The Human Eye and the Colourful World – MCQs

Correct answer: B. Retina.

The retina is the light-sensitive layer at the back of the eye. It contains photoreceptor cells called rods and cones, which respond to light and help convert it into electrical signals. These signals travel through the optic nerve to the brain, where visual information is processed. The image formed on the retina is real and inverted; the brain interprets the information so we perceive the world in its usual orientation.

Correct answer: A. To control the size of the pupil and regulate the amount of light entering the eye.

The iris is the coloured part of the eye surrounding the pupil. Its muscles change the pupil's diameter. In bright light, the pupil becomes smaller to reduce incoming light; in dim light, it becomes larger to allow more light to enter. The pupil is the opening, while the iris controls its size.

Correct answer: C. Eye lens.

The eye lens is a transparent, flexible, convex lens. The ciliary muscles alter its curvature and therefore its focal length. This adjustment helps focus light from near and distant objects onto the retina. The process is called accommodation. The lens does not normally move backwards and forwards significantly to focus; its shape changes instead.

Correct answer: D. The ability of the eye lens to adjust its focal length to focus on objects at different distances.

When viewing a nearby object, the eye lens becomes more convex and its focal length decreases. When viewing a distant object, it becomes less convex and its focal length increases. These changes help produce a focused image on the retina. The ciliary muscles control the change in lens shape.

Correct answer: B. It becomes more convex and its focal length decreases.

For a nearby object, the ciliary muscles contract and allow the eye lens to become thicker or more convex. This increases its converging power and reduces its focal length, enabling the image to be focused on the retina. For a distant object, the lens becomes thinner and its focal length increases.

Correct answer: C. 25 cm.

The least distance of distinct vision is the nearest distance at which a normal adult eye can see an object clearly and comfortably. For school-level calculations, it is usually taken as 25 cm. The actual near point varies with age and individual visual ability, so 25 cm is a standard reference value rather than a universal measurement for every person.

Correct answer: A. Rods.

Rods are highly sensitive to low levels of light and are important for night and dim-light vision. They do not provide colour vision in the way cones do. Cones function best in brighter conditions and are responsible for colour perception and fine detail. Both types of photoreceptors are found in the retina.

Correct answer: D. Cones.

Cones are photoreceptor cells that work best in relatively bright light. Different cone types respond most strongly to different ranges of wavelengths, allowing the visual system to distinguish colours. Rods are more sensitive in dim conditions but do not provide normal colour discrimination. Colour vision depends on the combined responses of cone types and brain processing.

Correct answer: B. To carry visual signals from the retina to the brain.

Photoreceptor cells in the retina respond to light and begin the process of converting it into electrical signals. These signals are relayed through retinal nerve cells and travel along the optic nerve to the brain. The brain interprets this information to produce our visual experience. The optic nerve does not act as the eye's lens or pupil.

Correct answer: C. The retina contains light-sensitive cells that detect the image.

The cornea and eye lens refract light to focus an image on the retina. Rods and cones detect the light and initiate signals that are sent to the brain. If light is not focused properly on the retina, the image may appear blurred. Corrective lenses help adjust the path of light so the image can be focused more accurately.

Correct answer: D. A defect in which distant objects appear blurred while nearby objects are seen clearly.

In myopia, light from a distant object is focused in front of the retina rather than on it. This can occur because the eyeball is too long or the optical power of the eye is too great. Nearby objects can still be focused on the retina. Myopia is commonly corrected using a suitable concave, or diverging, lens.

Correct answer: B. Concave lens.

A concave lens diverges incoming light before it enters the eye. This helps shift the final focus backwards onto the retina for a person with myopia. The lens power must be selected to suit the person's vision. In the school-level model, myopia is corrected using a diverging lens with negative power.

Correct answer: C. A defect in which nearby objects appear blurred while distant objects may be seen clearly.

In hypermetropia, light from a nearby object would be focused behind the retina if the eye could not accommodate sufficiently. The eyeball may be too short or the eye's converging power may be insufficient. A person may have difficulty reading close-up text. A suitable convex lens helps converge the light before it enters the eye.

Correct answer: A. Convex lens.

A convex lens is a converging lens. It brings light rays closer together before they enter the eye, helping the eye focus light from nearby objects on the retina. This corrects the focusing problem associated with hypermetropia. The required lens power depends on the person's specific vision needs.

Correct answer: D. In front of the retina.

In myopia, the eye converges light from distant objects too strongly, or the eyeball is too long. Consequently, the image forms in front of the retina. By the time the rays reach the retina, they have begun to diverge again, causing distant objects to appear blurred. A concave lens helps correct this by reducing the effective convergence.

Correct answer: B. Behind the retina.

In hypermetropia, the eye has insufficient converging power for the nearby object, or the eyeball is too short. The light rays would meet behind the retina if they could continue through the eye. A convex lens adds converging power to the incoming rays so that the final image can form on the retina.

Correct answer: C. An age-related reduction in the eye's ability to focus on nearby objects.

As a person ages, the eye lens may become less flexible and the accommodation mechanism becomes less effective. The near point moves farther away, making close-up reading difficult. This age-related condition is called presbyopia. Depending on a person's vision, reading glasses, bifocals or other prescribed corrections may be used.

Correct answer: A. The eye may not increase its converging power enough for nearby objects.

Focusing on a close object requires the eye lens to become more convex and shorten its focal length. With presbyopia, the lens becomes less flexible and accommodation decreases. Reading glasses provide additional optical power so that close objects can be focused more comfortably. The precise correction depends on the individual's vision.

Correct answer: D. Myopia is generally corrected with a concave lens, while hypermetropia is generally corrected with a convex lens.

Myopia causes distant objects to focus in front of the retina, so a diverging concave lens helps move the focus back. Hypermetropia makes close objects tend to focus behind the retina, so a converging convex lens helps bring the focus forward. Remember the defect, the location of the image and the corrective lens as a connected set of ideas.

Correct answer: B. Cataract.

A cataract occurs when the eye's natural lens becomes cloudy. This can cause blurred vision, glare and reduced visual clarity. Cataract is different from myopia and hypermetropia, which are focusing errors. Cataracts are a medical condition and should be assessed by a qualified eye-care professional.

Correct answer: C. The splitting of white light into its constituent colours.

Dispersion occurs because different wavelengths of light are refracted by different amounts when they pass through a dispersive medium such as a glass prism. White light separates into a spectrum of colours. In the familiar visible spectrum, these colours are often remembered as violet, indigo, blue, green, yellow, orange and red.

Correct answer: D. Violet.

In ordinary glass under visible-light conditions, violet light generally has a higher refractive index than red light and is deviated more by a prism. Red light is deviated the least among the visible colours. This difference in refraction causes white light to spread into a spectrum.

Correct answer: A. Red.

Red light has a longer wavelength than violet light and is generally refracted less by ordinary glass. Violet light is deviated more, while red is deviated less. The separation of colours occurs because the refractive index of glass depends on wavelength.

Correct answer: B. Refraction, dispersion and internal reflection of sunlight in water droplets.

Sunlight entering a raindrop is refracted and dispersed into different colours. The light undergoes internal reflection inside the droplet and is refracted again as it emerges. Because different colours leave at different angles, an observer can see a coloured arc when the Sun, water droplets and viewing direction are appropriately positioned.

Correct answer: C. Sunlight travels a longer path through the atmosphere, so much of the shorter-wavelength light is scattered out of the direct path.

Near sunrise and sunset, sunlight passes through a greater thickness of atmosphere before reaching the observer. Shorter wavelengths such as blue and violet are scattered more strongly by air molecules than longer red wavelengths. More of the red and orange light remains in the direct sunlight reaching the eye, making the Sun appear reddish. Dust and aerosols can also affect the colour.

Correct answer: D. Air molecules scatter shorter wavelengths of sunlight more strongly than longer wavelengths.

Air molecules scatter short-wavelength visible light, including blue and violet, much more strongly than red light. This is called Rayleigh scattering. Although violet is scattered strongly too, the Sun's spectrum, the eye's sensitivity and the atmosphere's transmission all influence the perceived colour, so the sky generally appears blue to us.

Correct answer: A. Atmospheric refraction caused by continuously changing layers of air.

Starlight passes through atmospheric layers with changing temperatures and refractive indices. These variations continually alter the apparent position and brightness of the star, creating the twinkling effect. Stars are so far away that they appear nearly point-like, making these changes noticeable. Planets usually appear less affected because they present a small disc rather than a point source.

Correct answer: B. Because atmospheric refraction bends sunlight towards the observer.

Earth's atmosphere becomes progressively denser closer to the surface, so the refractive index changes with height. Sunlight bends as it passes through these layers. The Sun's apparent position is therefore slightly higher than its actual position near the horizon. This allows it to be seen a little before actual sunrise and a little after actual sunset.

Correct answer: C. The scattering of light by particles in a colloid or fine suspension, making the path of a light beam visible.

When light passes through a colloid or a fine suspension, suspended particles scatter the light, making the beam's path visible from the side. Examples include a beam of sunlight visible in a dusty room and light passing through fog. The Tyndall effect is a demonstration of scattering by particles, rather than the formation of an image by a mirror or lens.

Correct answer: D. In Rayleigh scattering, scattering intensity is approximately inversely proportional to the fourth power of wavelength.

For particles much smaller than the wavelength of light, Rayleigh scattering approximately follows an inverse-fourth-power relationship. This means shorter wavelengths are scattered much more strongly than longer ones. The relationship helps explain the blue appearance of the clear sky and the reddish appearance of the Sun near the horizon.