Title Chapter 10 The Human Eye and The Colourful World.pdf ✅

SCIENCE (Physics) Chapter 10: The Human Eye and the Colourful World
(1) 10 THE HUMAN EYE AND THE COLOURFUL WORLD Human Eye and the Colourful World Human Eye ● The human eye is an important and valuable sense organ which uses light and enables us to see the colourful world around us. ● The various parts of the human eye and their respective functions include Part Function Cornea Protective layer of the eye Refraction of light rays entering the eye Eye lens Adjust the focal length and form an inverted image of the object on the retina Pupil Regulates the amount of light entering the eye Iris Controls the size of the pupil Retina Acts as a screen for forming the image Ciliary muscles Adjust the thickness of the lens Optic nerves Send signals to the brain ● The image of any object seen persists on the retina for 1 th 16 of a second, even after the removal of the object. This continuance of sensation on the eye for some time is called persistence of vision. ● The numerous light-sensitive cells contained in the retina of the eye are of two types: o Rod-shaped cells which respond to the brightness or intensity of light. o Cone-shaped cells which respond to the colour of light. .
(2) 10 THE HUMAN EYE AND THE COLOURFUL WORLD Power of Accommodation of the Human Eye ● Power of accommodation of the eye is the ability of the eye to observe distinctly the objects, situated at widely different distances from the eye, on account of change in the focal length of the eye lens by the action of the ciliary muscles holding the lens. ● The farthest point up to which the eye can see objects clearly is called the far point (F) of the eye. It is ideally infinity for a normal eye. ● The point of closest distance at which an object can be seen clearly by the eye is called the near point (N) of the eye. For a normal eye, the near point is 25 cm, which is called the least distance of distinct vision (d) of a normal eye. ● The distance between the far point (F) and near point (N) is called the range of vision of the eye. Defects of Vision (a) Myopia or Short-Sightedness A person with myopia can see nearby objects clearly but cannot see distant objects distinctly, as if the far point of the eye has shifted from infinity to some particular distance from the eye. (a) Myopic Eye (b) Far Point of a Myopic Eye This defect may arise due to (i) excessive curvature of the eye lens or (ii) elongation of the eyeball. To correct myopia, the person has to wear spectacles with a concave lens of focal length equal to the distance of far point of the myopic eye. (c) Correction for Myopia
(3) 10 THE HUMAN EYE AND THE COLOURFUL WORLD (b)Hypermetropia or Long-Sightedness A person with hypermetropia can see objects lying at large distances clearly but cannot see nearby objects clearly, as if the near point of the eye has shifted away from the eye. (a) Hypermetropic Eye (b) Near Point of a Hypermetropic Eye This defect may arise because (i) focal length of the eye lens is too long or (ii) the eyeball has become too small. To correct hypermetropia, the person has to wear spectacles with a convex lens of focal length f, given by f x′ d x′− d , where d is the least distance of distinct vision and x′ is the distance of near point N of the hypermetropic eye. (c) Correction for Hypermetropia eye (c) Presbyopia ● Presbyopia is a human eye defect because of which an old person cannot read and write comfortably. ● It occurs in old age when the ciliary muscles holding the eye lens weaken and the eye lens loses some of its flexibility. ● To correct presbyopia, an old person has to wear spectacles with a convex lens of suitable focal length (as in hypermetropia).