Two ripple-tank runs. Ripples cross slantwise onto a shallow shelf, and the path bends one way. Ripples cross slantwise off the shelf into deep water, and the path bends the other way.
Refraction has a direction. Which way does the path bend when the wave slows down, and which way when it speeds up?
Measure from the normal, as with reflection: the line perpendicular to the boundary.
Take the slowing case first. The first end of the front into the slow medium slows first, and the front swings so that the path beyond the line lies closer to the normal.
A wave that slows down bends toward the normal.
Now the speeding-up case. The first end in races ahead, the front swings the other way, and the path beyond the line lies farther from the normal.
A wave that speeds up bends away from the normal.
A head-on crossing still bends neither way: with no slantwise arrival, nothing swings.
Ripples cross slantwise from deep water onto a shallow shelf, where they travel more slowly. Does the path bend toward the normal or away from it?
The wave slows down at the boundary.
A wave that slows down bends toward the normal.
The path bends toward the normal.
Ripples cross slantwise from the shelf into deep water, where they travel faster. Toward the normal or away from it?
The wave speeds up at the boundary.
A wave that speeds up bends away from the normal.
The path bends away from the normal.
Ripples cross slantwise into a region where they travel more slowly. Does the path bend toward the normal or away from the normal?
A wave crosses slantwise into a medium where it travels faster. Does the path bend toward the normal or away from the normal?
Eirik says: 'Whether a refracted wave bends toward or away from the normal depends on whether it slows down or speeds up at the boundary.' Is Eirik correct?
A water wave crosses slantwise from deep water into a shallow bay, where it travels more slowly. Which description fits its path beyond the boundary?
A water wave crosses slantwise from a shallow bay into deep water, where it travels faster. Which description fits its path beyond the boundary?
What decides whether a refracted wave bends toward the normal or away from it?