Mirages and Fata Morgana

Cured by di Gabriele Calzà.

Translation by Liceo linguistico “S. Scholl” – Trento

Material needed

– 1 rectangular transparent container, high and wide, better if narrow in depth

– denatured alcohol

– sugar (or concentrated syrups for drinks)

– water

– darkened environment (even if just in part)

– laser pointer (recommended)

Equipment

a sink and a jug

Procedure (estimated time: 20 minutes)

MIRAGE

Pour 3-4 cm of water into the transparent container, and then a layer of alcohol of the same height. Make sure that the alcohol does not mix completely with the water: it should remain separated from a zone with medium alcohol concentration.

“FATA MORGANA”

–

Pour 1-2 cm of water into the container and add spoons of sugar, stirring until it is completely dissolved; add more and stir again…
Then pour some water slowly on top of this “syrup” to form the second layer, of the same height. Again, make sure that the syrup does not mix completely with the water, but remains separated by a layer with medium concentration.

How does it work?

Observe, thanks to these fluids and adjusting the height of the eye, the two phenomena mentioned above: in the first one, we can see an object that’s on top, doubled on the bottom (mirage); vice-versa, in the second one, we can see an object floating high, while it is actually on the bottom (“fata morgana”).

We could also trace the path of the rays going from the object to the eye, and thus understand the mechanism by which these two effects operate.

In order to do this, we just have to colour the water (by adding concentrated tea, for example), observe what happens from the side and use the laser pointer:

– in the first case, point the laser between the water and the alcohol, with an angle downwards, moving it up and down until you find the spot where the effect is more evident: you will notice a progressive deviation of the ray upwards, which will appear curved (convex);

– in the second case, point the laser between the water and the syrup, with an angle downwards. Once found the best spot, you will notice a phenomenon that’s completely the opposite of what you saw before: a progressive deviation of the ray downwards.

Explanation

When light passes from one medium to another with a different refractive index, it undergoes the phenomenon of refraction. If the separation of the two means is not sharp and the index change is gradual, the beam of light does not form a sharp angle with the interface (which is not there), but it deviates progressively and follows a gentle curvature path.

Water has a refractive index 1.33, while both alcohol and sugar solution have a higher refractive index (1.36 for alcohol and 1.53 for solid sugar). However, since the first layers over the water and the second one below it, two complementary situations are created.

In reality, layers of air at different temperatures create one or the other of the two situations experienced.

WARNING – The laser pointer’s light is very intense and collimated: avoid pointing it to the eyes or staring at its reflection on lucid metallic surfaces – THE DIRECT LASER POINTER’S LIGHT CAN DAMAGE THE RETINA.

Find out more

“Mirages in a bottle”- T. Lòpez-Arias, G Calzà, L. M. Gratton and S. Oss

[Physics Education, Volume 44, Number 6]