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

Albuquerque Journal - Parade - - Healthy - By Mar­i­lyn vos Sa­vant Send ques­tions to mar­i­lyn @ pa­

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.

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