Nilo streams a video. The data reaches his street not as electricity in a copper wire but as flashes of light racing down a glass thread thinner than a hair, under the sidewalk, for miles.
A glass thread can carry light around corners without losing it. This lesson explains the device that does it.
The glass thread is called an optical fiber.
Light travels down the middle of the fiber, in a glass channel called the core.
Around the core sits a second glass layer, made less optically dense than the core.
That pairing builds the trap from the last two lessons: light in the core heading for the outer layer is going from denser toward less dense.
The fiber is laid so that light runs nearly along it, striking the core's boundary at a grazing angle: far from the normal, past the critical angle.
The light zigzags down the core, bounce after bounce, each reflection total. After thousands of bounces it leaves the far end almost as strong as it entered.
Internet data rides this trap as light pulses: flash for 1, dark for 0, billions of times each second, down a fiber that can cross an ocean.
A doctor's camera probe carries light down a fiber to see inside a patient's stomach. The light strikes the core's boundary at 80° from the normal, and the core-coating critical angle is about 75°. Why does the light stay in the core?
The core is optically denser than its outer layer, so the light heads from denser toward less dense.
The 80° strike is past the 75° critical angle.
Both trapping conditions hold at every bounce.
At each strike the light totally internally reflects, so it stays in the core all the way down.
A technician bends a fiber around a sharp corner, and some light stops arriving at the far end. Explain what the sharp bend changed.
At a sharp bend, the light meets the core's boundary more steeply: closer to the normal.
A steep enough strike falls below the critical angle.
Below the critical angle, the trap opens: most of that light refracts out through the boundary instead of reflecting.
The bend let some strikes fall below the critical angle, so part of the light refracted out of the core and was lost.
Light travels down an optical fiber's core for miles without escaping. What keeps it inside at every strike on the core's boundary?
An optical fiber's core is wrapped in a second glass layer. For the fiber to trap light, how must that outer layer compare with the core?
Oona says: 'A light pulse can follow a gently curving fiber, because at each strike it still meets the core's boundary past the critical angle.' Is Oona correct?
Which statement explains how an optical fiber guides light along its length?
A fiber is clamped into a very sharp bend, and light begins escaping from the core at the bend. What changed there?
A video call crosses the ocean through optical fibers. In what form does the call's data travel inside a fiber?