Explaining visual illusions: What do they mean and how can they be dangerous?

Explaining visual illusions: A visual illusion is not an optical illusion. An optical illusion is caused by an optical device, a lens or prism or mirror etc. A visual illusion is created within the visual system. It is an incorrect interpretation made by the brain from data which is scanty, unreliable, or in conflict with commonly represented articles.

Another interesting visual illusion is created by exhaustion of visual cells. Driving on a highway for a long time at high speed causes fatigue of the motion direction cells. When you reach the city you keep feeling that you are going slow, though your speed, for the city, would be dangerously fast. Thus you keep on driving faster than you should, leading to an accident. It is the major cause for accidents which occur after a long drive. Thus after any spell of long-range fast driving, a short stop to let the cells reorient themselves, just before entering the city, is an important safety measure to prevent accidents.

The size of an object also influences your judgement as to how fast it is moving. A jumbo jet seems to land much slower than smaller aircraft. Similarly, driving on a foggy night, a smaller car seems to be further away than it actually is and can cause an accident from incorrect judgement.

Are there different cells in the retina for black, white and colour vision?

Yes. Retinal light-sensitive cells are of two types—rods for black and white vision and cones for colour vision. Most of the cones are packed at the point of clearest vision in the retina termed the macula. In the centre of the macula is a depression termed the fovea which has the sharpest sight.

The rods are more numerous than the cones, 125 million rods to 6 million cones. The rods are sensitive to low illumination while the cones can distinguish fine details and colour but need a brighter light. Thus in a dim light, colours and details are difficult to make out.