Petrine shines a laser slantwise from the air into a glass block. The ray kinks at the surface, and the path inside the glass lies closer to the normal than the arriving ray did.
OPT-026 explained why light bends. This lesson gives the rule that says WHICH WAY: toward the normal, or away from it.
The normal is the reference line: the line perpendicular to the boundary at the crossing point. Both verdicts are read against it.
You saw the direction rule for water waves: a wave that slows at a boundary bends toward the normal.
In optical-density language, the rule for light has two halves.
Petrine's laser shows the first half: glass is optically denser than air, so the ray entering the glass bends toward the normal.
So light does not always bend toward the normal. The direction depends on which way the speed changes: slower means toward, faster means away.
One case bends neither way: a head-on arrival, along the normal itself. The speed still changes, but the change arrives everywhere at once, so nothing swings, and the ray runs straight on.
A ray of light passes from water into a glass paperweight, arriving slantwise. Which way does it bend?
Compare the two media first: glass slows light more than water does, so the glass is optically denser.
The ray is entering the denser medium, so the light slows.
Slower means toward: the ray swings toward the normal.
The ray bends toward the normal as it enters the glass.
The same ray later leaves the glass and passes into water, arriving slantwise. Which way does it bend now?
Compare the media again: water is less optically dense than glass.
The ray is entering the less dense medium, so the light speeds up.
Faster means away: the ray swings away from the normal.
The ray bends away from the normal as it enters the water.
A ray of light passes slantwise from air into still pond water. Does the ray bend toward the normal, away from it, or not at all?
A ray of light traveling inside a glass block reaches the surface and passes slantwise out into the air. Does the ray bend toward the normal, away from it, or not at all?
A ray of light passes slantwise from water into a diamond. Light travels more slowly in diamond than in water. Does the ray bend toward the normal, away from it, or not at all?
A ray of light inside a sapphire reaches the flat top of the stone traveling exactly along the normal, head-on, and passes out into the air. Does the ray bend toward the normal, away from it, or not at all?
Oona says: 'Whenever light crosses a boundary slantwise, it bends toward the normal.' Is Oona correct?
Maud says: 'A ray entering a medium where light travels faster bends away from the normal.' Is Maud correct?
A ray of light passes slantwise from a sapphire into water. Light travels faster in water than in sapphire. What does the ray do at the boundary?
A ray of light crosses slantwise from honey into an unknown liquid, and it bends toward the normal. What does the bend tell you about the liquid?
A ray of light bends away from the normal as it leaves a plastic block and enters the air. What happened to the light's speed at the boundary?