Credit: Salon 24 http://arkadiusz.jadczyk.salon24.pl/
What happened to the end of the glass rod in the liquid? It’s invisible!
How?
To answer this question, it is first necessary to understand a few other things. First up: How do we see anything? One of three ways.
Mostly we see things by the light they reflect – light from the Sun, a desk lamp, whatever, falls on an object – your hand, shirt, desk etc. – light reflected from the object enters you eye & so you see it.
Some things we see because they emit their own light – Sun, stars, computer screen.
Transparent objects are a little bit different. What makes something “transparent” is that light passes right through it – so how can we see it at all?
Part of the answer is that light doesn’t actually “pass right through” – a small part of the light striking say, glass or water, is reflected.
Another part of the answer is that light doesn’t pass straight through a transparent object. As light moves from one transparent medium (e.g. air), into another transparent medium (e.g. glass), the light is bent. this is called refraction.
Light refracted as it passes from air into the transparent plastic block. As it passes from the plastic into the air again, it is refracted in the opposite direction. Note that as the beam leaves the clear plastic block, it is parallel with the beam entering the plastic.
Credit: Wikimedia Commons/
How does refraction work? As light passes from one medium to another, it changes velocity. E.g., as light travels from air into the plastic block, in the image above, it slows down & so changes direction. As it leaves the plastic block, it speeds up again & turns back to its original direction.
So why is the glass rod in the beaker at the top of this post invisible?
One more thing to understand before answering this question – How do we see the glass rod above the liquid? Refraction. As light passes from air to glass, is refracted. I.e. it slows down & changes direction. As light exits the glass it is refracted again in the opposite direction. This gives glass (or other transparent material) its unique appearance. It’s why a glass rod looks different from a steel rod or a pencil.
Finally – So why is the glass rod in the beaker at the top of this item invisible?
Because the velocity of light in the liquid is the same as the velocity of light in glass!
As light travels from the liquid (probably cooking oil) into the glass rod, it doesn’t change velocity, so it doesn’t change direction. Thus, light which passes through the glass rod is indistinguishable from light which misses it & passes only through the liquid. That is, the glass rod is invisible!
More about how to do it
http://www.exploratorium.edu/snacks/disappearing-glass-rods / refractive index index of refraction high dispersion glass low-dispersion glass refraction reflection optical medium velocity of light transparent invisible perspex polycarbonate transparent plastic /
