Test: The Eye & Ear - 1 - A Level MCQ

The term aperture represents a hole through which light enters the eye. It is known as pupil. Its size can be changed by iris. Iris is a thin circular structure in the eye. It controls the diameter and size of the pupil (aperture of the eye) and thus regulates the amount of light entering the pupil and reaching the retina.

So, the correct answer is 'Iris'.

The retina makes up the inner layer and occupies only the posterior two-third of the eye. The retina consists of several layers of cells, including the rods and cones, the sensory cells, that respond to light. The tips of the rods and cones are embedded in a pigmented layer of cells on the back of the retina. The pigment helps prevent light from scattering in the back of the eye.

All photoreceptors share a common visual pigment: a mixture of a vitamin A-based chromophore (retinal) and a seven-transmembrane-helix apoprotein (opsin). Together, they form rhodopsin. The pigments belong to a special class of receptors called as G protein-coupled receptors, characterised by signaling through heterotrimeric G proteins. 

In vertebrates, the rhodopsin is thermally unstable, breaking down with light into the opsin and chromophore; rhodopsins are maintained by an enzymatic process. Invertebrate rhodopsins are stable, although renewal also must take place there.

If the light source in front of eye becomes bright suddenly, then the amount of light entering the eye will increase. The pupil will then adjust to regulate the amount of light entering into eye. The lens will change the focus according to the light entering the eye. The size of pupil is regulated by iris which contracts to decrease the size of pupil. This regulation of light entering the eye by adjusting the size of pupil is called accommodation.
So, the correct answer is 'Focus of lens changes'.

The ciliary muscle is a circular muscle that relaxes or tightens the zonules to enable the lens to change the shape for focusing . The zonules are the fibre that hold the lens suspended in position and enable it to change shape during accomodation. It increase or decrease focal length of eye lens.

Myopia is a defect of vision in which far objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye’s lens too strong. Myopia can be corrected by wearing glasses with concave lenses these help to focus the image on the retina.

The semicircular canals are three tiny fluid filled tubes in your inner ear that help to keep your balance. When your head moves around , the liquid inside the semi- circular canals flashes around and moves the tiny hairs that lie in each canal.

Among many nocturnal vertebrates, the white compound guanine is found in the epithelium or retina of the eye. This provides a mirror-like surface, the tapetum lucidum, which reflects light outward and thereby allows a second chance for its absorption by visual pigments at very low light intensities at night. Tapeta lucida produces the familiar eyeshine of nocturnal animals.

 Eye is most sensitive to 5000 Angstrom. 
 At extremely low intensities of stimuli, when only rods are stimulated, the retina shows a variable sensitivity to light according to its wavelength, being most sensitive at about 5000 angstroms, the absorption maximum of the rod visual pigment, rhodopsin. 

Presbyopia is a normal part of the aging process. It occurs due to hardening of the lens of the eye, causing the eye to focus light behind rather than on the retina when looking at close objects.