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I had a Macbook Pro, and remember being intrigued when I noticed that the apple logo on the back of the screen allowed sunlight to shine through in an Apple shape on the front of the screen when turned off. But when the screen was turned on, it would go black. The black screen eliminated the apple shape that was visible when the screen was not lit.

I'm not quite sure I understand how black light works (and I assume black light is emitted, to achieve perfectly black color, darker than the screen itself), much less why it would mask white light shining through the emission surface. It seems like the white light would show through.

So. Why does projecting black light from a screen mask white light shining through it?

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  • $\begingroup$ Why do you assume they use black light? I do not really know what apple uses, but an easy way to do it is to turn on two perpendicular polarimeters, or just a black screen or electric filter that covers the apple behind the logo $\endgroup$ – Wolphram jonny Nov 12 '14 at 5:14
  • $\begingroup$ @julianfernandez I'm pretty sure the apple showing through was not intended. So I don't think they do anything special to hide the logo when the screen turns on. I think it's just black light. I don't really know how "black light" works, but projectors can turn a white wall black so.. $\endgroup$ – Viziionary Nov 12 '14 at 6:16
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    $\begingroup$ There's no such thing as black light (well, there sort of is, but that's a misnomer). Anything you see as black among non-black emits/reflects less light, not more "black" light. $\endgroup$ – Ruslan Nov 12 '14 at 8:08
  • $\begingroup$ @Ruslan I see. What about projectors? projectorreviews.com/review-tv/… They emit something pretty close to black light. $\endgroup$ – Viziionary Nov 12 '14 at 8:14
  • $\begingroup$ No they don't. They just increase the contrast by adjusting the brightness of surrounding pixels and trying to not emit stray light to supposedly black ones. See this thread, for an example. $\endgroup$ – Ruslan Nov 12 '14 at 8:22
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Macbooks use LCD screens. These have a liquid crystal layer with a backlight behind it. The LCD screen works by changing its opacity i.e. it controls the amount of light that can be transmitted through it from the backlight. If the LCD is not blocking any transmitted light then all the light from the backlight is transmitted and the screen looks white. If the LCD is blocking all the transmitted light then it lets no light through and looks black.

And this explains why a black screen blocks sunlight shining through the lid. It's because a black screen blocks all light.

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Perhaps it is working as a grease spot photometer as well. The display back light may just be much brighter than the diffuse light coming through the logo.

http://www.phy6.org/outreach/edu/greaspot.htm

As mentioned in the other answer it is possible that the LCD is transmitting less light when it is operating and so blocking out the back light (and the apple light)

To test which one is responsible it might be possible to test this by shining a bright light (much brighter than the back light) into the apple and seeing how a white or black area of the LCD passes this light to determine which effect is more pronounced.

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