The human eye is an organ of utmost importance. We perceive light, colour and shapes because of it.
(i) Structure of Human Eye
(ii) Image formation by Human Eye
(iii) Power of Accommodation of Human Eye
STRUCTURE OF HUMAN EYE
- The human eye is the most significant organ as it enables us to see the beautiful, colourful world around us.
- A diagrammatic representation of the human eye is shown above.
- The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina.
- Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball. Cornea protects the eyes from dust and particles.
- 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.
- The eye lens is composed of a fibrous, jelly-like material called crystalline lens. The crystalline lens by changing its curvature is able to focus the image on the retina.
- The curvature of the eye lens is controlled by the ciliary muscles. By stretching the lens, the ciliary muscles increase the focal length of the eye lens.
- Iris is behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil.
- The pupil regulates and controls the amount of light entering the eye.
- The eye lens 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 (photoreceptors) get activated upon illumination and generate electrical signals.
- The liquid between the cornea and crystalline lens is called Aqueous Humor. The liquid between crystalline lens and retina is called Vitreous Humor.
- There are two types of photoreceptors in the human retina- rods and cones.
- Rods are responsible for vision at low light levels. Cones are capable of colour vision.
IMAGE FORMATION BY HUMAN EYE
- The optical signals from the retina are sent to the brain by the optic nerves.
- The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
POWER OF ACCOMMODATION
- The change in the curvature of the eye lens changes its focal length. When the muscles are relaxed, the lens becomes thin and therefore, its focal length increases. This enables to see distant objects clearly.
- When looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens making the lens 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.
- However, the focal length of the eye lens cannot be decreased below a certain minimum limit.
- 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 to which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
- A normal eye can see objects clearly that are between 25 cm and infinity.