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Every laser laboratory in the universe has this sign on the wall:
Do NOT look into the laser beam with your one remaining eyeball.
This is meant to be funny. It is also meant to be a practical warning. Lasers can cause blindness when not used as directed. On the other hand, sometimes it is necessary and even prudent to shine a laser into your eye.
One such time is when you want to take a picture of the back of your eyeball, the retina. On the retina, there are cells that detect light, called photoreceptors. These cells come in two shapes: rods and cones. A lot of eye diseases are related to the rods and cones, so eye doctors would like to get clear images of them. However, the inside of the eyeball is not clear. Oh, maybe for short while, but the eyeball is filled with a fluid called the vitreous humour. It’s a clear liquid but it has some variation of index of refraction throughout it. It’s like our blobs of atmosphere, but blobs of funny vitreous stuff, instead. And unlike the atmosphere that is floating around us, this is the turbulence inside our eye. In the past, images of the retina were often too blurry to see any detail. To understand the structure and function of the eye, researchers would dissect eyes from deceased folks.
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