← Laboratorium Physics · optics

Experiment · light

Refraction light at the boundary between media

A teaspoon in tea looks broken, a pool seems shallower than it is, and a diamond sparkles. Behind all of it is one simple thing: light slows down in water and glass. See why that alone is enough to make a ray bend.

Angle of incidence

45.0°

← → change the angle by 1°, with Shift by 5°. You can also drag the handle on the ray.

Media

n₁
1.00
n₂
1.33

Light

550 nm
1.0×
Space

Overlays

Bright bands are wave crests, dark ones are troughs. The color matches the chosen wavelength.

Marchers

0.55

Step by step

0%

Bent spoon

−40°

Drag the eye in the drawing or use ← →.

Shallow pool

15°

Drag the eye — the more obliquely you look, the shallower the pool seems.

Optical fiber

25%
10°
core n = 1.50 · cladding n = 1.40

Mirage

55 °C

The effect is exaggerated in the drawing so that it is easy to see.

Did you know…?

  • Diamond has n = 2.42 — light slows down inside it to about 124,000 km/s. Its critical angle is only 24.4°, so light entering a cut stone bounces around inside many times before it escapes. That is where the sparkle comes from.
  • Light can be slowed to the speed of a car. In 1999, Lene Hau's team at Harvard sent it through an ultracold cloud of sodium atoms and measured about 17 m/s, or roughly 60 km/h.
  • Light takes the fastest path, not the shortest one (Fermat's principle, 1662). A lifeguard running to a drowning swimmer does the same: running longer on the sand and swimming less — and the route “bends” at the shoreline exactly as Snell's law says.

Challenge

  1. Find the critical angle for light going from glass into air.“Wave” mode: upper medium glass (1.50), lower medium air. Set α to within 0.5°.
  2. When does a lake become a mirror? Make more than 50% of the light reflect as it passes from air into water.“Wave” mode: watch the energy bar.
  3. Break the optical fiber. Bend it so sharply that less than half of the light reaches the end.“Everyday” mode → “Optical fiber”.