{"slug":"cbse-class-10-science-the-human-eye-and-the-colourful-world","title":"The Human Eye and the Colourful World","description":"The eye and accommodation, defects of vision and their correction, dispersion, atmospheric refraction and scattering, weighted towards myopia and hypermetropia, twinkling of stars and the prism and scattering questions set in the 2026 papers.","board":"CBSE","grade":"Class 10","subject":"Science","chapter":"The Human Eye and the Colourful World","url":"https://www.flipnlearn.app/deck/cbse-class-10-science-the-human-eye-and-the-colourful-world","studyUrl":"https://www.flipnlearn.app/deck/cbse-class-10-science-the-human-eye-and-the-colourful-world/study","cards":[{"id":"cornea","kind":"diagram-label","front":"In the human eye, what is the cornea, and where does most of the refraction of entering light take place?","back":"The thin transparent membrane forming the bulge on the front of the eyeball, through which light enters. Most of the refraction happens at its outer surface; the crystalline lens only provides the finer adjustment of focal length."},{"id":"iris","kind":"term-definition","front":"Iris","back":"The dark muscular diaphragm lying behind the cornea, which sets how wide the pupil opens. The pupil in turn governs how much light gets into the eye."},{"id":"retina","kind":"term-definition","front":"Retina","back":"The delicate light-sensitive screen in the eye on which the eye lens forms an inverted, real image. Its light-sensitive cells produce electrical signals when illuminated, and these travel to the brain through the optic nerves."},{"id":"power-of-accommodation","kind":"term-definition","front":"Power of accommodation","back":"The eye lens's capacity to change its own focal length. The ciliary muscles relax to make the lens thin, increasing its focal length for distant objects, and contract to make it thicker, decreasing the focal length for nearby objects."},{"id":"why-the-eye-cannot-see-closer-than-25-cm","kind":"concept-question","front":"Why can a normal eye not see objects clearly when they are closer than about 25 cm?","back":"The eye lens has a lower limit below which its focal length cannot be reduced. Closer than about 25 cm, the lens can no longer focus the image on the retina, so the image is blurred and the eye feels strain."},{"id":"near-point","kind":"term-definition","front":"Near point of the eye","back":"The closest distance at which an object is seen most clearly and without strain, also called the least distance of distinct vision: about 25 cm for a young adult with normal vision. The far point, the farthest point seen clearly, is infinity for a normal eye."},{"id":"myopia","kind":"term-definition","front":"Myopia","back":"Near-sightedness: nearby objects are seen clearly but distant ones are not, because the image of a distant object forms in front of the retina. It arises from excessive curvature of the eye lens or elongation of the eyeball, and is corrected with a concave lens of suitable power."},{"id":"hypermetropia","kind":"term-definition","front":"Hypermetropia","back":"Far-sightedness: distant objects are seen clearly but nearby ones are not, because light from a close object is focussed behind the retina and the near point lies beyond 25 cm. It arises when the eye lens's focal length is too long or the eyeball too small, and is corrected with a convex lens."},{"id":"presbyopia","kind":"term-definition","front":"Presbyopia","back":"The decrease in the eye's power of accommodation with ageing, so the near point gradually recedes and nearby objects cannot be seen comfortably without corrective glasses. It happens because the ciliary muscles grow weaker and the eye lens becomes less flexible."},{"id":"why-bifocal-lenses-are-needed","kind":"concept-question","front":"Why do some people need bi-focal lenses?","back":"They suffer from both myopia and hypermetropia, so they need different correction for distant and for near vision. A common bi-focal lens has a concave upper portion for distant vision and a convex lower portion for near vision."},{"id":"dispersion-of-light","kind":"term-definition","front":"Dispersion of light","back":"White light separating into the colours that make it up: violet, indigo, blue, green, yellow, orange and red (VIBGYOR). In a prism it happens because different colours bend through different angles, red the least and violet the most."},{"id":"why-a-glass-slab-does-not-disperse-light","kind":"concept-question","front":"Why does white light show dispersion through a glass prism but not through a rectangular glass slab?","back":"A prism's refracting faces are inclined, so colours that bend through different angles emerge along different paths and spread apart. A slab's faces are parallel, so the bending at the second face undoes the bending at the first, and the light emerges parallel to its original direction."},{"id":"how-newton-showed-white-light-is-made-of-colours","kind":"concept-question","front":"How did Newton show that sunlight is made up of seven colours?","back":"After producing a spectrum with one prism, he could not split the colours further with another. He then placed a second identical prism upside down in the path of the spectrum, and a beam of white light emerged from it, showing that the colours recombine into white light."},{"id":"rainbow","kind":"term-definition","front":"Rainbow","back":"A natural spectrum seen after a rain shower, always in the direction opposite to the Sun. Tiny water droplets act like prisms: they refract and disperse sunlight, reflect it internally, and refract it again as it comes out, so different colours reach the eye."},{"id":"atmospheric-refraction","kind":"term-definition","front":"Atmospheric refraction","back":"Refraction of light by the earth's atmosphere. Above a fire, the hotter air is less dense and has a slightly lower refractive index, and because conditions keep changing, objects seen through it appear to waver or flicker."},{"id":"why-stars-twinkle","kind":"concept-question","front":"Why do stars twinkle?","back":"Starlight is refracted continuously as it passes through air of gradually changing refractive index. Stars are so distant that they act as point sources, so as atmospheric conditions shift, the apparent position of the star and the amount of its light entering the eye keep fluctuating."},{"id":"why-planets-do-not-twinkle","kind":"concept-question","front":"Why do planets not twinkle?","back":"Being far nearer the earth, planets appear as extended sources, like collections of many point-sized sources. The variations in the light entering the eye from all these points average out to zero, cancelling the twinkling effect."},{"id":"advanced-sunrise-and-delayed-sunset","kind":"term-definition","front":"Advanced sunrise and delayed sunset","back":"Because of atmospheric refraction, the Sun is visible about 2 minutes before it actually crosses the horizon at sunrise and about 2 minutes after it actually sets. The same effect makes the Sun's disc look flattened when it rises and sets."},{"id":"tyndall-effect","kind":"term-definition","front":"Tyndall effect","back":"The scattering of light by colloidal particles, which makes the path of a light beam visible, unlike its path through a true solution. Examples are a thin sunbeam slanting through a small hole into a smoky room, or sunlight crossing the mist under a dense forest canopy."},{"id":"how-particle-size-affects-scattering","kind":"concept-question","front":"How does the size of the scattering particles affect the colour of the scattered light?","back":"Very fine particles scatter mainly blue light, while larger particles scatter light of longer wavelengths. If the particles are large enough, the scattered light may even appear white."},{"id":"why-the-sky-is-blue","kind":"concept-question","front":"Why the sky is blue","back":"Air molecules and other fine particles are smaller than the wavelength of visible light, so they scatter blue light, of shorter wavelength, far more strongly than red. This scattered blue light reaches our eyes; with no atmosphere, or at very high altitudes, the sky would look dark."},{"id":"why-danger-signals-are-red","kind":"concept-question","front":"Why are danger signal lights red in colour?","back":"Red light is the least scattered by fog or smoke, so a red signal can still be seen in the same colour from a distance."}]}