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Parts of Eyes: Exploring the Wonders Within the Human Eye
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- The human eye is one of the most valuable and sensitive sense organs. It is like a camera.
- Retina: Within the human eye, the lens system of eyes forms an image on a light-sensitive screen called the retina.
- The retina contains several nerve cells.
- Optical nerve: Within the human eye, sensations felt by the nerve cells are then transmitted to the brain through the optic nerve.
- There are two kinds of cells
- (i) Cones: These are sensitive to bright light
- (ii) Rods: These are sensitive to dim light.
- Blind spot: Within the human eye, at the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot.
- This is called the blind spot.
- The impression of an image does not vanish immediately from the retina.
- It persists there for about 1/16th of a second.
- So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.
- Cornea: Light enters the eye through a thin membrane called the cornea.
- It forms a 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.
- Iris: It is behind the cornea. It is a dark muscular diaphragm that controls the size of the pupil.
- Pupil: Within the human eye, it regulates and controls the amount of light entering the eye.
- Eye lens: It forms an inverted real image of the object on the retina.
- The retina is a delicate membrane having an 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.
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Focus and Flexibility: Understanding the Human Eye’s Accommodation and Vision Journey
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- The human eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles.
- The change in the curvature of the eye lens can thus change its focal length.
- When the muscles are relaxed, the lens becomes thin. Thus, its focal length increases.
- This enables us to see distant objects clearly.
- The ciliary muscles contract when objects are closer to the eye.
- This increases the curvature of the eye lens.
- The eye lens then becomes thicker.
- Consequently, the focal length of the eye lens decreases.
- This enables us to see nearby objects clearly.
- The ability of the eye lens to adjust its focal length is called accommodation.
- But the focal length of the eye lens cannot be decreased below a certain minimum limit.
- To see an object comfortably and distinctly it must be 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 infinite for a normal eye.
- When the crystalline lens of people at old age becomes milky and cloudy it is called cataract.
- This causes partial or complete loss of vision.
- It is possible to restore vision through a cataract surgery.
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