Our Eyes Trivia Quiz

Answer: A hole of variable size

The pupil is a hole through which light enters the eye. It is located at the centre of the front of the eyeball, and appears black in colour because all the light entering it is absorbed by the tissues inside the eye.

The size of the pupil determines the amount of light entering the eye. In conditions where there is bright light, the pupil contracts to prevent the light from damaging the eye, and in the dark it dilates, so that maximum light can be absorbed for best vision.

The constriction and dilation of the pupil take some time. If you step out of a well-lit room into the dark, then some time must elapse before you are well-adapted to seeing in the dark. Similarly, if you get up in the middle of the night and turn on the lights, it takes some time for your pupil to contract the required amount.

Answer: Iris

The iris generally is strongly pigmented, and may be coloured brown, blue, green, grey or hazel. When people talk about the colour of a person's eyes, what is actually meant is the colour of the iris. So if people say, "Mr XYZ has brown eyes", it actually means that his _iris_ is brown in colour; the part of his eye which appears brown is the iris. The colour, texture and patterns of a person's iris are unique, just like one's fingerprints.

The iris adjusts in size, and thus regulates the amount of light that enters the eye through the pupil. The action of the iris is completely involuntary.

Answer: Sclera

The sclera, which is connective tissue made of fibrin (a protein), is the outer protective covering of the eye. The sclera is thinner and less opaque in children, as a result of which the underlying tissue can be seen, thus giving it a bluish look. As one ages, the sclera usually becomes yellower.

Answer: Retina

The retina is a light-sensitive screen inside the eye which contains around 131 million photoreceptor cells. These cells capture light rays and convert them into electrical impulses, which travel to the brain through the optic nerve. The image formed on the retina is inverted, but is inverted once more by the optic nerve and the brain, so that the final image is erect, and not upside-down!

Answer: True

The choroid is a spongy tissue composed of layers of blood vessels between the sclera and the retina, which nourish the back of the eye. Choroidal melanoma is the most common type of cancer which originates in the eye. It is estimated that about 6 to 8 new cases of this cancer per 1,000,000 population are diagnosed annually in the USA and Canada.

Answer: Vitreous

Vitreous humour is comprised primarily of water, and is responsible for giving the eye its form and shape. Because of its viscous properties, the eye returns to its original shape when compressed. In children, the consistency of the vitreous humour is similar to that of the albumen (white) of an egg. As one ages, it gradually thins and becomes more liquid.

Answer: Rods and cones

The rods and cones are, as is fairly obvious, rod and cone-shaped respectively. The rods responds to the intensity of light, but cannot distinguish between various colours. Cones, on the other hand, respond to colour only. Different cone cells responds to red, green and blue (the three primary colours) light to different extents.

Colour-blindness is a much heard-of phenomenon. It is usually a genetic condition, where a person's eyes are lacking in cone cells which respond to a particular colour.

Answer: 400 - 700 nanometres

The section of the electromagnetic spectrum between 400 and 700 nanometres is called the "visible range" because we are capable of detecting the radiation of only these wavelengths. The visible range includes the seven colours of the rainbow- violet, indigo, blue, green, yellow, orange and red, of which red has the highest wavelength and violet the least. Beyond 700 nm is infrared radiation, and below 400 nm is the infamous ultra-violet (UV) radiation. Bees are capable of detecting UV radiation because they have special retinal cones which respond to it.

(One nanometre is one-billionth of a metre, or 10^-9 metres.)

Answer: Cornea

The cornea is a curved, transparent membrane in the front of the eye, responsible for the refraction of light along with the eye-lens. It, however, contributes much more to the total refraction undergone by the light. Unlike the eye-lens, though, its curvature / focal-length is fixed and cannot be adjusted.

Answer: Eye-lens

The eye-lens, also called the crystalline lens, is composed of a fibrous, jelly-like material. Its focal-length / curvature can be adjusted by the ciliary muscles. When the ciliary muscles are relaxed, its focal length is around 2.5 centimetres. At this stage, the lens is relatively thin, and distant objects can be seen clearly.

While looking at nearby objects, ciliary muscles contract, thus making the focal-length decrease, so that the image is brought into focus on the retina.

Answer: A small area of the retina where the optic nerve enters the eye

The blind spot of the eye is the part of the retina where the optic nerve enters the eye. As there are no photoreceptor cells in this region, this part of the field of vision is obscured. However, with the help of the input from the surrounding regions of the retina and that of the other eye, the detail of this region can be seen.

As a result of this, the blind spot is not usually perceived.

Answer: 7

These seven tiny extra-ocular muscles together control the movements of the eye. Four of these are responsible for controlling the eye's movements from left to right and from top to bottom, and vice versa. Two of the other EOMs move the eye inward or outward and the last one, levator palpebrae superioris, controls the eyelid and is not always included as an EOM. All these muscles control the movements of the eye in unison: as one muscle contracts, the opposing one relaxes, thus creating a smooth motion. Also, the muscles of both eyes work in a co-ordinated manner so that the eyes are always aligned.

Answer: tears

The tears of the eye are composed of three (3) layers- oil (produced by the Meibomian glands), water (produced by the lachrymal glands) and mucous (produced by goblet cells). The lower mucous layer serves as an adhesive and helps the watery layer stick to the eye-surface. The upper oil layers seals the tear-film, and thus helps prevent the evaporation of the watery layer.

The tear-film has several functions- keeping the eye moist, maintaining a smooth surface for the easy propagation of light, nourishing the front of the eye, and protecting the eye from infection and injury.

Answer: About 1/16th of a second

The sensation of an image remains on the retina of the eye for only 1/16th of a second. This is known as "persistence of vision", and is made use of in movies. When pictures depicting individual stages of a movement are shown in quick succession at about 24 frames per second (the current industry standard), one cannot identify any individual frame/image, the movement seems to take place "smoothly".

However, in some really old films, the frame-rate is much less than 24 frames/second and so they appear choppy.

Answer: Perception of depth (radial distance)

It is the lateral displacement of the eyes which provides two slightly different views of the same object, which the brain is able to analyse and thus calculate the rough distance of that object from us. It is, however, possible to perceive depth even with only one eye, with the help of factors like relative sizes, light and shade, etc., but not to the extent that is perceptible with binocular (i.e., two-eyed) vision.

Try keeping one eye closed, and putting the lid of a pen on to the pen while holding both of them at a fairly large distance from your body. It should take you at least a couple of attempts before getting it right.

This is why you should NOT go driving while wearing an eye-patch; you won't be able to tell how far the vehicle ahead is from you!

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Now that you're done playing this quiz, give your eyes a bit of rest. Blink a few times and give your eyes a much-needed coat of tears; while staring at the computer-screen, one has the tendency not to blink at the required frequency. Turn off the monitor and keep your eyes closed for a few seconds.