Refraction of Light

When light travels from air into water, glass, or another medium, it “bends.” How much it bends depends on the refractive index and the angle of incidence.

Key Points

• Refractive index n: how much a medium slows light down. Air is about 1.0, water about 1.33, and ordinary optical glass about 1.5.
• The larger the angle of incidence, the more pronounced the deflection; when light goes from a higher refractive index to a lower one at a sufficiently large angle, total internal reflection occurs.
• The cornea and the lens form a natural “refraction system” that brings parallel rays to a focus step by step inside the eye.

A Small Experiment (to think about)

• Put a chopstick into a half-full glass of water and watch it appear to bend: this is the visual effect of refraction.
• Imagine light slowing down and changing direction as it enters the cornea, then being fine-tuned by the lens so that the focus lands on the retina.

Understanding refraction is the foundation for making sense of lenses and eye imaging later on.