The World Through Our Senses

THE WORLD THROUGH OUR SENSES Topic : Sensory Organs and Their Functions

THE WORLD THROUGH OUR SENSES Topic : Sensory Organs and Their Functions LIGHT AND SIGHT A .

Astigmatism

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Astigmatism is caused by the irregular curvature of the cornea. All the light rays from an object do not meet at a point on the retina. On the other hand, some light rays are focused on the retina while others are focused either in front or behind the retina. In many cases, astigmatism causes blurred vision for either near or distant objects. To correct astigmatism, the optician recommends cylindrical lenses.

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Limitation of the sense of sight Optical illusion

1 Sometimes what we see may not appear to be the real thing. This is because the brain cannot interpret accurately what is actually seen by the eye. 2 This limitation of the sense of sight is known as optical illusion (confusion of the brain).

Laboratory Activity Aim Material Method

To study optical illusion Ruler

1 The pictures (a) to (f) in Figure 1.27 are observed carefully. 2 A ruler is used to measure if necessary Observation

Picture (a) (b) (c) (d) (e)

(f)

Guide

Observation

Are the lengths of lines AA’ and BB’ the same? Is the length of the base of the hat the same as its top? Are the pair of lines C and D straight? Are the lines E, F, G and H parallel to one another? Does the shape of the figure change after viewing for a long time?

no

What can you observe from picture (f)?

no no no

Observation after careful study Lines AA’ and BB’ are of the same length. The lengths of the base and the top of the hat are the same. The pair of lines C and D are straight. The lines E, F, G and H are parallel to one another.

yes

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vase/ shadow of faces of two persons

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Discussion 1 The limitation of the sense of sight seen in the above activities is known as optical illusion. 2 The brain cannot accurately interpret what is seen by the eye. Conclusion Optical illusion happens when the brain cannot accurately interpret objects viewed.