Ask Marilyn The smoke detector in my bedroom emits a soft green light. If I look at it in the dark, I can’t see it. But if I look slightly to one side, I can see the light clearly. Why is that? —Jim Hester, Fairview, Texas
For the same reason it seems you can never see a shooting star where you’re looking: It’s always in your side vision. Your retina contains two kinds of light-detecting cells. Cones, which are highly concentrated in the center of your eyes, detect brighter lights; rods detect lower levels. So to see a dimmer source of light, you must avert your eyes just a bit. That visual sweet spot varies from person to person.