Why do sunglasses work?

[budfox]

Possibly a silly question, but isn't the point of the pupil in your eye to open and close, allowing for different light levels? If that's the case then how come putting on a pair of shades doesn't just force your pupil to open a bit and negate the effect?

[budfox]

Yes, but even in conditions where my iris would be within opening and closing range, sunglasses stil make things darker.

[budfox]

Thanks all.