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.

Scene 1: 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.

Scene 2: 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.

I captured Scene 1 and Scene 2 using a camera. I compared these two images using a program. The shadows are of different intensity in the two images as it was observable by the naked eye.

A practical example: Sun spots appear black, but they aren't actually black

It seems camera also adapts and works similar to our eyes. Can somebody formally explain this to me? I am more interested in the image formation by the camera in this case, not the human eye and biology related things.

  • 6
    $\begingroup$ I think that's just your eyes adapting to the mean ambient light: you increase the ambient light, the shadow stays the same, your eyes adapt to the extra light, the shadow looks darker. $\endgroup$ – rodrigo Dec 13 '15 at 11:07
  • $\begingroup$ @rodrigo: Well, I don't think so. There is a significant change in the darkness. Can you please elaborate 'adapt to the extra light'? Moreover, if I take a picture of the scene, there is a significant difference in the shadow darkness in the images. $\endgroup$ – kunal18 Dec 13 '15 at 11:09
  • 1
    $\begingroup$ Significant how? Are you using your eyes? Human eyes measure intensity by comparing with the surrounding objects, and that can trick you. Check this illusion, for example, squares A and B are exactly the same! $\endgroup$ – rodrigo Dec 13 '15 at 11:12
  • 5
    $\begingroup$ I'm voting to close this question as off-topic because it is about perception of light intensity in the human eye/brain (as becomes clear in the comments) and thus about biology rather than physics. $\endgroup$ – Norbert Schuch Dec 13 '15 at 13:57
  • 1
    $\begingroup$ Could you provide the data set showing the intensity differences? $\endgroup$ – Kyle Kanos Dec 13 '15 at 21:26

Ok, let's put some numbers to the scene. They are arbitrary numbers, in an arbitrary scale, just for illustration purposes, but I hope that will make my explanation clear.

I will use light units (lu) as an absolute scale for light intensity, and apparent units (au) for the perceived light intensity.

Scene 1: You have your room lit with a room light. The toy projects a shadow to the wall. The un-shadowed wall will have a light intensity of 100 light units (lu), while the shadowed wall will have 10 lu (because of ambient light, diffuse reflections in other walls, etc.).

Scene 2: You switch on your torch and point to the toy. The torch will make the un-shadowed wall brighter, with extra 1000 lu, that added to the room lamp gives a total of 1100 lu. The shadowed wall will get a bit of extra diffuse light, but not much because the torch has a narrow beam, so it will be about 15 lu.

That is:

  • Shadow 1: Wall of 100 lu, shadow of 10 lu.
  • Shadow 2: Wall of 1100 lu, shadow of 15 lu.

Now, your eyes, and your camera, compensate for the full intensity of the room, that is they usually try to normalize the light intensity of the full scene (also the colors but that is out of topic).

And since the wall is what dominates the scene, your eyes/camera will normalize the situation in this way, more or less. This is actually an oversimplification, since human eyes are non-linear, that is their response to light is logarithmic, but that detail is not necessary for this explanation, and will only complicate the computations without changing the broad conclusion.

  • Scene 1: divide by 100.0

    • 100 lu -> 1.0 au (apparent units)
    • 10 lu -> 0.1 au
  • Scene 2: divide by 1100

    • 1100 lu -> 1.0 au
    • 15 lu -> 0.014 au

As you can see, although in scene 2 the shadow is absolutely brighter (10 lu vs 15 lu), it looks a lot darker (0.1 vs 0.014 au).

  • $\begingroup$ After reading you answer, it seems to me that I didn't put my question clearly. Actually in ambient conditions there is no shadow on the wall. Only when the toy is illuminated with torch, the shadow is formed. If the intensity of torch light is increased, the shadow formed becomes darker. But I think the answer will hold true in this scenario too. $\endgroup$ – kunal18 Dec 13 '15 at 12:47
  • $\begingroup$ @stalin: Oh, you have a regulable torch! But you are right, the idea is basically the same. $\endgroup$ – rodrigo Dec 13 '15 at 12:51
  • $\begingroup$ In case of the scenario which you have considered while writing the answer, the ambient shadows actually go away when the scene is illuminated with torch! They don't become darker as you have said. $\endgroup$ – kunal18 Dec 13 '15 at 12:55
  • $\begingroup$ @stalin: I'm not sure about what you say here... I was assuming that the room light and the torch were coming more or less in the same direction, so both shadows mostly overlap. If they do not, then your toy would have two different intensity shadows, and things would get complicated. $\endgroup$ – rodrigo Dec 13 '15 at 13:03
  • $\begingroup$ Yes. So, The ambient lighting doesn't cause any shadow on the wall currently. The shadow on the wall appears only because of torch light and then it gets darker when torch light intensity is increased. If there are any ambient shadows, they disappear in presence of torch light! $\endgroup$ – kunal18 Dec 13 '15 at 13:06

Not the answer you're looking for? Browse other questions tagged or ask your own question.