Q.Differentiate between rod and cone cells.

The blind spot, or optic disc, is where the optic nerve fibres leave the retina; since it has no rods or cones, light falling on it cannot be perceived.

The retina is the light-sensitive inner layer of the eye, containing two types of photoreceptor cells: rods (for dim-light, black-and-white vision) and cones (for colour and sharp daylight vision).

At one specific point on the retina, nerve fibres from all the retinal ganglion cells converge and exit the eyeball as the optic nerve, carrying visual information to the brain. Because this point is occupied entirely by converging nerve fibres and blood vessels, it contains no rods or cones.

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(a), where rays converge behind the retina, is Hypermetropia — corrected with a convex lens; (b), where rays converge in front of the retina, is Myopia — corrected with a concave lens.

The eye normally focuses light exactly on the retina to produce a sharp image. In refractive errors, the focal point falls either behind or in front of the retina:

• Diagram (a): the parallel light rays converge to a point BEHIND the retina. This is Hypermetropia (Hyperopia, far-sightedness) — it occurs because the eyeball is too short (front-to-back) or the lens's focusing power is too weak, so near objects cannot be brought into focus on the retina. Remedy: a CONVEX (converging) lens is used in spectacles, which adds extra converging power so the image is pulled forward onto the retina. …