I am in psychology class, the light’s are on and I clearly see this black squares on white background. How is this possible? https://www.instructables.com/Optical-Illusion-Black-Squares-and-Gray-Dots/
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Projectors don’t actually project black color, they just don’t project anything where the black parts are. It’s just our perception, because relative to the white/bright parts, those areas seem darker.
The projector obviously can’t remove light from the wall. It can only add more. When we observe an image, we look for relative differences and accept the darkest portion to be black. Pure black almost never exists. Materials always reflect some light. Common video monitors with a backlight usually emit light when fully dark.
Weirdly enough, there’s another optical illusion, [the checker shadow illusion](https://en.wikipedia.org/wiki/Checker_shadow_illusion), that might help you visualize what’s going on. In the image on that page, the dark checkers outside of the cylinder’s shadow are exactly the same shade of grey as the light checkers inside the shadow. Despite that, your brain’s visual centres are making your perceive all of the dark checkers to be the same shade and all of the light checkers to be the same shade, compensating for the shadow’s effect.
That’s the same thing you’re experiencing on the projector screen. The projector isn’t projecting black. It’s just not projecting anything in those spots. Your brain is then filling in the blanks, making you “see” the squares as being black, relative to the illuminated region.
Another [optical illusion](https://en.wikipedia.org/wiki/Checker_shadow_illusion) explains that.
The projector, of course, can’t project darkness. However, it can make everything else brighter, so the parts that aren’t illuminated seem black. That’s also the reason why projectors don’t work in sunlight because they’d have to be blindingly bright to get enough contrast.
I am an engineer and I have worked a lot with projectors. There are 3 ways you could be seeing the black square. I will start with the least likely and work my way up to the most likely.
1. Reflection: It is possible that the black square is a reflection. If there is a pane of glass or a whiteboard that is reflecting the black square (that might be hidden to you) then the reflection might be strong enough to see from where you are. This one is really not likely. Modern projectors do not create a reflection that is strong enough to see from a distance unless you are looking at the surface of the screen.
2. Scatter of the light: Light is never perfectly collimated (meaning that it is a laser that is all going in exactly one direction) so it will scatter a little bit once it hits a surface. That little bit of scatter could be going into your eyes. This is somewhat likely but if you are seeing black squares from further away then the projector is that will decrease the likelihood of this being a cause.
3. You are not seeing the light: This is the most likely cause. Your brain takes in a **lot** of information and processes it. Think about how you can hear your name being called out even though you are in a crowded room having a conversation, or how you can focus on one person’s conversation in a room with a lot of people talking. Your brain filters out the things that you do not want to pay attention to. It filters out most of the things that you are looking at. (For example, how often do you notice what color the outside of the car you are driving is, or the color of the shirt that the cashier is wearing when you go to the grocery store?) When you are watching a movie or a presentation, your brain will likely prioritize the things that you are watching over the objects in the room around you that are not as important. So, while the light from the projector could be hitting the entire wall (and maybe spilling over a little bit further), you are only paying attention to (and therefore are only aware of) the area that the movie (or presentation) is in.
I hope this helps. Let me know if you have any more questions!
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