|MadSci Network: Physics|
Hi, Ernest – you asked why snowflakes appear to radiate from a point in front of you as you drive, but the effect disappears when you stop. This optical illusion is based on the property of perspective. Think about looking down a very long square corridor. As you know, perspective makes the walls converge – eventually to a point. But of course, the walls are actually parallel. If small lights are placed along the walls and floor and ceiling at even intervals they would appear to radiate from a point – but in this case, that effect occurs while you are stationary – you don’t have to move forward to see the effect. Now, in a snowstorm, the flakes are falling, but do so fairly slowly – compared say, to raindrops. If you are driving fast into them, they fall a lot more slowly than you take to reach them from first sighting them. Assume that the flakes are more or less evenly distributed – i.e. are equidistant from each other. Then imagine a small cell of snowflakes some distance in front of you. They will seem much closer to each other than they actually are, because of perspective. As you rapidly bear down on them they will appear to separate, as the distance between them becomes visually larger, again, due to perspective. This applies to all groups of flakes in your forward vision and so they all appear to radiate from your point of focus. If you stop the car, all you see is the flakes falling vertically – unlike a corridor there is no fixed structure to see converging to a point. Meteor showers or stars flashing by at “Warp Speed” would behave just like snowdrops. The effect does not depend on persistence of vision.
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