# Light Bender

High\-school optics in a lighthouse lamp room at dusk: reflection, refraction and Snell's law, total internal reflection, dispersion, lenses and focal points, and the Fresnel lens

By Numa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/0difa6ucCKrH)

## Transcript

### 1. The Light That Saved Ships

At dusk the keeper lights the lamp, and a ring of glass turns its glow into beams that sweep the sea\. This is a Fresnel lens, first lit in France in 1823, and its beam could be seen more than 20 miles out\. The keeper's bench shows how it works\.

The clockwork turns the lens 30 times faster than in real life\.

### 2. Reflection

A mirror sends light back at the same angle it came in\. Both angles are measured from the normal, the dashed line at right angles to the mirror\. Drag the mirror to turn it, and watch the two angles stay equal\.

### 3. Refraction

Light slows down in glass, so a ray that enters at an angle bends\. The index, n, tells you how much the light slows\. Turn the block and change the material\. When the ray leaves the far side, it bends back to its first direction\.

Snell's law gives the exact angle: n₁ sin θ₁ = n₂ sin θ₂\.

### 4. Total Internal Reflection

Now the light starts inside the glass, heading out into air\. Past the critical angle, 41\.8° here, no light can escape and all of it reflects back inside\. Drag the ray box around its track and watch the reflected ray brighten\.

Optical fibres carry internet signals this way\.

### 5. Dispersion

Glass bends each colour by a different amount\. Violet slows most and bends most, so white light fans out into a spectrum on the card\. Turn the prism to move the colours\. Turn it too far and they vanish, trapped by total internal reflection\.

### 6. Lenses and Focal Points

A converging lens bends rays so they meet again and form an image\. Slide the lens until the spot on the card shrinks to a point\. Then put the bulb exactly at the focal point: the rays leave parallel\.

The focal length of this lens is 25 cm\.

### 7. Inside the Great Lens

Fresnel cut one huge lens into thin rings, so it bends light just as well with far less glass\. The bullseye belt bends the lamp's light into a level beam\. The prism rings catch steeper rays by total internal reflection\. Drag the lens to turn it\.

### 8. Focus and Sweep

The lamp must sit exactly at the lens's focal point\. Too low and the beams aim up into the sky\. Too high and they aim down into the sea\. Drag the lamp, or press Focus the lamp, until the beams lie level, then watch them sweep out to sea\.

Reflection, refraction, total internal reflection and focus: one machine uses them all\.