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Light on earth

  1. Dec 1, 2012 #1
    A question that just popped into my head..

    If during the day blue wavelength gets scattered more in the atmosphere,
    why isn't everything we see on earth is more blueish?

    Kind regards,
    AviiNL
     
  2. jcsd
  3. Dec 1, 2012 #2

    K^2

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    Have you noticed that there are shadows on the ground during the day? That tells you that most of the light comes directly from the sun, not scattered by the sky. But in the shaded areas open to the sky, there probably is a bit more blue light. Thing is, your eye adjusts to these sort of things.

    This illusion is fairly well known. Squares A and B are actually the same shade. Your brain simply interprets B as brighter, because it corrects for the shadow. When a tinted light is used, human brain also tends to correct for it. Up to a point, of course.
     
  4. Dec 1, 2012 #3

    Bandersnatch

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    Compare pictures from places like Finland with those from the Tropics. There's a visible blue tinge to those further north.
    Similar difference can be seen between midday and sunset times anywhere.
     
  5. Dec 1, 2012 #4

    sophiecentaur

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    You have to remember that the blue is very de-saturated. It's only 'blueish'. Likewise, the evening Sun is 'reddish'. Our brain / sense of vision has conflicting requirements and it evolved accordingly to get the most relevant information out of what we see. The fact that the illusion (in the above post) fools us means that it is more important to our survival that we recognise the pattern and the contrast than the absolute light level involved. The same goes for colour.
    If we had evolved with a built in calibrated colour measuring system in our heads, then we would also need to have a look-up table in our memories to take account of the time of day, lattitude, position of shadows etc. etc. in order to recognise what we are actually looking at. This is the problem that colour film has (you have no control on colour balance). Digital takes care of it to some extent but can still make faces look wrong because it looks at the whole scene and 'integrates to grey'. That statement may not be accurate any more, now that face recognition is available in cameras - they may have got cleverer still by now - but I still wouldn't necessarily trust them.

    On the subject of smart colour systems, I was in a supermarche in mid France this summer. On the veg counter they had a weighing machine (usual self-op). It recognised the colour of the item you put on the scales and gave you a set of green options on the touch screen when you put a green apple and a set of red possibliities when you put a tomato on. I played with it for some while and a queue began to form . . . . (Ze crazy Anglais)
     
  6. Dec 1, 2012 #5
    So, if I understand correctly, it is actually blueish, but our brain just compensates for it..
     
  7. Dec 1, 2012 #6

    sophiecentaur

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    Yep. And it's reddish in the late afternoon. We can cope with almost any state of sunlight and sky but we cope very badly with some forms of artificial light because the spectrum can be very different from sunlight.
     
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