Why do objects in the distance appear to converge?

23 November 2008

Question

Why do objects in the distance appear to converge?

Answer

Chris - If you see two railway track in the distance they seem to come to a point. The same thing happens with sunlight - these corpuscular rays or "God's fingers", coming through the clouds they also seem to be coming from a focal point just beyond the clouds. Why is that? Dave - This is all to do with geometry. How big something looks on the back of your eye is to do with how big of an angle it takes up in front of you. If two things are a very small angle apart then they're going to look very close together in the back of your eye. If there's a big angle then it's going to use more of your retina so they'll look bigger. How do you make that angle? It's all geometry done at school. If you imagine a big triangle with one side how far away something is and then a right angle between how far apart the two objects are. The farther away something is, the smaller angle they're going to make between the two of them. The smaller they're going to be on the back of your eyes. The reason why railway tracks look as if they're getting closer together is that they're a metre apart closer to you and a metre apart a mile away but the angle it makes closer to you is much larger than a metre will make a long way away from you. Chris - Why do you think the visual system doesn't compensate and tell us that things are just the same distance apart, even though they might be getting smaller?

Dave - I think your visual system does, to some extent, compensate because the moon looks different sizes depending on how high it is in the sky. If it's right on the horizon or it's near objects you think are much farther away you think it's much bigger than when it's up in the sky. You have no concept of how far away it is so you think it's smaller which is how you get this optical illusion that the moon looks huge on the horizon but tiny up in the sky.

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