TwiddleLab / Physics / Light and Sight
Total Internal Reflection: Light in a Pipe
What you will learn
- Going from glass or water toward air, a ray bends away from the normal. Past the critical angle (about 42° for glass, 49° for water) no light gets out: total internal reflection.
- It only happens when light heads into a material with a smaller n.
- An optical fibre traps light by total internal reflection, so it follows gentle bends. Too sharp a bend and light leaks out.
How the lesson goes
Your internet probably travels as light in glass thinner than a hair. How does light follow a bent cable?
Light inside glass, heading out into air. Raise the angle.
The laser shines from inside a half disc of glass at the middle of its flat face.
Now a glass fibre round a corner. How tight can the bend be?
Total internal reflection.
From glass or water toward air, a ray bends away from the normal, so the angle in air grows faster.
The critical angle is the angle in the glass that sends the ray along the surface: about 42° for glass, 49° for water.
Past it, no light gets out: total internal reflection. It only happens heading into a material with a smaller n.
An optical fibre traps light this way. Simplified: real fibres are about 0.1 mm thick, with a glass core inside glass cladding of slightly lower n.
Game: Fiber route. Connect the house to the server.
Route the cable round the walls and lose less than 20 % of the light.
Where can light be trapped?
Light goes from the first material into the second.
Total internal reflection needs the outgoing angle to reach 90° before the incoming one does. That happens only when the ray bends away from the normal, which means moving into a material where light is faster: a smaller n.
Code: fibres carry bits as flashes of light.
On is 1, off is 0. Build the byte for the letter H.
Each lamp is one bit worth 128, 64, 32, 16, 8, 4, 2 or 1. The letter H is number 72 in the ASCII table, and 72 = 64 + 8, so the byte is 01001000. You will meet bits and letters properly in the Code track.
Which rays stay inside the glass?
For glass (n = 1.5) the critical angle is about 42°: there, the ray in the air would be at 90°. Any steeper angle in the glass has no way out, so the light reflects back in completely.
Send your letter down your cable.
Previous lesson: Refraction: Light Bends · Next lesson: Lenses Make Images · Part of the course: Light and Sight
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