The Flying Dutchman, Pirates of the Caribbean

The responsible of this kind of story is the physical phenomenon of the refraction: when a wave that is moving in a medium with some conditions makes it to a different medium, the wave experiences a change of its velocity of propagation. In this case, the medium is the air, and the other medium would be an air with different density.

Well, probably not all of us have the experience of seeing a ghost ship but we all have heard about mirages in the desert. In the desert you can remember your name ♫, and moreover it’s very hot. That makes the air located just over the sand get hotter than the rest of the air. This temperature gradient implies a density gradient, so it also implies a gradient of the refraction index. This gradient curves the rays of the light, so we can see in the ground the reflection of objects located far away, despite they seem to be closer. That’s the optical illusion of the oasis, the inferior mirage. A less fancy example would be the false water that we see on a long road on a hot day.

In the case of the boat, the air touching the sea is the colder one, so it’s a superior mirage. In this case, if the gradient is strong enough to make the curvature of the light bigger than the curvature of the Earth, we could even see islands floating in the sky. This may sound as a fairytale, in fact, it’s called Fata Morgana in reference to the fairy of the Arthurian Legend.

But I want to talk about another optical phenomenon: the green flash. Seconds after dusk, or seconds before the dawn, if the sky is clean enough, you can see a green ray coming from the horizon, just from over the Sun. When the light of the Sun is traveling around the atmosphere, it moves slower on the inferior air, denser than the air of the top layers. Due to this, the light follows a curved path. The light of higher frequencies, as the blue and green colors, are more curved than the rest of the light, so the green remains in the sky while the red is gone.

Jules Verne said that in these few seconds you can see “a green which no artist could ever obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot be but of this shade, which most surely is the true green of Hope”.