Optical illusion

solution to cointrick

When light passes from air into water it bends(refracts).

In air light travels in a straight line to the eye. The coin is obscured by the side of the cup. As the cup is filled with water the light is refracted so that the coin is visible. The brain interprets the light as being straight and creates an image of the coin that appears to be closer to the surface than it actually is.

Look at the animation on the left.

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Clear section of glass. Cancle on one side of the glass placed equal distance from the glass as the burning candle on the other side. Burning candle on the other side of the glass. Reflected image of flame. Unlit candle Burning candle Clear glass