CBSE – 10th Standard Science

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Class 10 Chapter 11 – The Human Eye and the Colourful World

The Human Eye and the Colourful World

THE HUMAN EYE

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  • The human eye is one of the most valuable and sensitive sense organs. It enables us to see the world and colours around us.
  • Eye lens forms an image on a light-sensitive retina.
  • Light enters the eye through a thin membrane called the cornea.It forms the transparent bulge on the front surface of the eyeball..
  •  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 light-sensitive cells get activated upon illumination and generate electrical signals.
  • The signals which are sent to the brain via the optic nerves. The brain interprets these signals.

Power of Accomodation

  • The eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified to some extent , the ability of the eye lens to adjust its focal length is called accommodation.

DEFECTS OF VISION AND THEIR CORRECTION

  • When the crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision.
  • Three common refractive defects of vision are (i) myopia (ii) Hypermetropia (iii) Presbyopia.
  •  Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
  •  In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect can be corrected by using a concave lens of suitable power.
  • Hypermetropia is also known as far -sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
  • From a close by object are focused at a point behind the retina This defect can be corrected by using a convex lens of appropriate power.

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  • Presbyopia arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens due to aging. A person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses.
  • Splitting of light into its component colours is called dispersion

REFRACTION OF LIGHT THROUGH A PRISM

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PE is incident ray, EF is the refracted ray and FS is the emergent ray. A ray of light is entering from air to glass at the first surface AB. The light ray on refraction has bent towards the normal.

At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. Compare the angle of incidence and the angle of refraction at each refracting surface of the prism.

The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In this case ∠D is the angle of deviation.

ATMOSPHERIC REFRACTION

  • A rainbow is a natural spectrum appearing in the sky after a rain shower It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere.
  • A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract
  • Twinkling of stars is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.
  • The planets are much closer to the earth, and are thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light
  • Variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect.
  • Scattering of light by colloidal particles causes its path becomes visible through a colloidal solution where the size of the particles is relatively larger.

Colour of the Sun at Sunrise and Sunset

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Advance sunrise and delayed sunset

Tyndall Effect

  • The earth’s atmosphere is a heterogeneous mixture of minute particles of water, smoke, air n dust. When a beam of light strikes such fine particles, the path of the beam becomes visible
  • Sky is blue because :The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light
  •  When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes.
  • Light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes the Sun appears red at the sunrise or the sunset

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