Revision Tool11. Human Eye and Colourful World Human Eye
Hubman Eye See human eye
The human eye is one of the most significant and sensitive sense organs which allow us to see the beautiful and colourful world around us. The functioning of the eye is explained below:
Light rays get reflected from an object and reach the eye.
The transparent cornea has the refractive power to bend the light ray.
The bent light rays pass through the pupil and reach the lens.
The lens focusses the light rays onto the retina.
Receptors present in the retina detect the light stimulus.
Parts of the Human Eye See parts of human eye
The important parts of the human eye are:
Cornea: It is the clear, transparent, thin membrane at the front portion of the eye through which light enters.
Sclera: It is a membrane of tendons which protects the inner sensitive parts of the eye such as the retina. It is also called "white of the eye".
Conjunctiva: It is a mucous membrane which covers the sclera and the inside of the eyelids.
Iris: It is a coloured, circular membrane which regulates the amount of light entering the eye by adjusting the size of the pupil.
Pupil: It is the opening or hole at the center of the iris that allows the light to enter.
Aqueous Fluid: It is the fluid present between the lens and the cornea which supplies both with oxygen and nutrients.
Lens: It is the transparent tissue behind the iris which focuses light rays onto the retina.
Vitreous body: It is the transparent, colourless soft, gel-like material that fills the center of the eye behind the lens.
Retina: It is the light-sensitive tissue at the back of the eye which transmits visual signals through the optic nerve to the brain.
Macula: It is the area near the retina that consists of closely-packed visual cells.
Optic Nerve: It is the nerve that sends signals from the eye to the brain, where these signals are interpreted into images.
Ciliary Body: It is the part of the eye that consists of the ciliary muscles that controls the shape of the lens.
Power of Accommodation See Power of accommodation
Power of accommodation is the ability of the eye to focus objects lying at different distances by adjusting its focal length.
The minimum distance at which the human eye can see objects clearly is called the Near Point and the location of the farthest object on which a fully relaxed eye can focus is called the Far Point.
Defects of Vision
Any object can be seen clearly by the human eye if the image of the object falls accurately on the eye’s retina. If the image does not fall at the correct location in the eye, it could be due to one of the following defects of vision:
Myopia: It is also known as near-sightedness. It is the defect of vision in which nearby objects are seen clearly, but distant objects are not. This happens because the image of the object is formed in front of, rather than on, the retina.
Hypermetropia: It is also known as far-sightedness. It is the defect of vision in which distant objects are seen clearly, but nearby objects are not. This happens because the image of the nearby object is formed behind, rather than on, the retina.
Presbyopia: It is the defect of vision in which the nearby and sometimes distant objects cannot be seen clearly due to old age. This happens because the lens loses elasticity due to age.
Correction of Vision
Defects of vision can be corrected by using the correct choice of lens.
Concave Lens: Concave lenses are used to correct Myopia as they are diverging lenses. They diverge the light before it enters the eye, so that the cornea and lens are then able to converge the light onto the retina.
Convex Lens: Convex lenses are used to correct Hypermetropia as they are converging lenses. They refract the light before it enters the eye so that the eye’s cornea and lens are then able to focus the already refracted light onto the retinal surface.
Bi-focal Lens: Bi-focal lenses are used to correct Presbyopia. They are concave in the upper portion for distance vision and convex in the lower portion for reading.
Refraction of Light Through a Glass Prism
Refraction is the bending of light when it enters a medium with a different density from the one through which it has been traveling, such as a glass prism. Depending on the two mediums, light will bend at different angles. A few important parameters of refraction are:
Angle of the Prism: The angle between two lateral faces of the prism is called the angle of prism.
Angle of Incidence: The angle at which the light ray enters the medium.
Angle of deviation: The angle between incident ray and the refracted ray after entering a new medium.
Refractive index of prism: The measure of bending of light when it passes from one medium to another. It is calculated by dividing the speed of light in a vacuum by the speed of light in the medium.
Dispersion of White Light
The splitting of white light into its component colours is called Dispersion of Light. White light consists of seven colours - Violet, Indigo, Blue, Green, Yellow, Orange and Red. When white light passes through a prism, each colour bends at a different angle. Red has the largest wavelength and bends least, whereas violet has the shortest wavelength and bends most. The band of different colours is called a spectrum.
Formation of a Rainbow
A rainbow gets formed as a result of dispersion of white light by tiny water droplets present in the atmosphere. The necessary components for viewing a rainbow are bright sunlight, suspended droplets of water and proper angle of sighting.
A light ray from the sun passes through suspended water droplets which serve as a refractor of light. Upon entering the droplet, the light is dispersed into its component colours. These colours are then reflected internally in the droplet and when the light then exits the droplet, it exits is again refracted. Depending on where you are standing, you will see only a part of the spectrum emerging from each droplet. Therefore, if there is an abundance of droplets in the sky and you are standing with the sun behind you, you will see an entire rainbow. This rainbow has been formed as a result of light refracting twice and reflecting once in a droplet of water.
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Atmospheric Refraction
Due to variation in the air density at different points of the atmosphere, white light is refracted several times while passing through the atmosphere. This refraction of white light from the sun by different layers of the atmosphere is called Atmospheric Refraction.
Air is denser near the surface of earth. Light thus travels from a rarer to a denser medium while coming from an object like a star. So it bends towards the normal and hence the light appears to be coming from a different point.
Twinkling of Stars
Star twinkles because its light passes through layers of atmosphere on earth’s surface and undergoes refraction. On entering the earth’s atmosphere, the star light bends several times in random directions due to change in the density of various layers of atmosphere. As these various layers are also moving, the refraction is not consistent and so the stars appear to twinkle.
Stars that are closer to the horizon appear to twinkle more than stars that are overhead. It is because the light of stars near the horizon has to travel further through layers of the atmosphere than the light of stars overhead and so is subject to more refraction.
Apparent Position of the Sun
An observer on earth sees the sun two minutes before the sun actually rises over the horizon and two minutes after the sun actually sets due to atmospheric refraction. During sunrise and sunset, sunlight enters the atmosphere and bends towards the earth due to atmospheric refraction. Thus, what the observer sees in the sky is the apparent position of the sun during sunrise and sunset, which is due to bending of the light by the layers of the atmosphere.
Scattering of Light
The deviation of light from its straight path by one or more irregularities in the medium through which it passes. When sunlight passes through the atmosphere, it gets scattered by dust and gas molecules. Because the particles responsible for scattering are very small, the extent of scattering depends on the wavelength of the light. Blue light, which has a shorter wavelength, scatters or deviates more than red light which has larger wavelength. This is why the sky appears blue.
Red Sunrise and Sunset
During sunrise and sunset, the sun is lower on the horizon and so light from the sun has to travel a greater distance through the layers through the atmosphere. Light scatters in the atmosphere and blue light gets scattered more than red, because blue has a shorter wavelength. Since the light is travelling a greater distance than during the day, more blue light gets scattered and so the light reaching our eyes tends to be more from the red and orange frequencies of light. For this reason, the sky has a reddish-orange hue during sunrise and sunset.
White Colour of Clouds
When light is scattered by tiny particles in the atmosphere, the amount of scattering depends on the wavelength of the different constituent colours of light. However, because the water droplets in clouds are much larger compared to the wavelength of light, all constituent colours of white light get scattered equally, due to which they appear white in colour. When the clouds grow thicker, more sunlight gets reflected from them. Since less sunlight reaches the underside of the cloud, less light is scattered, and the base of the cloud appears grey.
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