So on the macbook air, there is an ambient light sensor (Photo detector). when the room is bright you can see the keys, so the backlight is not needed and turned off to save power. when its dark, it turns on so you can see the keyboard. you can also manually change it by pressing these keys:
typically, (at least for my macbook air and laptops I've seen) its somewhere in the red box.
If you cover it with your finger, you should see the screen brightness go lower and the keyboard up.
What is a Photodetector?
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