Snell’s Law & Refraction

Follow the light

  1. Start with the rectangular block. Drag the laser, or use the incidence slider, to aim at the center of the top face. Angles are measured from the dashed normal, perpendicular to the surface.
  2. Measure the bend. Drag the transparent protractor onto the point where the ray meets the block. It snaps into position with its zero line on the normal. The Snap to normal button does the same thing. Toggle angle labels to compare your measurements.
  3. Change the materials. Compare acrylic, glass, water and diamond, or choose a custom index. Watch both the entry and exit rays. Water represents a water-filled cell with ideal, optically negligible walls.
  4. Find total internal reflection. Choose the semicircular block. The laser enters the curved face along a radius without bending and meets the flat face from inside. Select a block with a higher index than its surroundings and slowly increase the angle through the critical angle.

Open Try an investigation for short exploration prompts. Turn off Show measurements to use the protractor before revealing the calculated angles. Use Zoom to entry to read the protractor on a small screen. The dashed secondary ray is reflected light. Use Just below and Just above to compare the critical-angle boundary while keeping materials fixed.

Try this: Compare air → glass at 45° with glass → air at 45°. What happens to the transmitted beam?

Use the controls with a keyboard or drag the equipment with a mouse or touch. This simulation does not collect observations. In a real lab, never look into a laser beam or aim it toward anyone’s eyes.

LASER + OPTICAL BLOCK

The refraction bench

Try an investigation

Choose a starting point, then explore freely. Predict what will happen before changing a control.

Change materials ↓
TOP VIEW / OPTICAL BREADBOARDRECTANGULAR BLOCK
Laser refraction apparatusA laser aimed at a glass block on an optical breadboard. LASER650 nm ZORNSCIENCE / OPTICS
Solid: incident & transmitted · Dashed: reflected · Arrows: light direction
REFRACTIONPROTRACTOR PARKED · DRAG TO THE ENTRY POINT

AIM THE LASER · ANGLES FROM THE NORMAL

45°
0° · normal80°

INCIDENCE · θ₁45.0°
REFRACTION · θ₂28.1°
CRITICAL ANGLE-
Connect the rays to Snell’s law

n₁ sin θ₁ = n₂ sin θ₂

n is the refractive index: a larger n means a lower light speed (v = c/n). Angles use the normal, not the surface.

The physics behind the bench

n₁ sin θ₁ = n₂ sin θ₂. Angles are measured from the normal at the first flat interface. Total internal reflection occurs only when n₁ > n₂ and θ₁ exceeds arcsin(n₂/n₁). At the critical angle, the refracted ray travels along the surface.

The rectangular block traces the primary ray through each face, including side exits and internal reflections. A faint ray shows reflection at the first interface. The semicircular block uses radial entry so its curved face does not change the incidence angle at the flat face. Water is an ideal water cell; the same geometry is used for the virtual diamond block.

Indices are approximate values for red light: air 1.000, water 1.333, acrylic 1.490, glass 1.500, diamond 2.417. The model uses uniform, lossless, isotropic materials and a fixed 650 nm laser. Beam width, brightness and equipment proportions are illustrative; secondary reflections, dispersion and evanescent fields are omitted. Arrowheads indicate propagation direction. The dashed secondary ray is reflected light; the solid outgoing ray is transmitted light. At the critical angle the surface ray shows the limiting direction, not transmitted power.