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The Human Eye acts as a natural optical instrument providing us the ability to detect light and see the beautiful world around us. The human eye has a significant similarity to a camera due to the presence of a screen system and a lens. Various parts of the eye work collectively leading to our normal visual functioning. The ability to differentiate between colors and depth is made possible through the eyes. Although delicate and small, eyes are an extremely complex organ.
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Keyterms: Light, camera, Colors, Internal reflection, White fibers, Optic nerve, Rod Cells, Retina, Cornea, Lens
Read Also: Light-Reflection and Refraction Revision Notes
Fun Fact
- The human eye contains 7 million cone cells that allow the eyes to see colors and details.
- The rod cells on the other hand are 100 million in numbers, allowing the eyes better vision in the dark.
- The optic nerve passes through the retina resulting in small blind spots in our eyes. The brain utilizes information from the other eye to fill these blind spots.
Structure of the Human Eye
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- It is a spherical ball with a transparent bulge in the front.
- It has a diameter of 2.3 cm.
- The eye has an outer covering known as a scleroid. It mainly consists of white fibers and protects all parts of the eye.
- Another covering called choroid darkens the eye from the inside and prevents internal reflection.
The video below explains this:
Human Eye One Shot Detailed Video Explanation:
Parts of the Human Eye
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Various important parts of the human eye and their functions are mentioned below.
- Retina- It is a delicate membrane where an image is formed. The retina contains a huge amount of light-sensitive cells and is also popularly known as the light-sensitive screen. It consists of rod and cone cells that allow vision and light perception.
- Cornea- It is a thin membrane that forms the transparent frontal bulge. The cornea refracts the light entering the eye, acting as a lens.
- Aqueous humour- The gap between the lens and cornea is filled by a liquid called aqueous humour. It flows into the anterior chamber and is produced by the ciliary muscle.
- Crystalline lens- The crystalline lens simply delivers the finer adjustment of focal length to focus on intermediate and near objects.
- Iris- It is a pigmented tissue found behind the cornea. It is the colored portion of the eye that controls pupil constriction and dilation.
- Pupil- The amount of light entering the eye is controlled and regulated by the pupil. It is the hole in the middle of the iris through which the light passes to the retina. The light entering the eye is absorbed by the retina making the pupil black.
- Ciliary muscle- The ciliary muscle is attached to the crystalline lens. It contracts to adjust the shape of the lens, allowing people to focus on nearby objects with greater precision.
- Optical nerve- The light-sensitive cells get activated and generate electrical signals or nerve impulses as soon as light enters the eye. These signals are transported to the brain through the optic nerves. It is a network of nerve cells.
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Defects of Vision and their Correction
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The following defects are most commonly found in vision
Myopia or short-sightedness occurs due to excessive curvature of cornea or elongation of the eyeball due to which a variation in focal length is observed. The image is formed before the retina, therefore a person is unable to see distant objects clearly whereas the nearby vision is clear.
This defect can be corrected by concave lenses as they can diverge incoming rays.
Hypermetropia or farsightedness occurs due to the increase in the focal length of the eyeball and/or when a shortening of the eyeball is observed. The image is framed past the retina resulting in a poor vision for distant objects.
This defect can be corrected by convex lenses as they can converge incoming rays.
Presbyopia is caused when the flexibility of the lens is reduced, the lens becomes hardened and weakening of ciliary muscles is observed. It is an age-related defect that causes a person to suffer from both myopia and hypermetropia.
This defect can be corrected by bifocal or progressive lenses.
Refraction of Light through a Prism
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At the point when a beam of light is incident on a rectangular glass slab, after refracting from the slab it gets displaced horizontally. Thus, the emergent ray comes out corresponding to the incident ray.
In contrast to a rectangular glass slab, the sides of a glass prism are slanted at a point called the angle of prism.
Dispersion of White Light by a Glass Prism
The phenomenon due to which the white light splits into its seven constituent colors when passed through a glass prism is known as dispersion of white light. The colors observed are Violet, Indigo, Blue, Green, Yellow, and Red; the band of these seven colors is called a spectrum.
Isaac Newton was the first to observe the spectrum and conclude that the sun is made of seven colors VIBGYOR.
Concerning the incident angle, different component colors bend at different angles. Red light bends the most whereas violet light bends the least.
Rainbow is observed as the spectrum of sunlight. It is formed when the sunlight is dispersed by the tiny atmospheric water droplets acting as a prism.
Atmospheric Refraction
The refraction of light brought about by the Earth's atmosphere (having layers of air of fluctuating optical densities) is called Atmospheric Refraction.
Twinkling of Stars
It is caused due to atmospheric refraction. As the starlight continues to deviate from its way, the position of stars continues to change stating that the earth's atmosphere isn't fixed.
Thus, the measure of light entering our eyes varies, becoming bright and at some point faint. This is the ‘Twinkling effect of the star”.
Read More About Stereoscopic Vision
Frequently Asked Questions
Ques. Which part of our eyes helps us to focus on nearby and distant objects? (1 mark)
Ans. Ciliary Muscle
Ques. What type of particles acts like a prism in the formation of a rainbow in the sky? (1 mark)
Ans. Atmospheric water droplets act as a prism.
Ques. State the function of: (A)Pupil (B) Iris. (1 mark)
Ans. (A) Pupil- The amount of light entering the eye is regulated and controlled by the pupil.
(B) Iris- It controls the size of the pupil.
Ques. How can myopia be treated? (1 mark)
Ans. Myopia occurs due to excessive curvature of cornea or elongation of the eyeball due to which a variation in focal length is observed. This defect can be corrected by concave lenses as it can diverge incoming rays.
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