英语时差：黄色太阳镜

Look at a cloud-filled sky, then look again through a pair of amber sunglasses.

You'll be surprised to see the hazy, bluish clouds jump suddenly into focus, revealing a world of unexpected detail. Many skiers use amber sunglasses too, to help see detail in white banks of snow. On today's Moment of Science, we'll learn how amber-tinted sunglasses make things appear so much sharper.

The answer has as more to do with your eyes than it does with the sunglasses. Behind the pupil of your eye is a natural lens like a tiny bag of transparent jello. This lens focuses images on the back of your eyeball the same way a camera's lens focuses images onto film.

Unfortunately, even healthy eyes can't focus everything perfectly. This is because each color of light focuses slightly differently through a lens. Imagine a slide projector showing a red and a blue image on the same screen. If you focus the red image sharply, the blue one will be slightly blurry. If you twist the knob to focus the blue image, the red one will drift slightly out of focus. You can only get ultra-sharp focus of one color at a time.

Healthy eyes are set to focus yellow, the middle of the color spectrum. This is great for yellow--and pretty good for green and orange--but it makes red and blue appear slightly out of focus. White sunlight contains all the colors, so the red and blue extremes will make any white object seem somewhat hazy. Amber sunglasses block the blue and red parts of sunlight, letting you concentrate on the yellow colors that are easier to focus. Without blue and red, everything seems sharper.