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The Human Eye and the Colourful World

The Human Eye

  • When you visit a beautiful hill station, beach, or even a garden, you are amazed by the beauty of these places. It is possible only because of our eyes. The human eyes are one of the most important sense organs. Our eyes consist of a light-sensitive screen known as the retina, where the image gets formed. The cornea is a thin membrane that allows light to enter the eye.
  • On the front surface of the eyeball, the cornea forms a transparent bulge. Our eyeball is almost spherical and has a diameter of about 2.3cm. The outer surface of the cornea is where the majority of the refraction for light rays entering the eye occurs. Behind the cornea, we have an iris. It is a dark muscular diaphragm. Iris controls the size of the pupil.
  • The pupil adjusts and controls the amount of light entering the eye. The eye lens makes an inverted image of the object on the retina. The retina has several light-sensitive cells. Upon illumination, these cells get activated and generate electrical signals. These signals are sent to the brain through the optic nerves. These signals get interpreted by the brain. The brain processes the information and, thus, we perceive the objects as they are.

Power of Accommodation

  • Do you know that our eyes contract and extent to see objects at a different distance? The curvature of the eye lens gets modified to some extent by the ciliary muscles. The change in curvature, in turn, changes its focal length. The lens becomes thin when the muscles are relaxed. It leads to an increase in the focal length of the eye lens. And this helps us to see distant objects.
  • The ciliary muscles contract when you look at an object near your eyes. The lens becomes thick when the muscles contract. It leads to a decrease in the focal length of the eye lens. And this helps us to see nearby objects.
  • The ability of the eye lens to change its focal length according to the position of the object is called accommodation. But, beyond a minimum limit, we cannot decrease the focal length. The near point of an Eye is the minimum distance at which an object can be seen without strain. The near point for a Normal Eye is 25 cm. The maximum distance at which an object can be seen is called the far point of the eye. The far point of a Normal Eye is infinity. Thus,a normal eye can see objects between 25 cm and infinity.
  • Cataract is a condition in older persons. In this condition, the crystalline eye lens of old people becomes milky and cloudy. It leads to complete or partial loss of vision. Cataract surgery is a way to restore the vision of such old people.

Defects of Vision and their Correction

  • In some people, the eyes may lose their power of accommodation. In such situations, the person cannot see the objects. Their vision gets blurred due to the refractive defects of the eyes. There are three significant defects of vision, and they are (i) Myopia (ii) Hypermetropia (iii) Presbyopia. Each of these defects can be corrected using a suitable spherical lens.
    • Myopia
      • It is also called near-sightedness. People with myopia cannot see distant objects clearly but, they can see nearby objects. Such people have their far point nearer than infinity. For a person with myopia, the image of the distant object forms in front of the retina. The major reason for this defect is (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. A concave lens of required power is used for correcting myopia. This lens helps in bringing back the image to the retina.
    • Hypermetropia
      • It is also called far-sightedness. People with hypermetropia cannot see nearby objects clearly, but they can see distant objects. Such people have their near point farther from the Normal near point. For a person with hypermetropia, the image of the nearby object forms behind the retina. The main reason behind this defect is (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. A convex lens of suitable power is used for correcting hypermetropia. The convex lens provides the extra focusing power needed for forming the image on the retina.
    • Presbyopia
      • As we get old, the near point of eyes slowly moves away. People often find it tough to see nearby objects. This condition is called presbyopia. But this is not to be confused with hypermetropia. Presbyopia happens due to ageing. Presbyopia is mainly due to the weakening of the ciliary muscles and decreasing flexibility of the eye lens. In certain cases, a person may suffer from both myopia and hypermetropia. These people use bi-focal lenses. A bi-focal lens generally consists of both concave and convex lens. The upper portion of the lens has a concave lens that helps in seeing distant objects. The lower part has the convex lens that helps in seeing near objects. In addition to using these lenses for correction of these defects, these days, we can use contact lenses or even undergo surgery.

Dispersion of white light by Glass Prism

    • Have you seen what happens when a ray of sunlight falls on a prism? When a ray of sunlight falls on the prism, the incident white light gets split into a band of colours. The band of colours is Violet, Indigo, Blue, Green, Yellow, Orange, and Red.
    • You can remember the sequence of colours using the acronym VIBGYOR. The splitting of light into different colour components is known as dispersion. The band of the colours formed from the light beam is called its spectrum.
    • But you know how we get these seven colours? When the light passes through the prism, each colour of light bends at a different angle with respect to the incident ray. The violet light bends the most while the red bends the least.
    • The spectrum of sunlight was first obtained by Sir Isaac Newton. Do you remember where you came across these seven colours earlier? Yes, it is the rainbow that we see in the sky after a rain shower. The rainbow is a natural spectrum. A rainbow is formed due to the dispersion of sunlight by tiny water droplets in the atmosphere. Here the water droplets act like a small prism.

Atmospheric Refraction

  • Atmospheric refraction is the refraction of light by various layers of the earth’s atmosphere. The twinkling of stars, the advanced sunrise, and delayed sunset are examples of this phenomenon.
        • Twinkling of stars
          • The atmospheric refraction of starlight causes the stars to twinkle. Before the starlight reaches the earth, it undergoes refraction on entering the earth’s atmosphere. The density of the atmosphere varies from one place to another. So is the refractive index. The atmospheric refraction happens in a medium of changing refractive index. When more starlight gets refracted by the atmosphere towards us, stars appear bright. On the other hand, when less starlight gets reflected by the atmosphere, it appears dim. So, the stars appear bright at times and at times fainter, thus the twinkling effect.
        • Advanced sunrise and delayed sunset
          • The sun is seen 2 minutes before the actual sunrise and about 2 minutes after the actual sunset. It is because of atmospheric refraction. Here the actual sunset means the actual crossing of the horizon by the sun. Fig: the actual and apparent position of sunlight with respect to the horizon.

Scattering of light

  • When light travels through a medium, it deviates in a different direction due to the particles present in the medium. This phenomenon is called the scattering of light.
  • Tyndall Effect
    • The earth’s atmosphere consists of minute particles like smoke, tiny water droplets, suspended particles of dust, and molecules of air. When a beam of light strikes these particles, the path of the beam becomes clear. The light gets reflected by these tiny particles. The scattering of light by the colloidal particle causes a phenomenon called the Tyndall effect. This effect is observed when sunlight enters a smoke-filled room through a tiny hole. It is also seen when sunlight passes through a canopy of the forest. The scattering of light makes the particles visible during the Tyndall effect.
    • The scattered light may have some colour and this depends on the size of the scattering particles. Small particles scatter light of shorter wavelength, large particles scatter light of longer wavelength.
  • Why is the colour of the clear Sky Blue?
    • The fine particles and the molecules in the atmosphere are smaller in size than the wavelength of visible light. They are more effective in scattering lights of shorter wavelengths i.e., blue, than the light of longer wavelength i.e., red. When sunlight passes through the atmosphere, the blue colour is scattered more than the red by the fine particles. Thus, the sky appears blue.
  • Colour of the Sun at Sunrise and Sunset
    • The colour of the sun during sunrise and sunset is reddish and this is due to the scattering of light. Light from the sun close to the horizon goes through a thicker layer of air and a larger distance in the atmosphere before reaching our eyes. But, the light from the sun overhead (during noon) travels a shorter distance. During noon hours, the sun appears as white as the amount of blue and violet colour scattered is less. Most of the blue light and light of shorter wavelengths near the horizon gets scattered by the particles. Thus, only light of a longer wavelength like red reaches our eyes. It gives a reddish appearance to the sun.