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Why shadows become dark when light intensity increases?

  1. Dec 13, 2015 #1
    I observed following phenomena today: in my room with some lightings, there is a stuff toy placed in front of a wall. The ambient lighting doesn't cause any shadow on the wall currently. Now, I switched ON a torch and pointed it to a stuff toy which was kept in front of a wall and a shadow appears on the wall. Later, I increased the light intensity of the torch falling on the toy and observed that the shadow on the wall has become darker. However, I didn't observe this phenomena when I switched of all the lights of my room and repeated the experiment.

    EG7fo.gif

    As far as I have understood, the directed light from torch tries to cut-off the ambient light on the wall. The more the intensity of torch light is, the more ambient light gets cut-off.

    I will be really grateful if somebody can formally explain this phenomena to me with all the physics laws involved, preferably with some links or illustrations on the topic.
     
  2. jcsd
  3. Dec 13, 2015 #2

    mfb

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    It does not. Just the contrast increases, which our brain perceives as "darker" shadow. Use a photodiode or some other objective measurement and you'll see the shadow does not get darker.
     
  4. Dec 13, 2015 #3
    I will be grateful if this phenomena is explained to me formally in detail. I mean why at all this is happening and our brain is doing like that?
     
  5. Dec 13, 2015 #4

    jim mcnamara

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    You are really asking about how the visual cortex in our brain operates. AFAIK there is no completely correct, testable model for this problem. In a very, very general persepective: our vision is adapted to be very efficient. It evolved two primary vision systems: color vision for daylight conditions, and 'black and white' low level light (night) vision. There are intermediate light conditions where both can be turned on and functioning: What you saw when there is clear vision in both shadow and light.

    Have you ever walked out from a very dark room and then been blinded by daylight? This happens because the biochemistry of your eye has to change to the new light level. From only using night vision.

    When there is a large disparity in light levels your eye adjusts to use the bright 'setting' only, and turns off the low level (night) system. Your brain knows the lowest signals are turned off so it fills in those areas with black. BTW most of what you see perpheraly is the same kind of fill - the brain knows there are few signals to process so it make up stuff that was there before. Ever read something on a sign intently, then look up and see that the people or cars in your field of view instantly changed? Your brain got a new dose of information for what was previously just fill-in.

    Think of vision this way: your brain is creating a megapixel image using a slow input channel, so it has to fill in lots of parts of what you see. If this wasn't true magicians would be out of business, for example.

    See: https://en.wikipedia.org/wiki/Night_vision
     
  6. Dec 13, 2015 #5

    jtbell

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    A closely related factoid is that sunspots are not really black.

    https://van.physics.illinois.edu/QA/listing.php?id=28682&t=are-sunspots-really-black
    http://image.gsfc.nasa.gov/poetry/workbook/sunspot.html

     
  7. Dec 13, 2015 #6
    What I have understood after reading other answers as well: "less brighter objects surrounded by brighter area, appear darker and if the brightness of the brighter area increases further, then the less bright objects appear more dark."

    Am I right? The same thing also happens when we take photos using a camera. Is it easier to explain this if we consider camera instead of a human eye?
     
  8. Dec 13, 2015 #7

    mfb

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    Here are guessing questions. What is brighter?

    (1)
    - the surface of a light bulb (like this one) at night
    - a white house in direct sunlight

    (2) which circled square in this image?
    (3) A or B in this image?

    (1) It depends a bit on the parameters, but they are very similar. It does not appear so, because the light bulb surface is by far the brightest object, while the street is darker than many other surfaces seen at the same time.

    2+3: they are exactly the same. Take a graphics program to move them next to each other if you don't believe me.

    Right.

    Cameras are accurate - they record whatever they get (but their exposure time will depend on the light conditions). Humans looking at pictures are the same problem as before.
     
  9. Dec 13, 2015 #8

    FactChecker

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    A single-point light source makes very sharp shadow edges. Ambient lighting contains a much higher percentage of reflected light from many directions. So the shadow edge is much less sharp. There might not be any visible shadow at all.
     
  10. Dec 13, 2015 #9
    Well, I used a program to check the pixel intensities of the marked portions in (2) and (3). They were exactly same! However, these images are artificially created.
    If we use a camera to capture the two scene about which I am talking about, even in the images the shadow appears to get darker. It means the camera also might be doing something similar to eye! So can someone explain this phenomena in terms of camera (assuming it is easier than human eye!)
     
  11. Dec 13, 2015 #10

    mfb

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    Did you check with a program or by eye?

    Exposure time of the camera is another thing to consider.
     
  12. Dec 13, 2015 #11
    I checked by a program also, there is a intensity difference of around ~15 (8-bit images, intensity ranging from 0 to 255). By naked eye, one can quite easily observe this.
     
  13. Dec 13, 2015 #12

    jim mcnamara

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    You can validate actual light levels with a light meter (some cell phones have that builtin to the camera hardware), a camera and other light sensing equipment.
    You can NOT do his reliably with your eyes. Your brain enhances and plays with images, as do the light processing cells in your retina.

    Example: we can 'see' a lot of colors and intermediate color values. There are three light sensing pigments in your eyes, so you should be able to see only a limited number of colors assuming that pigments are the only perceptual mechanism. They are not - the retina and the brain mess around hugely with the raw color input. Like they do with all of your vision input data.

    BTW - your eyes are actually derived from the same precursor cells as your brain's precursor cells, and can be thought of as part of the brain. Some models of human vision use that concept to teach vision theory. Which is very complex.

    Boy, Biology loses something when I try to make it Physics-palatable. Sorry.
     
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