Human Eye: Human eye is a natural optical instrument which is used to see objects by human beings. It acts like a camera which has a lens.
Parts of Human Eye
- Cornea: The cornea acts as the eye’s outermost lens. It functions like a window that controls and focuses the entry of light into the eye.
- Retina: The retina is the innermost layer in the eye that is responsible for the visual processing that turns light energy from photons into three-dimensional images.
- Pupil: The round opening in the center of the iris (the colored tissue that makes the “eye color” at the front of the eye). The pupil changes size to let light into the eye. It gets smaller in bright light and larger as the amount of light decreases.
- Iris: The colored tissue at the front of the eye that contains the pupil in the center. The iris helps control the size of the pupil to let more or less light into the eye.
- Eye Lens: It is a convex lens made of transparent and flexible jelly like material.
- Optic nerve: The optic nerve transmits electrical impulses from your eyes to your brain.
- Ciliary muscles: These are the muscles which are attached to eye lens and can modify the shape of eye lens which leads to the variation in focal lengths.
- Aqueous humour: It is fluid which fills the space between cornea and eye lens.
Power Of Accommodation Of The Eye
When the ciliary muscles are relaxed, the eye lens becomes thin, the focal length increases, and the distant objects are clearly visible to the eyes. To see the nearby objects clearly, the ciliary muscles contract making the eye lens thicker. Thus, the focal length of the eye lens decreases and the nearby objects become visible to the eyes. Hence, the human eye lens is able to adjust its focal length to view both distant and nearby objects on the retina. This ability is called the power of accommodation of the eyes.
Defects Of Vision And Their Correction:
There Are Three Common Defect Of Vision
- Myopia: It is also called nearsightedness. It is a kind of defect in the human eye due to which a person can see near by objects clearly but he cannot see distant objects clearly. Myopia is due to
(i) excessive curvature of the cornea.
(ii) elongation of the eyeball.
Correction of Myopia: Since, a concave lens has an ability to diverge incoming rays, it is used to correct this defect of vision.
- Hypermetropia(Long sightedness): It is a kind of defect in the human eye due to which, a person can see distant objects properly but cannot see the nearby objects clearly. It happens due to
(i) decrease in the power of eye lens i.e., increase in focal length of eye lens.
(ii) shortening of eyeball.
Correction: Since a convex lens has the ability to converge incoming rays, it can be used to correct this defect of vision.
Presbyopia: It is a kind of defect in the human eye which occurs due to aging. It happens due to the following reasons
(i) decrease in flexibility of eye lens.
(ii) gradual weakening of ciliary muscles.
In this, a person may suffer from both myopia and hypermetropia.
Correction: By using a bifocal lens with appropriate power. Bifocal lenses consist of both concave and convex lenses, upper position consists of the concave lens and lower portion consists of a convex lens.
Refraction Of Light Through Prism
- The direction of light changes as it passes from one medium to another. In other words, light bends as it travels from one medium to another. This phenomenon is known as refraction.
- When light waves travel from one optical medium to another, they get refracted due to differences in optical density of the two media.
- The degree of refraction varies with the wavelength of light.
- The speed of light in different media varies inversely with the refractive index of a medium.
When a ray of light passes through a prism, it bends towards the base of the prism: The incident ray AB is incident is going from air(rare medium) into glass (denser medium), so it bends towards the normal BN’ and goes along the direction BC inside the glass prism. Thus, BC is the refracted ray of light which bends towards the base QR of the prism.
Dispersion Of White Light
When white light is passed through a glass prism it splits into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colours is termed as dispersion.
Recombination
When white light is passed through a prism, it disperses, and when another prism is kept inverted, the dispersed light returns to white light. This is referred to as light spectrum recombination. This is caused by the different velocities of different colors passing through the prism.
Application Of Refraction Of Light
- Twinkling of stars: The twinkling of stars is due to atmospheric refraction of star-light. The atmosphere is made of several layers and the refractive indices which keep on changing continuously due to this path of light rays from the star keep on changing their path continuously
- Advance sunrise and delayed sunset: Advanced sunrise and delayed sunset are phenomena caused due to atmospheric refraction. In this the Sun appears to rise early by 2 minutes and set late by 2 minutes. The diagram below shows this phenomenon. When the rays from the Sun hit the atmosphere they get refracted.
Scattering Of Light
When white light from the sun enters the earth’s atmosphere, the light gets scattered i.e, the light spreads in all directions by the dust particles, free water molecules and the molecules of the gasses present in the atmosphere. This phenomenon is called scattering of light.
Sky Appears Blue In Daytime
The sky appears blue to the human eye as the short waves of blue light are scattered more than the other colours in the spectrum, making the blue light more visible.
The Human Eye
The human eye uses light and enables us to see the Colourful world, beautiful nature and natural phenomenon.
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