Why does the light below the optical mouse brighten up?

See, If you follow the steps below, you will see the light below the optical mouse brighens up.

1. Lift up your optical mouse.
2. Rotate it upside down. It’s not required. You can move your finger near to the light after lifting it up .
3. Just move your finger near to the blinking light below the mouse (Don’t touch the finger to that portion. Just keep your finger 2 to 3mm near to it).

I wonder, how can this happen?
Is there any sensor provided ?