Maud shines a laser from the air onto a pond at a grazing 80° from the normal. She expects the surface to act like a mirror and keep the beam out of the water. Most of the beam refracts straight through and heads down into the pond.
Total internal reflection is picky. This lesson gives the two-condition test that says when it can happen at all.
Condition 1: the light travels in the optically denser medium, heading toward the boundary with a less dense one. Denser toward less dense, never the other way.
Condition 2: the angle of incidence is past the critical angle for that pair of media.
Run the test on a clear case. Light in a glass block (critical angle with air: about 42°) strikes the boundary with air at 55°. Denser toward less dense: yes. Past the critical angle: yes. Total internal reflection can occur here.
Now change ONE thing: the same glass-air boundary, struck from inside the glass at 30°. Denser toward less dense: yes. Past the critical angle: no, 30° is below 42°. No total internal reflection: the light refracts out.
Now change the other thing: the boundary struck at 55°, but from the AIR side, heading into the glass. Denser toward less dense: no. The test already fails, whatever the angle. The light refracts into the glass, bending toward the normal.
That is why Maud's beam got into the pond: it was traveling in the LESS dense air toward the denser water, so condition 1 failed before any angle mattered.
Light travels inside a sapphire toward its boundary with air, striking at 40° from the normal. The critical angle for sapphire and air is about 34°. Can total internal reflection occur?
Condition 1: sapphire is denser than air, and the light heads from the sapphire toward the air. Yes.
Condition 2: 40° is past the 34° critical angle. Yes.
Both conditions hold.
Yes: the light totally internally reflects back into the sapphire.
Light travels through water toward the glass wall of its tank, striking at 70° from the normal. Glass is optically denser than water. Can total internal reflection occur?
Condition 1: the light is heading from the LESS dense water toward the DENSER glass.
Denser toward less dense fails, whatever the angle.
Condition 2 never needs checking.
No: light moving toward a denser medium can never totally internally reflect. It refracts into the glass.
Light travels in water toward the boundary with air, striking at 55° from the normal. The critical angle for water and air is about 49°. Can total internal reflection occur?
Light travels in air toward the surface of a pond, striking at 65° from the normal. Can total internal reflection occur?
Petrine says: 'Light moving from air into glass can never be totally internally reflected, however large the angle of incidence.' Is Petrine correct?
In which situation can total internal reflection occur?
Light travels from air toward a diamond's surface. Why can total internal reflection never occur at this boundary crossing?
A glass window pane (n = 1.50) sits underwater. Light inside the glass strikes the glass-water boundary at 75° from the normal. The critical angle for glass and water is about 62°. What happens?