Where Mindful Learning Begins
Where Mindful Learning Begins
Class 10 Science NCERT Solutions Chapter 10 (CBSE 2026-27) — every in-text question, every embedded Activity question, and all 12 back-of-chapter Exercises on this one page, solved with full working.
The Human Eye and the Colourful World applies everything learnt about lenses in Chapter 9 to the eye itself — the cornea, iris, pupil, retina, and the eye lens's remarkable power of accommodation — before covering the three common refractive defects of vision (myopia, hypermetropia, presbyopia) and exactly how concave and convex corrective lenses fix each one. It then turns to the optics of everyday natural phenomena: how a prism disperses white light into its seven colours, how a rainbow forms, why stars twinkle but planets don't, why sunrise and sunset appear a little early and late, and why the sky is blue. These NCERT solutions work through every Activity-based conceptual question and the full 12-question Exercise set with complete step-by-step working for every lens-power calculation.
The Human Eye and the Colourful World is really two chapters in one, connected by the same optics from Chapter 9. The first half treats the eye as a living camera — a cornea that does most of the refracting, an eye lens whose curvature the ciliary muscles adjust (accommodation) to keep images sharply focused on the retina, and the three refractive defects — myopia, hypermetropia, presbyopia — that show up when this system can't quite do its job, each corrected with the right concave or convex lens. The second half turns outward, to what light does once it leaves a prism or the atmosphere: why a prism splits white light into a band of seven colours (dispersion), how that same idea explains a rainbow, why the atmosphere bends starlight enough to make stars twinkle but not planets, why the sun appears to rise a little early and set a little late, and why scattering by air molecules paints the sky blue. Every question woven through the chapter is solved here, in the order the textbook presents it: the one "QUESTIONS" box after the defects-of-vision section, the questions embedded directly inside the prism and dispersion Activities, and the full 12-question Exercise set at the end, with complete numerical working for every lens-power calculation.
Cornea, iris, pupil and retina; how the ciliary muscles adjust the eye lens's focal length to focus near and far objects.
Why distant or nearby objects blur, and exactly which lens — and what power — corrects each defect.
Prisms, rainbows, twinkling stars, early sunrise and late sunset, and why a clear sky looks blue.
The power of accommodation is the ability of the eye to adjust the focal length of its lens, using the ciliary muscles, so that it can focus clearly on objects placed at different distances — both near and far.
A concave lens of suitable power should be used. It diverges the incoming light rays so that the image of a distant object forms exactly on the retina instead of in front of it, correcting the myopia (near-sightedness).
Near point: about 25 cm from the eye. Far point: infinity. A normal eye can see objects clearly anywhere between these two points.
The student is likely suffering from myopia (near-sightedness) — they can see nearby objects clearly but not distant ones, like the blackboard from the back of the class. This can be corrected by using spectacles with a concave lens of suitable power.
At the first surface (air to glass), the ray bends towards the normal — the angle of refraction is smaller than the angle of incidence. At the second surface (glass to air), the ray bends away from the normal — the angle of emergence is larger than the angle of refraction at that surface.
This is not the same as a glass slab. In a slab, the two refracting surfaces are parallel, so the emergent ray comes out parallel to the incident ray (only laterally displaced). In a prism, the two refracting surfaces are inclined to each other, so the emergent ray is not parallel to the incident ray — it bends at an angle, called the angle of deviation.
A beautiful band of colours — Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR) — appears on the screen. This happens because white light is actually made up of these seven colours, and each colour bends through a different angle as it passes through the prism (violet bends the most, red the least), due to their differing refractive indices. This causes the colours to separate and emerge along different paths — a phenomenon called dispersion.
Red light is scattered the least by fog, smoke, or other particles in the atmosphere (since it has the longest wavelength among visible colours). This means red light can travel the farthest distance while remaining visible in its original colour, making it ideal for danger/warning signals that need to be seen clearly from far away.
Answer: (b) accommodation
Answer: (d) retina
Answer: (c) 25 cm
Answer: (c) ciliary muscles
(i) Distant vision:
(ii) Near vision:
A concave lens is needed so that an object at infinity forms a virtual image at the person's far point (80 cm = 0.8 m). Here u = −∞, v = −0.8 m.
A concave lens of power −1.25 D is required.
Diagram: for the uncorrected hypermetropic eye, rays from a nearby object (at 25 cm) converge to a point behind the retina. Placing a convex lens in front of the eye converges the rays earlier, moving the focus point back onto the retina.
A convex lens is needed so that an object at the normal near point (25 cm) forms a virtual image at the person's actual near point (1 m). Here u = −0.25 m, v = −1 m.
A convex lens of power +3 D is required.
The ciliary muscles of the eye can only increase the curvature (and reduce the focal length) of the eye lens up to a certain maximum limit. Beyond this limit, the lens cannot become curved enough to further decrease its focal length, so it cannot properly focus the image of objects closer than 25 cm onto the retina — the image would form behind the retina and appear blurred.
The image distance stays essentially unchanged — the image is always formed on the retina, at a fixed distance from the eye lens, since the retina's position doesn't move. Instead, it is the focal length of the eye lens that changes (via accommodation, adjusted by the ciliary muscles) to keep the image sharply focused on the retina as the object distance changes.
Stars twinkle due to atmospheric refraction of starlight. As starlight enters the earth's atmosphere, it undergoes continuous refraction through layers of air with gradually and randomly changing refractive index, caused by varying physical conditions (temperature, etc.). Since stars are extremely distant, they act as point sources of light, so this continuously changing refraction causes the apparent position and brightness of the star to fluctuate — appearing alternately brighter and fainter, which is the twinkling effect.
Planets are much closer to Earth than stars, and are seen as extended sources of light rather than point sources — a planet can be thought of as a collection of many point-sized sources of light. Even though atmospheric refraction affects light from each of these individual points, the total variation in the amount of light reaching our eyes from all the points averages out to zero (the fluctuations cancel each other out), so the twinkling effect is not observed for planets.
The blue colour of the sky as seen from Earth's surface is due to the scattering of sunlight by fine particles/molecules in the atmosphere (blue light is scattered more than other colours). At very high altitudes, where astronauts are, there is little to no atmosphere present, so there is no scattering of sunlight. Without scattering, no light is redirected towards the observer's eyes from all directions, so the sky appears dark/black instead of blue.
Next up is Chapter 11 — Electricity, moving into the Effects of Current unit. Or explore the full chapter list, browse the Class 10 Science hub, or book a free demo class for personalised coaching.
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