Refraction.
One boundary. Two light speeds. Follow the rays, or watch the wavefronts change direction.
At the boundary
Arrowheads show the direction in which light travels.
Incident / refracted lightReflected lightNormal to the boundary
Fractions of incident power · ideal transparent materials · unpolarised light
SAME FREQUENCY. DIFFERENT SPEED AND WAVELENGTH.
Light in each material
Speed of light
Wavelength
Incident and reflected light have the same speed and wavelength in medium 1.
Speed of light
Wavelength
Snell’s law
Measure both angles from the normal, not the surface. Light bends towards the normal when it enters a material with a higher refractive index, and away from it when it enters a lower-index material.
Frequency stays the same across a stationary boundary. The speed and wavelength both decrease by the same factor when the refractive index increases.
From internal reflection to total reflection
Send light from glass into air. Below the critical angle, part reflects inside the glass and part emerges into air.
At the critical angle, the refracted direction lies along the boundary: r = 90°. Above it, all the incident power is reflected. Total internal reflection requires n₁ > n₂ and i > i꜀.
About the model and material presets
Two transparent, non-magnetic media separated by a flat, stationary boundary. Rays obey Snell’s law and the law of reflection. Reflected and transmitted power are calculated with the Fresnel equations for unpolarised light, without absorption.
Preset indices are representative visible-light values: vacuum 1; air 1.000293; water 1.333; benzene 1.501; fused quartz 1.458; crown glass 1.520. Indices are held constant when the wavelength changes, so this model isolates refraction without modelling dispersion. Fused quartz is used to avoid the birefringence of crystalline quartz.
Wavefronts are successive crests, perpendicular to each branch’s travel direction. Their spacing and slowed animation speed use the same scale in both media. Solid and dashed crests can overlap; their interference amplitudes and reflection phase shifts are not shown. The finite width of the illustrated beam is schematic. At the critical angle, a dashed grazing line shows the limiting refracted direction, with no transmitted normal power. During total internal reflection, no propagating transmitted ray is drawn; the evanescent field is outside this model.
Wavelength and speed in medium 2 remain listed as material properties during total internal reflection. They do not imply a travelling transmitted beam. Optical colour is illustrative and tied to vacuum wavelength.
Reference: OpenStax: refraction and material indices; OpenStax: total internal reflection.
Explore refraction and total internal reflection
Choose air, vacuum, water, benzene, fused quartz or crown glass, or adjust either refractive-index slider. Change the incidence angle, compare rays with moving wavefronts, and inspect light speed, wavelength and the critical angle. The light keeps the same frequency across the stationary boundary.
Supports refraction and optical-wave topics in IB Physics, A Level Physics and AP Physics 2. The model uses Snell’s law and lossless, unpolarised Fresnel power fractions. Fixed material indices omit dispersion; no propagating transmitted ray is shown during total internal reflection.