Why can't we see UV light?

Answer: Our eyes lack cells that respond to UV wavelengths

Our eyes lack cells that respond to UV wavelengths ✓ — Correct! The retina's light-sensing cells are tuned to roughly 380–700 nm, so UV light below that range doesn't trigger them. The lens and cornea also absorb most UV before it reaches the retina. Some animals, like bees and birds, do have cells tuned to UV and can see it.

UV light is simply too weak for our eyes to detect — Wrong. UV light can be very intense—think of sunlight on a bright day. The problem isn't strength; it's that our eyes have no cells tuned to respond to UV wavelengths, just as we can't see infrared.

The brain filters out UV wavelengths before we notice them — Wrong. The brain doesn't filter UV; it only processes signals the retina sends. Since the retina has no cells that respond to UV, no such signal is ever generated for the brain to filter.