CHAPTER 2 THE HUMAN EYE AND THE COLOURFUL WORLD
THE HUMAN EYE
The human eye is like a camera. Light enter the light through a thin membrane called the cornea. It forms a transparent bulge on the front surface of the eyeball. Behind the cornea, the iris is present. The iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate member having enormous number of light sensitive cells. The light sensitive cells get activated upon illumination and generate electric signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals and finally, processes the information.
POWER OF ACCOMODATION
The ability of the eye lens to adjust its focal length is called the power of accommodation. The change in the curvature of the eye lens can thus change its focal length. This modification in the curvature of the eye lens is done by the ciliary muscles.
The smallest distance at which the eye can see objects clearly without strain is called the near point of the eye or the least distance of distinct vision. For a young adult with normal vision, it is about 25cm.
DEFECTS OF VISION
The common refractive defects of vision include myopia, hypermetropia and presbyopia.
Myopia is also called short-sightedness. This happens when the image of distant objects is focussed before the retina. This defect is corrected by using a concave lens of suitable power.
Hypermetropia is also called far-sightedness. This happens when the image of nearby objects is focussed beyond the retina. This defect is corrected by using a convex lens of suitable power.
Presbyopia is when the eye loses the power of accommodation with age. It happens due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
DISPERSION OF LIGHT
The splitting of white light into its component colours is called dispersion. White light is composed of seven colours- Violet, Indigo, Blue, Green, Yellow, Orange and Red. The acronym used for these colours is VIBGYOR.
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