Connect a change in speed at a boundary with a change in the direction of light.
Select a page to enlarge it. The PDF contains the complete set.
At an oblique boundary from air to water, light slows and bends toward the normal. From water to air, it bends away when a transmitted ray exists. Use refractive index rather than assuming mass density alone predicts every material’s optical behaviour.
A ray entering along the normal changes speed but not direction. Refraction therefore does not mean that every crossing must produce a visible bend. Both angles are measured from the normal.
Light from an underwater object refracts before reaching an observer in air. A straight backward extension of the incoming ray places the apparent object in a different location. This helps explain a pencil’s apparent bend and a pool’s apparent shallowness.
From a higher to lower refractive index, a sufficiently large incident angle produces total internal reflection. This is important in optical fibres. A mirage involves light passing through layers with differing refractive indices; the scene is not a puddle that is actually present.
Predict bending relative to the normal and explain apparent displacement without moving the object.
Physics Girl · Demonstration
Explain how repeated internal reflections can guide light along a curved route. Angles in our lesson are measured from the normal.
Change materials and the incoming angle. Use the normal and the angle tools to compare the paths.
Keep the materials fixed. Compare 0° and 45° incidence, then swap the materials. Explain the direction of bending.
Sketch one before-and-after comparison. Label what you changed and what stayed the same. Use the lesson’s vocabulary to explain the result.
Flat-boundary ray model using refractive indices 1.00 (air) and 1.333 (water). Snell’s law computes the path; using that equation is optional extension work. A partially reflected ray is omitted except during total internal reflection. Colours identify paths, not changes in light colour.
A ray enters water from air at an angle. State toward or away for its bending relative to the normal and explain your reasoning.
Compare the refractive indices.
It bends toward the normal.
At zero degrees to the normal, does its direction change? State yes or no and explain your reasoning.
The ray is perpendicular to the boundary.
Its speed changes, but its direction does not.
Does the fish move when the viewing direction changes?
No. The paths reaching the eye change. Apparent position is an interpretation of those paths; it is not the physical position of the fish.
The light path changes; the pencil does not. The image can shift while the object stays straight.
Commit to an answer, then use the feedback to check your reasoning.
