CAN WE TRAP LASER INSIDE WATER?!?💧⚡️🌈

Lasers are science's secret weapon; they're extremely strong, flexible, and capable of travelling great distances. But, as this simple experiment demonstrates, a laser beam can be 'trapped' in water despite its immense strength. Yeah, you can do this at home (as long as the laser is kept away from everyone's eyes).

All you need is a plastic bottle filled with water, a pin, and a laser pointer, as demonstrated in the video below by the University of Utah's Department of Physics and Astronomy (once again again, not for eyes).

Simply poke a hole in the side of your bottle to allow a stream of water to flow out smoothly, then aim your laser through the bottle to see it become entangled in the cascading waterfall and bend downwards with it, interrupting its normally straight direction.

"Placing one's hand into the stream can expose the laser light reflecting on your skin," the department's YouTube channel writes. "Mixing in a little scattering agent (such as Pine-Sol or milk) will make the beam more apparent."

So, aside from a badass party trick, what's going on here?

Complete internal reflection happens when a beam of light strikes a medium boundary (the edge of the water spout) at a specific angle, causing it to bounce rather than move straight through. When the light reaches each boundary, this happens again and again, trapping the laser in the waterfall.

There are two conditions for complete internal reflection to occur from a physics standpoint. To begin with, the light must be within a denser medium and entering the boundary of a denser medium - for example, travelling from water to air.

Second, the light must strike the medium boundary at a greater angle than what is known as the 'critical angle.' The angle varies depending on the medium you're working with, but a schematic of how it works is shown below. (Those with a keen eye may use Snell's law to measure the critical angle of a specific medium boundary.)

It's all interesting, but the most fascinating aspect of the experiment is that it's a great example of how optical fibres operate - you know, the fibres that make up the internet cables.

These fibre optic cables can catch beams of light, just like the waterfall in the video above, and direct them wherever they're needed because they're made of long glass or plastic fibres. That's useful because we can use it to encrypt stuff like the new Game of Thrones episode.

So remember to thank the poor laser that got trapped inside your cable to send the information to you the next time you stream something on your screen or download (legally, of course) an episode of your favourite TV show.

You're amazing, physics.

REFERENCES:

https://www.britannica.com/science/total-internal-reflection

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/totint.html

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