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The Human Eye and the Colourful World
THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colours around us.
How we see with our eyes ?
- The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina.
- Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball .
- The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
- Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.
- The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina.
- We find a structure called iris behind the cornea.
- 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 membrane having enormous number of light-sensitive cells.
- The light-sensitive cells get activated upon illumination and generate electrical signals.
- These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
Power of Accommodation
The ability of the eye lens to adjust its focal length is called accommodation.
To see an object comfortably and distinctly, you must hold it at about 25 cm from the eyes.
- The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eye. For a young adult with normal vision, the near point is about 25 cm.
- The farthest point upto which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye. You may note here a normal eye can see objects clearly that are between 25 cm and infinity.
Cataract: Sometimes, 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. It is possible to restore vision through a cataract surgery.
DEFECTS OF VISION AND THEIR CORRECTION
There are mainly three common refractive defects of vision. These are
- myopia or near-sightedness,
- Hypermetropia or far-sightedness, and
These defects can be corrected by the use of suitable spherical lenses.
- Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
- A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres. In a myopic eye, the image of a distant object is formed in front of the retina
This defect may arise due to
- excessive curvature of the eye lens, or
- elongation of the eyeball.
This defect can be corrected by using a concave lens of suitable power.
A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
(a), (b) The myopic eye, and (c) correction for myopia with a concave lens
- Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
- The near point, for the person, is farther away from the normal near point (25 cm). Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a closeby object are focussed at a point behind the retina.
This defect arises either because
- the focal length of the eye lens is too long, or
- the eyeball has become too small.
This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focussing power required for forming the image on the retina.
(a), (b) The hypermetropic eye, and (c) correction for hypermetropia
- The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
- Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.
These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
REFRACTION OF LIGHT THROUGH A PRISM
Prism: It has two triangular bases and three rectangular lateral surfaces. These surfaces are inclined to each other. The angle between its two lateral faces is called the angle of the prism.
Reflection of light through prism :-
Here PE is the 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.
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.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM
Desperion of light: When white light dispersed through prism it get divided into seven colours this is called desperion of light.
The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red .
- The acronym VIBGYOR will help you to remember the sequence of colours.
- Spectrum: The band of the coloured components of a light beam is called its spectrum.
- Rainbow is an example of Desperion of light.
Refraction of light: is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends thelight ray toward the normal to the boundary between the two media.
Twinkling of stars
The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.
– the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.
Apparent star position due to atmospheric refraction
- Planets don’t twinkle because The planets are much closer to the earth, and are thus seen as extended sources.
Advance sunrise and delayed sunset
The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.
The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the refraction.
Atmospheric refraction effects at sunrise and sunset
SCATTERING OF LIGHT
Light scattering is a form of scatteringin which light in the form of propagating energy is scattered. Light scattering can be thought of as the deflection of a ray from a straight path.
1. Tyndall Effect
The phenomenon of scattering of light by the colloidal particles gives rise to
This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible.
Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.
2. Why is the colour of the clear Sky Blue?
A clear cloudless day-time sky is bluebecause molecules in the air scatterblue light from the sun more than they scatter red light.
3. Colour of the Sun at Sunrise and Sunset
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.
Reddening of the Sun at sunrise and sunset
However, light from the Sun overhead would travel relatively shorter distance. At noon, the Sun appears white as only a little of the blue and violet colours are scattered.
Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun.