I have a strip of film which I shone a light behind with my phone flashlight. But the image projected onto a wall comes out blurry. But why is it blurry and how would a lens make it clearer?
In: Physics
With the phone flashlight, light can go from any point of the LED (which is a rectangle) to any point on the film. So you have light coming from multiple directions, which will project the color from the film to multiple locations on the screen. That makes it blurry.
To fix this, you can use a pinhole: Punch a small hole into a piece of paper or cardboard and put that in front of the camera light. This way, there’s only one path that the light can take from the light source to the film, and your image should be less blurry. However, it will also be a lot less bright, since most of the light is absorbed by the paper.
This also works in reverse: You can put the film behind a tiny hole in a box and get a reasonably sharp image. This is how the earliest photographic cameras worked, [and still an interesting hobby or school project](https://pinholemanual.wordpress.com/making-a-pinhole-camera/shoebox-pinhole-camera/).
Imagine arrows coming out of the phone light, through the film onto the wall. If you have a point light source, then everything is fine, each arrow can only go through a single possible point on that film and that film sets the amount of light that goes through at that point.
Now imagine the same thing but two points close to each other, now each of those points projects these rays of light, the key is each point maps the entire film then the entire wall, so two points means there are two different rays going to the same point on the wall, each going through a different part of the film.
Now add in another two points of light, 4 points, each point on the wall gets 4 different rays to it, each from different parts of the film.
Keep adding points.
Your phone camera is not a point source of light, it is likely a round shape a centimeter across. Were it a point source of light there would be no problem. But it isn’t.
So we use a lens to correct the trajectory of the rays.
Latest Answers