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cornea (Fig. 13.14). Behind the cornea, Class VII. The lens focuses light on the we find a dark muscular structure back of the eye, on a layer called retina called iris. In the iris, there is a small (Fig. 13.14). The retina contains several opening called the pupil. The size of nerve cells. Sensations felt by the nerve the pupil is controlled by the iris. The cells are then transmitted to the brain iris is that part of eye which gives it its through the optic nerve. There are two distinctive colour. When we say that a kinds of cells– person has green eyes, we refer actually (i) cones, which are sensitive to bright to the colour of the iris. The iris controls light and the amount of light entering into the (ii) rods, which are sensitive to dim light. eye. Let us see how. Cones sense colour. At the junction Ciliary of the optic nerve and the retina, there muscle are no sensory cells, so no vision is possible at that spot. This is called the Iris blind spot. Its existence can be Optic Lens Nerve demonstrated as follows: Cornea Activity 13.9 Make a round mark and a cross on Retina a sheet of paper with the spot to the right of the cross (Fig. 13.15). The Fig. 13.14 : Human eye Caution : For this activity, never use distance between two marks may be 6-8 cm. Hold the sheet of paper at a laser torch. an arm’s length from the eye. Close Activity 13.8 your left eye. Look continuously at the cross. Move the sheet slowly Look into your friend’s eye. Observe towards you, keeping your eye on the size of the pupil. Throw light on the cross. What do you find? Does her eye with a torch. Observe the the round mark disappear at some pupil now. Switch off the torch, and point? Now close your right eye. observe her pupil once again. Do you notice any change in the size Look at the round mark now and repeat the activity. Does the cross of the pupil? In which case was the disappear? The disappearance of the pupil larger? Why do you think it cross or the round mark shows that was so? there is a point on the retina which In which case do you need to cannot send messages to the brain allow more light in the eye, when when light falls on it. the light is dim or bright? Behind the pupil of the eye is a lens which is thicker in the centre. What kind of lens is thicker at the centre? Fig. 13.15 : Demonstration of blind spot Recall what you learnt about lenses in 172 SCIENCE 2024-25