Bermouli makes waves!The blue liquid (water saturated with salt) is denser than the other liquids (oil silicone). When the tube is tilted, the blue liquid “goes down” and the transparent liquid “goes up”. This relative movement of the two fluids is accompanied by the appearance of small waves to the interface. When two fluids (liquids of gas) in parallel move one with the other at different speeds, the interface which separates them can be unstable.
It is a consequence of the Bernoulli effect; when a fluid moves, the greater the speed, the weaker the pressure.
Let us imagine wind blowing on the surface of a lake. If the surface of the water incidentally grows hollow somewhere, then the speed of the wind in the trough is weaker as the pressure is greater.
The air presses thus more on the water at the bottom of the trough than at the top of the bumps and the initial deformation is accentuated.
Fortunately, there are mechanisms which moderate this instability and prohibit that the waves do not become infinite… In particular, the interfacial tension prevents that the surface of separation of the two fluids increases too much and its force of gravity supports the horizontality of the interface.
The sky is blue!
What to do?
Put the lamp in front of the small tube, then in front of the big tube, and observe the change of the colour. Do it again and look now through the extremity of each tube. What is the colour of the lamp?
Put the lamp in front of the small tube, then in front of the big tube, and observe the change of the colour. Do it again and look now through the extremity of each tube. What is the colour of the lamp?
What we learn?
The light that comes from the Sun is a mix of colours: blue, yellow, green, red… the atmosphere of the Earth doesn’t entirely transmit all those colours and radiations. During daytime, part of the radiations is absorbed and diffused again: the blue colour is the most diffuxed one. It is the case in the whole atmosphere, and that is why the sky is blue.
In the evening, on the horizon, the light crosses a thicker layer of atmosphere. The blue colour does not reach us anymore, and the red one is still diffused. This sometimes happens also during the day, when the air is full of industrial dust in suspension which accentuates the phenomenon of diffusion.
The light that comes from the Sun is a mix of colours: blue, yellow, green, red… the atmosphere of the Earth doesn’t entirely transmit all those colours and radiations. During daytime, part of the radiations is absorbed and diffused again: the blue colour is the most diffuxed one. It is the case in the whole atmosphere, and that is why the sky is blue.
In the evening, on the horizon, the light crosses a thicker layer of atmosphere. The blue colour does not reach us anymore, and the red one is still diffused. This sometimes happens also during the day, when the air is full of industrial dust in suspension which accentuates the phenomenon of diffusion.
No comments:
Post a Comment