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

An optical mouse has a sensor that reads a picture of what’s underneath. Sensor has a lense that focuses light in a way so that the image is focused when the mouse lays flat on the table. Lift the mouse – lose focus and get an blurry mess on the sensor.