Snell's Law & Refraction
Light into glass — adjust angle.
You shine a flashlight beam at an angle into a swimming pool. The beam travels from air (n = 1.00) into water (n = 1.33). You can clearly see the beam bending at the water surface.
Which way does the beam bend as it enters the water?
The beam bends TOWARD the surface normal. When light enters a medium with a higher index of refraction (water, glass), it slows down and bends toward the perpendicular. Snell’s law says n₁ sinθ₁ = n₂ sinθ₂. Since n₂ > n₁, the refracted angle θ₂ must be smaller than the incident angle θ₁ — meaning the beam tilts toward the normal.
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Adjust the incident angle and watch the beam refract at the boundary. See how the wavefronts explain WHY it bends — one side slows down first!
n₁ sinθ₁ = n₂ sinθ₂
Going into a denser medium (higher n): light bends toward the normal. Going into a less dense medium: light bends away. At the critical angle, total internal reflection occurs.
Diamond’s high index (n = 2.42) creates a very small critical angle (24.4°). Most light entering a well-cut diamond gets internally reflected many times, making it sparkle brilliantly.
A swimming pool always looks shallower than it is. Light bending at the water surface makes the bottom appear closer, typically looking about 3/4 of its real depth.
Total internal reflection keeps light bouncing inside glass fibers for hundreds of kilometers. Your internet data rides on light trapped by Snell’s law.
Hot air near a road surface has lower n than cool air above. Light from the sky refracts and curves upward, creating a mirror-like mirage of sky on the road.
“Snell’s law — four variables, one equation — explains why pools look shallow, diamonds sparkle, and fiber optics carry the internet at the speed of light.”
Refraction Lab
Explore Snell’s law with full control. Adjust the angle of incidence and the refractive indices of both media. See refraction, watch for total internal reflection, and try preset materials like air, water, glass, and diamond.
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Start with light going from air (n=1.0) into glass (n=1.5) at 45°. Note the refracted angle. Now reverse it: glass into air at 45°. What happens? Increase the angle until you find the critical angle — total internal reflection! Now try diamond (n=2.42) — the critical angle is tiny, trapping light inside.
Total internal reflection only happens when light goes from a denser to a less dense medium, AND the angle exceeds the critical angle. This one phenomenon makes fiber optics, diamonds, and prisms possible.