Typically, it is not wavelengths from the visual light spectrum. It is common to use infrared. For example, high-power infrared light can penetration at least 3 cm into the brain.
Uhh, I'm sure it's fine, but how high power are we talking?
Just when you say "high-power infrared light" my brain thinks "cooking an egg"
Would that not risk heating the brain and injuring the neurons?
"Non invasive" in this context means not physically opening the skull and inserting a device.