very specific angle of incidence, where the refracted ray doesn't actually emerge into the air anymore. Instead, it travels right along the boundary, or interface, between the water and the air.

At this exact moment, the angle of refraction is 90 degrees. This special angle of incidence in the denser medium is what we call the 'critical angle'.

Think of it like a threshold. If the light hits the surface at an angle less than the critical angle, it will refract out. If it hits exactly at the critical angle, it skims the surface. And if the angle of incidence becomes even larger than the critical angle, something remarkable happens: the light doesn't refract at all; instead, it's completely reflected back into the water.

This phenomenon is known as Total Internal Reflection (TIR). So, the critical angle is the gateway to TIR. It's a crucial concept for understanding how light behaves when moving from a denser to a rarer medium, and it forms the basis for many optical technologies like fiber optics and the sparkling of diamonds.