Your vision and optical illusions

1. Your visionand optical illusions - A story about the most important sense and the way we use it.

2. Content • The eye • Vision and brain • Optical illusions • Try it - optical illusions

3. The eye The human eye weighs 7½ g and measures 2½ cm in diameter. It is a fantastic piece of precisions mechanic with a sophisticated construction. In principle it is like a camera – but the eye functions are superior due to the image processing by an advanced computer – the brain. Sclera – The white of the eye Iris – The colour

4. The human eye is complex – it is composed of many fine layers and areas with specialized functions. The lens is elastic and used to focus on what we decide to look at. The retina is situated at the posterior of the eye – here the image is formed. A special area of the retina - Fovea – is specialized to make a very sharp image of what we see. The images are sent to the brain via the optic nerve – the we have a blind spot. Fovea – the yellow spot The lens The blind spot – optic nerve head

5. In the retina you find the photoreceptors termed cones and rods. The human eye has 3 kinds of cones to detect red, green and blue colours. We mix these 3 colours to produce all the other colours we are able to see. Defective cones result in colour-blindness, which is mostly seen among males. The rods produce a blurred image, but they are 10.000 times more sensitive to light than the cones. The Retina Fovea – the yellow spot Cones red, green and blue colour vision Rods greyscale night vision

6. Vision and brain The visual impression is transmitted from the eye to the brain via the optic nerve system. The optic nerves from the left and right eye intersect to mix the visual impressions into one image. The human visual sense is highly advanced – you are able to detect colour, intensity of light, contrast, details and movements in 3D. In fact it is the brain that do most of the work.

7. Optical illusions The brain process our visual impressions into images, and the power of the graphic processor of the brain is 10.000.000.000 times that of an ordinary PC ! But the brain is a neural network – is learns to make the images of our visual impressions. That is why the brain can be fooled when it anticipate a certain known image and the visual information is wrong. Optical illusions use the experience of our brain to create fake images. How many legs does the elephant have ?

8. Can these constructions be made in the real world ?

9. Shades and colours can fool the brain ! The horses have the same colour – it is the background that differs. A and B are the same grey – the shadow cheats you.

10. The brain tries to make recognizable images. How many bars do you see ? How many triangles do you see ? There are two – the rest is illusions. There are no triangles – there are pies and angles.

11. Colour and shadows make an illusion of movement.

12. It is the small movements of your eyes that make the image alive.

13. By focusing on a point you may freeze a part of the image.

14. Try to focus at one centre – the movement freezes.

15. Choose an illusion at the top row – either geometry or colourUse the slide bar (blue dot) to make the illusion disappear.