Question

Describe how light is focused on the retina of the general vertebrate eye

Answer 1

The construction plan of all vertebrate eyes is the same. It basically is a hollow ball which consists of an outermost layer (sclera), a middle layer (choroid) and an innermost layer (retina). Sclera is composed of bone, cartilage and fibrous tissue. Choroid is a dark and vascularized layer and retina is composed entirely of sensory cells. In front of sclera, another transparent layer is present known as cornea, which allows the passage of light rays inside the eye ball.

A soon as the object is viewed by the eye, the path of light begins a journey through eye's optical parts which are important for adjusting and focusing light to the optic nerve.Optic nerve carries the perceived signals in the form of images to the brain.

When light first enters the eye, it first gets bent as it passes through cornea. From cornea, the light gets passed on to the pupil. Pupil is located in the center of iris (colored portion of the eye) and effectively regulates the amount of light entering the eyes. As it dilates, it increases in size so as to receive more light under dim light conditions, and vice versa. The light waves move further to reflect on the lens. Lens inverts the image and focuses it on retina. Retina is made up of two types of photoreceptor cells: rods which let you see in dim light and hence, are more sensitive to light, and cones which transmit colored and sharp images. Once the images have been perceived by retina, the impulses are sent over to the brain through optic nerve. The brain automatically turn the image upside down so as to make it upright again.