The Human Eye and the Colourful World

The Human Eye

• The human eye uses light to enable us to see objects around us.
• The working of the human eye is based on the refraction of light through a natural convex lens.
• The human eye is the most significant as it lets us see the beautiful, colourful world.
• The human eye is like a camera. It is one of the most sensitive sense organs.
• A human eyeball is almost spherical in shape with a diameter of about 2.3 cm.

Parts of Human Eye and their Functions

The human eye performs three functions:

1. It makes adjustments to admit the appropriate amount of light inside.
2. It bends/refracts the light rays to form a sharp image.
3. It collects and sends information about the image to the brain for further processing.

The human eye has various parts each performs different functions, as discussed below:

• Iris: It is a dark circular diaphragm that controls the size of the pupil.

• Pupil: It is the central circular opening in the iris. it regulates and controls the amount of light entering the eye, i.e., it contracts in excess light and widens in dim light.

Power of Accommodation

• The focal lens of the eye lens can be decreased or increased.
• The lens becomes thin when the muscles are relaxed. This increases the focal length. This helps to see distant objects.
• Similarly, when the eye lens becomes thick, the focal length decreases and we are able to see nearby objects.

So, power of accommodation is defined as ability of the lens to adjust the focal length.

• The minimum distance at which a particular object can be seen clearly is known as the least distance of distinct vision. This is also called the Near Point of the Eye. For young individuals, the near point is about 25 cm.
• The farthest point to which an object can be seen is known as the Far Point of the Eye. This is about 25 cm to infinity.
• In old age people, the lens becomes milky and cloudy, this is known as Cataract. This can lead to complete or partial loss of vision.

Defects of Vision and their Correction

The three common defects of vision are as follows-

Myopia (also known as Near Sightedness).

• In this case, distant objects cannot be seen clearly but nearby objects are clearly visible.
• Image is not formed on the retina, instead, it is formed in front of the retina.
• Using the concave lens of suitable power can be used to correct this defect.

Hypermetropia (also known as Far-sightedness)

• Nearby objects are not seen clearly whereas distant objects can be seen clearly.
• The image is formed behind the retina instead of on the retina.
• A Convex lens of appropriate power can be used to correct this defect.

Presbyopia 

• It is defined as the one in which the power of accommodation decreases with ageing.
• They have difficulty in near vision.
• Ciliary muscles get weakened and the flexibility of the eye lens gets diminished.
• Generally, bifocal lenses are used for correcting the defects.