why does myopia cause night blindness?!

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Without going into too much detail, here is an answer that will hopefully suffice. Myopia basically means that the person is nearsighted. They have trouble focusing on objects far away. So night blindess is basically caused by the fact that it is harder to see at night to begin with, and for those that are myopic they have even more trouble focusing on objects far away. They have two things working against them at night: 1) Lack of light 2) their myopia.

The myopia itself is caused by the angle the light transmits on the retina. It has to do with the shape of their eye, which affects how the light rays focus on the eye. So the mechanism of night blindness in myopic patients is the same mechanism for myopia in patients during the day time, it has to do with the shape of the eye affecting the way light rays are focused on the retina. That's why lasik surgery (corrective eye surgery) works, because it changes the shape of the eye so the light rays focus better.

Hope this helps!

Medical school/ physics