ICSE Class 10 Answered

• The two eyes are located in deep sockets called orbits.
• The upper and lower moveable eyelids protect the front surface of the eyes.
• There are 6–12 tear glands.

The wall of the eyeball is composed of the following three concentric layers:

1. Sclerotic Layer (Outer Layer)

• The white visible portion of the eyeball is nothing but the sclera.
• The sclera covers the coloured part of the eye, i.e. the cornea.

2. Choroid Layer (Middle Layer)

• Richly supplied with blood vessels to provide proper nourishment.
• Choroid expands in the front to form a ciliary body.
• Iris is also a part of the choroid.
• The iris partially covers the lens. It leaves a circular opening in the centre called a pupil.
• The muscles of the iris regulate the size of the pupil. Thus, the pupil regulates the amount of light entering the eye.

3. Retina (Inner Layer)

• It has two types of sense cells—rods and cones.
• The rod cells are sensitive to dim light and do not respond to colour.
• The cone cells are sensitive to bright light and are responsible for colour vision.

Four Major Steps in Seeing an Object

• Light rays reflected from the object enter the eyes through transparent structures
• First, the curvature of the cornea converges the rays to some extent, and then the lens converges them further.The image on the retina is inverted and real.
• The light energy produces chemical changes in rods and cones which send the nerve impulse. This nerve impulse is sent to the cerebrum through the optic nerve. The cerebrum gives the sensation of sight.
• The brain interprets the inverted image on the retina, and the obect is seen upright.

Common defects of the eyes include myopia, hypermetropia, astigmatism, presbyopia, night blindness, squint etc.