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.

Why does this happen?

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?