Human Eye, Lenses and Dispersion of Light: Notes and PYQs

The correct answer is C. Myopia, or short-sightedness, is the defect in which nearby objects are seen clearly but distant objects look blurred. It arises either because the eye lens has become too converging, that is its focal length is too short, or because the eyeball has grown too long from front to back. In such an eye the rays from a distant object converge too early and the image falls in front of the retina, not beyond it, so statement C is the incorrect one and is therefore the answer. Option A is a correct statement, because a concave or diverging lens of suitable negative power moves the image back on to the retina. Option B is correct, since near sightedness is simply the common English name for myopia. Option D is also correct, because the far point of a myopic eye shifts in from infinity to a few metres.

The correct answer is B, cornea. The cornea is the thin, transparent, slightly bulging membrane that forms the front surface of the eyeball, and light must pass through it before reaching anything else. Because the change of medium from air to cornea is the sharpest in the whole eye, most of the refraction of incoming light also happens here. Option A is wrong because the retina is the light-sensitive screen at the back of the eye on which the image is finally formed, so light reaches it last and not first. Option C is wrong because the pupil is not a membrane at all but an opening, the central hole through which light passes after crossing the cornea. Option D is wrong because the iris is the coloured muscular curtain that widens or narrows the pupil to control the quantity of light entering the eye.

The correct answer is B, interference. A soap film is extremely thin, and light reflected from its front surface meets light reflected from its back surface. The two waves have travelled slightly different distances, so for each wavelength they either reinforce or cancel one another. This thin-film interference strengthens one colour and removes another at every point of the film, which is why a bubble shows shifting bands of colour. Option A, scattering, is wrong because scattering redirects light by particles and explains the blue sky and the red sunset, not the colours of a film. Option C, dispersion, is wrong because dispersion separates colours by refraction through a medium such as a prism or a raindrop, and a soap film is too thin to disperse light this way. Option D, diffraction, is wrong because that is the bending of light around obstacles and through narrow slits, as in a CD track or a grating.

The correct answer is B, the iris. The iris is the coloured ring of muscle lying just behind the cornea. By contracting and relaxing it makes the central opening, the pupil, smaller in bright light and larger in dim light, and in this way it regulates how much light falls on the retina. Option A is wrong because the cornea is the fixed transparent front membrane; it refracts light strongly but cannot change the size of any opening. Option C is wrong because the retina is the screen of rod and cone cells at the back of the eye where the image is formed and converted into nerve signals. Option D is wrong because the ciliary muscles act on the crystalline lens, changing its curvature and focal length to focus objects at different distances, which is accommodation and not light control.

The correct answer is C, 25 cm. The least distance of distinct vision, also called the near point, is the smallest distance at which an object can be held and still be seen clearly without strain. For a normal adult eye this distance is taken as about 25 centimetres, and it is the standard value used in every numerical problem on magnifying glasses and microscopes. Option A, 10 cm, is closer than an adult eye can comfortably focus, although a young child can manage something near it. Option B, 15 cm, is likewise too small for the standard adult value. Option D, 50 cm, is far beyond the near point of a healthy eye and is nearer to the receded near point of a person with hypermetropia or presbyopia, whose ciliary muscles can no longer thicken the lens enough.

The correct answer is B, dioptre. The power of a lens measures how strongly it converges or diverges light, and it is defined as one divided by the focal length expressed in metres. Since the focal length is in metres, the unit of power is the reciprocal metre, which is given the special name dioptre and the symbol D. Option A is wrong because the newton is the unit of force and has nothing to do with the focusing strength of a lens. Option C is wrong because the lumen measures luminous flux, that is the total visible light given out by a source. Option D is wrong because the candela is the base unit of luminous intensity, the light emitted by a source in a particular direction. Only the dioptre is a property of the lens itself rather than of the light.

The correct answer is B, minus 2 dioptres. Two steps are needed. First, the focal length must be converted to metres: 50 centimetres is 0.5 metre. Second, the power is one divided by the focal length in metres, which gives two dioptres in magnitude. Finally the sign convention is applied: a concave or diverging lens always has a negative focal length and therefore a negative power, so the answer is minus 2 D. Option A has the right magnitude but the wrong sign, and would describe a convex lens of the same focal length. Option C comes from forgetting to convert centimetres to metres and dividing by 2 instead, and option D repeats that error with the correct sign. Candidates who keep the unit conversion and the sign convention separate rarely lose this mark.

The correct answer is D, violet. The refractive index of a transparent medium is not the same for every colour; it rises as the wavelength falls. Violet has the shortest wavelength among the visible colours, so glass offers it the highest refractive index, it bends the most at both faces of the prism and it emerges with the greatest deviation. That is why violet lies at the lower end of the spectrum on the screen. Option A is the opposite case: red has the longest wavelength, the lowest refractive index in glass and therefore the least deviation. Options B and C, yellow and green, lie between red and violet in wavelength and so are deviated by intermediate amounts, more than red but less than violet. Remembering VIBGYOR with violet deviated most settles every question of this family.

The correct answer is B, cone cells. The retina carries two kinds of photoreceptor. Cone cells need reasonably bright light to work and respond selectively to different wavelengths, so they give us the sensation of colour; when the cones are defective a person is colour blind. Option A is wrong because rod cells are sensitive to very dim light but cannot distinguish wavelengths, which is why colours fade and the world looks grey at dusk when only the rods are working. Option C is wrong because goblet cells are mucus-secreting cells of the lining of the intestine and the respiratory tract and are not found in the retina. Option D is wrong because mast cells belong to the immune system and release histamine during allergic reactions. Only rods and cones are retinal receptors, and of these only the cones see colour.

The correct answer is B, a convex lens. In hypermetropia the eye lens is too weak or the eyeball is too short, so rays from a nearby object are not converged enough and the image falls behind the retina. A convex or converging lens of suitable positive power brings the rays together a little before they enter the eye, so the eye lens can now form the image exactly on the retina and the near point returns to about 25 centimetres. Option A is wrong because a concave lens diverges light further and is the correction for myopia, the opposite defect. Option C is wrong because a cylindrical lens corrects astigmatism, where the cornea is not perfectly spherical. Option D is wrong because a plane mirror only reflects light without converging or diverging it and cannot change where an image forms inside the eye.

The correct answer is C, scattering of light. The molecules of the atmosphere are much smaller than the wavelength of visible light, and such particles scatter short wavelengths far more strongly than long ones. Blue light from the sun is therefore thrown sideways again and again and reaches the eye from every direction in the sky, which makes the sky look blue; the same mechanism makes the setting sun look red. Option A is wrong because dispersion needs refraction through a medium that separates colours, as in a prism or a raindrop forming a rainbow. Option B is wrong because total internal reflection explains mirages, optical fibres and the sparkle of a diamond, not the colour of the sky. Option D is wrong because interference explains the colours of soap bubbles and oil films, which arise from thin layers rather than from the open atmosphere.