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Refraction / Snell's Law

Watch one light ray cross a boundary, connect refractive index to speed change, and see Snell's law set the refracted angle, bending direction, and critical-angle limit on the same live diagram.

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Why it behaves this way

Explanation

Refraction is a boundary story. Light reaches an interface, the wave speed changes because the refractive index changes, and the ray direction shifts just enough to keep the boundary geometry consistent on both sides.

This concept keeps one compact boundary-and-ray picture in charge. Incident angle, refractive indices, speed ratio, graph previews, prediction mode, compare mode, and the worked examples all read from the same interface state so the direction change never drifts away from Snell's law.

Key ideas

01A larger refractive index means a lower wave speed in that medium, so entering the higher-index side bends the ray toward the normal.
02Snell's law, n_1 sin(theta_1) = n_2 sin(theta_2), connects the two boundary angles quantitatively rather than by a memorized picture alone.
03At normal incidence the speed can still change while the direction does not, and when light tries to go from higher n to lower n too steeply there is no real transmitted angle.

Frozen walkthrough

Step through the frozen example

Frozen walkthrough
Solve the current boundary, not a detached worksheet. The substitutions track the live incident angle and refractive indices, and the same values stay visible on the ray diagram and graphs.

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View plans
Frozen valuesUsing frozen parameters

For the current interface, what transmitted angle follows from Snell's law?

Incident angle

50 °

Top-medium index

1

Bottom-medium index

1.5

1. Start from Snell's law

Use .

2. Rearrange for the transmitted sine

.

3. Interpret the result

.

Transmitted result

The lower medium is slower, so the transmitted angle is smaller and the ray bends toward the normal.

Common misconception

The boundary bends the ray because it gives the light a sideways push at the interface.

The directional change comes from the speed difference between the two media, not from a separate sideways force at the boundary.

That is why the same boundary can change the speed without changing the direction at normal incidence, and why a bigger index contrast changes the bend even when the interface itself stays fixed.

Mini challenge

A ray goes from air into glass at the same incident angle you see now. Before you adjust anything, what should happen to the transmitted ray?

Make a prediction before you reveal the next step.

Decide whether the ray should move closer to the normal or farther away, and whether the speed should rise or fall.

Check your reasoning against the live bench.

It should slow down and bend toward the normal.
A larger refractive index means a smaller speed. Snell's law then requires the transmitted angle to shrink relative to the normal, so the ray bends inward rather than outward.

Quick test

Variable effect

Question 1 of 4

Use the speed story, the angle story, and the graph story together. The goal is to reason from the live boundary, not to quote a slogan.

What happens when light crosses from a lower-index medium into a higher-index medium at the same incident angle?

Use the live bench to test the result before moving on.

Accessibility

The simulation shows a horizontal boundary separating a top medium from a bottom medium, with a dashed normal line through the point where the ray hits the interface. One incoming ray approaches from the upper left, and the transmitted ray leaves into the lower right unless the current setup exceeds the critical-angle limit.

Optional overlays show the angle markers, the medium-speed guide, and the critical-angle threshold used to introduce total internal reflection honestly.

Graph summary

The first graph plots refracted angle against incident angle for the current media pair, so hovering it previews a different boundary setup rather than a later moment in time.

The second graph plots the signed bend relative to the normal, with positive values meaning toward the normal and negative values meaning away. When a critical angle exists, the plotted branch stops where no real transmitted angle remains.