Structure of Human Eye:
The human eye is a spherical structure which fits in the eye socket in the skull bone. There are following main parts in the human eye.
Pupil: Pupil is the round black spot in front of eye. It regulates the amount of light entering the eyes. Pupil works like aperture of a camera. In case of dim light pupil dilate to allow more light to enter the eyes. In case of strong light pupil constrict allowing less light to enter.
Irish: Irish is made of muscles. They control the size of opening of pupil.
Lens: Lens lies just behind the pupil. Lens becomes thin to increase its focal length. This enables us to see distant objects clearly. To focus on nearer objects, lens becomes thick to decrease its focal length. But there is a limit. The minimum distance of clear vision is 25 cm. Below this distance, we cannot see things clearly.
Retina: Retina works like a screen or camera film. Retina is full of light and colour sensitive cells. These cells, upon receiving image send electrical signals to the brain, which processes these information to make a mental image of what we see. The photoreceptor cells in the eye are of two types, viz. rod cells and cone cells. The rod cells are sensitive to dim light. The cone cells are sensitive to bright light and colour.
Fig: Structure of Human Eye
Benefits of two eyes: One eye is having a field of vision of about 150 degrees. Both the eyes enable us to see upto a field of 180 degrees. Moreover, as two different images get juxtaposed in the brain, so we are able to see a three dimensional view of the world.
Power of Accommodation of Human Eye: The human eye can clearly see a nearby object as well as an object on infinity. This ability of the human eye is called the power of accommodation of human eye.
Malfunctions of Eyes:
Cataract: In old age the cornea becomes cloudy. This reduces the vision in old age. Cataract can be cured by eye surgery. Sometimes, artificial lens is also transplanted during cataract surgery. This is called Intra Ocular Lens Transplantation.
Myopia: 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 may arise due to
excessive curvature of the eye lens, or
elongation of the eyeball.
Correction of Myopia: 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.
Hypermetropia: 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 nearby object are focused 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.
Correction of Hypermetropia: This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Presbyopia: 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 bifocal 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.
Refraction of Light Through a Prism:
Prism is a transparent optical element which refracts light. An optical object to be defined as prism must have at least two faces with an angle between them. Triangular prism is the most common type of prism. It has a triangular base and rectangular sides.
When a ray of light enters the prism, it bends towards the normal; because light is entering from a rarer medium to a denser medium. Similarly, when the light emerges from the prism, it follows the laws of refraction of light. Due to the angle of the prism and due to different wavelengths of different components of white light; the emergent ray gets segregated into different colours. Finally, a colourful band of seven colours is obtained. This phenomenon is called dispersion of white light by the prism.
Formation of Rainbow: Raindrops work like a prism. When white light enters a raindrop, it experiences refraction and total internal refraction inside the raindrop. The emergent light experiences dispersion of light. As a result, rainbow is formed against the backdrop of sky.
When light enters from one medium to another, there is a deviation in its path. This phenomenon is called refraction of light. Atmosphere is composed of layers of various optical densities. Because of this, light rays passing through various layers of atmosphere; get deviated. Many interesting phenomenon can be observed because of atmospheric refraction. Some of them are given here.
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 atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon. Further, this apparent position of the star is not stationary, but keeps on changing slightly, since the physical conditions of the earth's atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers, i.e. the star sometimes appears brighter, and at some other time, fainter, which gives the twinkling effect.
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. By actual sunrise, we mean the actual crossing of the horizon by the Sun. The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun's disc at sunrise and sunset is also due to the same phenomenon.
Scattering Of Light
When light hits a particle, it scatters in different directions. Refraction happens because of non-uniformities of particles of a medium. Many interesting phenomenon can be observed because of scattering of light. Some of them are given here.
The optical effect because of scattering of light from the particles of colloid or suspension is called Tyndall Effect. For Tyndall effect to be possible, the size of particles should be less than or equal to the wavelength of the visible spectrum. So, the size of particles should be between 40 and 900 nanometer. Tyndall effect is responsible for many natural phenomena. The white beam of light which appears to come through the ventilation or through a slit in the door is because of Tyndall Effect and the dust particles in the air cause the scattering of light in this case. The white beam appears because scattering of light makes the dust particles visible in the light.
Why is the colour of the clear Sky Blue?
We know that the wavelength of red colour is more than that of blue colour. The size of particles in air is smaller than the wavelength of visible light. Hence, these particles scatter the light of shorter wavelength more effectively than light of longer wavelength. The blue end of the visible spectrum has shorter wavelength than the red end. Due to this, blue colour is scattered more strongly in the atmosphere; compared to the red colour. This is the reason sky appears blue. Since red colour is scattered the least hence it is used in traffic lights for showing the danger signal.