Track refraction without changing the source frequency

Establish the model and evidence

Refraction changes ray direction when light passes between media with different speeds, except at normal incidence where direction can remain unchanged. Refractive index is n=c/v, with c the vacuum speed and v the phase speed in the stipulated medium. A larger index means a lower speed, not a larger frequency supplied by the source.

Snell’s law is n₁ sinθ₁=n₂ sinθ₂, using angles from the interface normal. Entering a larger index bends the ray toward the normal; entering a smaller index bends it away when a transmitted ray exists. Comparing angles from the interface plane instead can reverse a conclusion if the complements are not handled consistently.

Frequency remains unchanged across a stationary interface. Since v=fλ, a slower wave at the same frequency has a shorter wavelength. The activity uses vacuum wavelength 600 nm and nondispersive indices for a single specified color. It is not a model that all wavelengths share one index in real glass; dispersion is a separate wavelength dependence.

Both reflection and transmission can occur at an interface. The drawn rays give directions only, not equal energy shares. A complete brightness or polarization calculation would require additional physics. The geometric model also omits absorption and wave interference. No real laser alignment, bright-light viewing or eye exposure is part of this virtual investigation.

An independent index inference can compare several angle pairs: plot sinθ₁ against sinθ₂ with a known incident index. The expected slope is n₂/n₁. A normal-direction zero error affects both angle interpretation and the inferred slope, so a straight fit alone does not verify alignment.

Worked evidence

With n₁=1.00, n₂=1.50 and θ₁=30°, sinθ₂=1/3, so θ₂≈19.47°. Speed becomes 2.00×10⁸ m/s. Frequency stays 5.00×10¹⁴ Hz and wavelength becomes 400 nm.

Predict and investigate

At normal incidence, can speed and wavelength change even though the ray does not bend?

Compare indices 1.2 and 1.8 at the same 40° incident angle, then repeat at 0°. Retain angle, speed, frequency and wavelength; explain which quantities change and which do not.

Transfer to changed conditions

A 500 nm vacuum wave enters a medium of index 1.25. Find its speed, frequency and wavelength, taking c=3.00×10⁸ m/s.

Compare the worked transfer

Speed=2.40×10⁸ m/s, frequency=6.00×10¹⁴ Hz and wavelength=400 nm. These statements concern one given index at that wavelength; real dispersion requires further data.

Explain the boundary

Why is a change in wavelength not evidence that the stationary interface changed the source frequency?

Sources and authorship

Physical relationship reference. Current AP Physics 2 framework, checked 30 October 2026. Original text, rays and computed/synthetic records, not completed optical measurements. All investigations are virtual: no lasers, intense-light viewing, sunlight focusing, flames or eye tests are requested. Any real practical work needs qualified supervision and independent safety review.