The ciliary muscle to which eye lens is attached (see fig-3) helps the eye lens to change its focal length by changing the radii of curvature of the eye lens.
When the eye is focussed on a distant object, the ciliary muscles are relaxed so that the focal length of eye lens has its maximum value which is equal to its distance from the retina. The parallel rays coming into the eye are then focussed on to the retina and we see the object clearly.
When the eye is focussed on a closer object, the ciliary muscles are strained and focal length of eye-lens decreases. The ciliary muscles adjust the focal length in such a way that the image is formed on retina and we see the object clearly. This process of adjusting focal length is called “accommodation”. However these muscles cannot strain beyond a limit and hence if the object is brought too close to eyes, the focal length cannot be adjusted to form an image on the retina. Thus there is a minimum distance for distinct vision of an object which is roughly equal to 25 cm as we have learned in activity-1.
The eye-lens forms a real and inverted image of an object on the retina. The retina is a delicate membrane, which contains about 125 million receptors called ‘rods’ and ‘cones’ which receive the light signal (cones- identify the colour: rods-identify the intensity of light). These signals are transmitted to the brain through about 1 million optic-nerve fibres. The brain interprets these signals and finally processes the information so that we perceive the object in terms of its shape, size and colour.
In our previous discussion, you have learnt that eye-lens itself changes its focal length in accordance with distance of the object with the help of ciliary muscles.
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