Why does an optical mouse need a surface to work on?

Question

Here is how I understand the mechanic of the optical mouse. An optical mouse uses an optical part (LED or infrared light) to capture the surface beneath. It does this process multiple times per second, then calculates how far and fast it has moved and finally transforms to a coordinate of the cursor on the screen. But why it cannot work if I lift the mouse up and move along (not touch) a surface? Is it hard to process the images in that way or is it not friendly to users?

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Answer 1

the optics on the bottom need to be able to see & track tiny variations on the table surface. it has to have good optical focus on the surface in order to see these variations. the optics are generally simple and cannot re-focus when you change the distance like a camera can.