Watch what happens to the relative heights of the two men when they switch places on an abolutely level plank.
What's Going On?
The Plank illusion involves two people standing on either side of a level plank.
The two men's relative height to each other appears to dramatically change when they switch places.
Not only does this illusion work for those people who are watching it from the side, but it also works for the two participants on the plank!
It is important to separate the Plank illusion into two separate illusions.
The first, involves the perception of bystanders who judge the relative heights of the two people on the plank.
The second, involves the two participants who judge their respective heights to each other as they exchange places on the plank.
What accounts for these strange effects? First of all, it is important to understand the basic set-up of the illusion.
The plank is made level on a slope.
The two participants standing on it should be able to see both down and up the hill some distance. Their toes should be at least twenty inches apart.
The camera is on a tripod with its lens about three feet above the level of the plank, approximately eleven feet away from the plank, and offset to the right by about thirty inches.
With the camera in this position the person on the right will present a larger visual angle to the camera.
The camera is also slightly tipped so that the plank is square with the picture.
This size illusion, like many such size illusions, is dependent upon your confusing size with distance, which interferes with your ability to do proper size scaling.
The Plank illusion combines features that are inherent in the Ames Room size illusion and in an anti-gravity house.
For example, as in an Ames Room, the person standing on the left is actually further away from the camera than the person on the right, but they are perceived to be the same distance away. Therefore, the person on the left has a much smaller visual angle than the person on the right.
The person on the right is also at the same elevation as the person on the left, just as in the Ames Room.
How is this accomplished?
The camera is tipped so that the horizontal is lined up with the plank, and not with the true horizontal.
This changes your frame of reference, just like what happens in the vicinity of the anti-gravity house.
In this case, the plank, and the framing of the picture, set the apparent horizon.
This has the affect of causing the receding figure to maintain a constant level to the horizon, which negates your ability to perform proper size-scaling
The Plank illusion is an example of the Ames room size illusion without the room!
A separate effect accounts for why this illusion also works for the two individuals standing on the plank.
When the two participants exchange places, they also see a change in their respective heights when they are asked to look straight ahead.
Experience, however, has always taught us that ìstraight aheadî is usually on a line of sight parallel to the ground ahead.
In this case, the ground under the level plank slopes.
The two people who are standing on the plank tend to have their line of sight parallel with the sloping ground.
When the uphill person is asked to look straight ahead and say where his eyes hit on the other person, he will misjudge and be looking slightly downhill, thinking that he is looking straight ahead.
The opposite is true for the person looking uphill.
The Plank illusion is usually found at tourist attractions that have the tilted anti-gravity houses.
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