AMASE: Bending Light with Optics

Posted on Monday, April 20th, 2020


[Videogame sounds with Orbax and Pepper do Science logo]

[Video cut to Orbax in his office]

Orbax: Hello everybody, Orbax here. I hope this video finds you safe and healthy.

Our question today comes from a canoe in Erieau, near Chatham.

[Video cut to a child in a canoe]

Forest: Why do the paddles look like they're bending when they're in the water uncle Orbax?

[Bird and water sounds]

[Video cut back to Orbax in his office]

Orbax: That's a great question Forest!

I hope you made it off, of that canoe and I keep telling you that man is not my brother.

Light. It wakes us up in the morning, it keeps our plants growing, but most importantly it allows us to see. For as much as a constant as light is in our daily lives it still behaves very weird at times. 

We see an object because light is coming from the object into our eyes. Now light travels in tiny packets of energy called photons and whether those photons bounce off, of an object or whether they're absorbed and they're re-emitted by that object, either way those photons continue to travel into your eye, are absorbed and tell your brain that

that object is there. 

Now the reason we can see something that we can even reach out and touch is because not only does light move really, really, really fast but it also moves in a straight line. If it didn't move in a straight line, if it bent in the air, then I could actually see something that was hiding right behind me!

[A cat emerges from a box behind Orbax, scary music]

Orbax: I mean I think we all know that light travels in a straight line you're all familiar with lasers.

[Video cut to a dark space with laser lights and dance music]

[Video cut back to Orbax in his office]

Orbax: So light travels from an object to our eyes and light travels in a straight line so why does the paddle look like it's bent?

Well the only thing that can affect the light that comes from the paddle is the water. 

[Orbax splashes some water from a pot on his desk]

Orbax: We all understand that water is different than air... but how is it different with respect to light? We say that water is more optically dense than air but what does that mean? 

Well it just means that there's more stuff in water for the light to interact with than there is an air. What happens is light in water gets absorbed and re-emitted. It bounces off the molecules that make up the water and all that interaction means that the light actually travels slower in water than in air. The physical result of that is that a ray of light actually bends when it enters water. 

So let's take a look. 

[Orbax holds up a clear glass jug half full of liquid]

Orbax: What I've got here is this cool bottle of what my wife informs me is a very expensive lemonade. Now personally I don't see the cost, I just see the benefits that it has on our local economy, but if I was to fill this with water we could actually take a look at laser light entering into it.

So let's fill this bottle with water and see how the beam from the laser is affected. 

[Video cut to a close up of the empty bottle with a green light shining through it, water is slowly poured in and the direction of the light changes]

Orbax: As you can see there's a definite change in the direction of the light it... looks like it's bent up a little, right? Can you see it? 

[Orbax brings a ruler into view to see the change in the direction of the line]

Orbax: Let's put a straightedge in there.

[Arrows indicating the original path of the light, and the path of the light through water appear on the jug]

Orbax: As you can see the light is bent upwards or towards this line that we call the normal. That's the line that's perpendicular to the interface between the air and the water. This is because the water is optically more dense. If it was in reverse, from the air to the water, it would bend the other way. 

You see it's based on this law that goes back to ancient Egypt but was rediscovered in the 1600s. 

[Snell’s law appears on the screen: \(n_1 \sin \theta_1 = n_2 \sin \theta_2\)]

Orbax: It's called Snell's law, and it tells you how a beam of light bends when it enters a different material. 

[Video cut back to the glass jug with liquid and a laser beam passing through]

Orbax: All right well what about something other than water?

[Video cut to a bottle of Olive Oil]

Orbax: Let's try something much thicker and a more viscous material like olive oil. Can you see how it bends even more? 

Let's draw a picture.

[A graph is drawn on screen over top of the liquid filled bottle as an axis for the laser beam passing through]

Orbax: We'll use this green line to represent the path of the laser in the water. If we now place that on top of the image of the olive oil and draw the new path of the laser in blue [video switch to Olive Oil bottle], we see that the beam has bent upwards towards this white line that we call normal. This is because the olive oil is more optically dense and this result is actually predicted by Snell's law. 

Now a lot of people wonder if this is the same as why a pencil looks like it's bent in water? 

[Video cuts to a close up of a glass of water with a marker in it]

Orbax: So... kinda.

[Video cuts back to Orbax in his office]

Orbax: You see when we look at the surface of a lake, or a rectangular piece of glass, we're looking at what we call a planar surface. That means a flat surface.

There's one big thing that a planar surface can't do and that's magnify an object. When we look at a pen in a glass of water not only does the light bend but because the curves of the glass it actually magnifies the pen just like a lens would. And the pen not only looks like it's in a different place, but the part of the pen that's underwater actually looks bigger! 

[Video close up of pen in water, then cut back to Orbax]

Orbax: There's lots of tricks that you can do with curved surfaces and light. 

[Video cuts to an image placed behind the empty glass jar, water is poured in and the image looks larger]

Orbax: You've probably seen a ton of them online just like this one so play around a little bit and see what you can come up with and send us a video!

[Video cut back to Orbax at his desk]

Orbax: If you have any more questions just ask, we're always happy to answer what we can!

And don't forget to #SCIENCE!


In today's video we go through the introductory theories of optics to explain why a paddle looks bent when it is underwater.

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