Question

Which of the following statements is correct about rods compared to cones?
(i) Rods are most numerous in the fovea.
(ii) Rods contain rhodopsin.
(iii) Rods produce a general outline of objects rather than sharp images.
(iv) Rods produce black and white but not colour images.

A. (i) and (ii)
B. (ii) and (iii)
C. (i), (ii) and (iii)
D. (ii), (iii) and (iv)

Answer 1

Hint: Our eyes are located in the sockets of the skull called orbits. The eye ball is composed of three layers:- Sclera is the external layer which is a layer of dense connective tissue. The middle layer is called the choroid layer which are rich in blood capillaries and appears bluish in colour. The innermost layer is called the retina which inturn has three layers of innermost ganglion cells, the middle layer bipolar cells and outermost photoreceptor cells.

Complete Step by Step Answer:
The rods and cones are the photoreceptor cells present in the outermost layer of retina. These cells contain photopigments which are light-sensitive proteins. Cones are responsible for the daylight (photopic) vision and the colour vision and thus produces colourful images. Rods are responsible for the nightlight vision (twighlight) and thus produces only black and white vision. A purplish-red protein called rhodopsin or visual purple which is a derivative of vitamin A is present in the rods. Three different types of cones are present in the retina which responds to red, green and blue light. Blind spot is a region where no photoreceptor cells arr present. A thinned out portion of retina where visual resolution is the greatest is called the fovea. It is densely packed with only cones. Rods does not produce a sharp image but rather produces an outline of the image.

Thus, the correct answer is option D:- (ii), (iii) and (iv).

Note: If the photoreceptor cells damaged due to any injury it will lead to decreasing clarity of image, highly sensitive to light, blind spots in the centre of the visual field and partial loss of peripheral vision. As photoreceptor cells are non dividing cells if once damaged, it results in irreversible visual impairment.