‘रचना-प्रयोग’ का मुख्य उद्देश्य क्या है?

व्याकरण का व्यावहारिक प्रयोग सिखाना

‘द्वितीयः’ किस प्रकार की संख्या है?

क्रमवाचक (दूसरा)

Why is the sky blue on a clear day?

Air molecules scatter the shorter-wavelength blue light far more than red, so the scattered light reaching our eyes is predominantly blue.

Why does the sun appear reddish at sunrise and sunset?

Light travels through more atmosphere, scattering away most blue light and leaving the longer-wavelength red to reach us.

What is the power of accommodation of the eye?

The ability of the eye lens to change its focal length (by the action of the ciliary muscles) so as to focus on objects at different distances.

What is myopia, its cause and correction?

Myopia (short-sightedness) is the inability to see distant objects clearly because the image forms in front of the retina; it is corrected with a concave (diverging) lens of suitable power.

What is hypermetropia, its cause and correction?

Hypermetropia (long-sightedness) is the inability to see nearby objects clearly because the image forms behind the retina; it is corrected with a convex (converging) lens.

What is the least distance of distinct vision for a normal eye?

About 25 cm; this is the closest distance at which the eye can see an object clearly and comfortably.

Why does a glass prism split white light into colours?

Different colours of light bend by different amounts (they have different speeds in glass), so white light is dispersed into its spectrum — violet bends the most and red the least (refer to the prism diagram in your NCERT textbook).

Why do stars twinkle but planets do not?

Starlight is refracted continuously by the changing atmosphere, making stars appear to twinkle; planets are nearer and appear as a collection of point sources, so their light averages out and they do not twinkle.

Why is the sky blue on a clear day?

Air molecules scatter the shorter-wavelength blue light much more than red, so the scattered light reaching our eyes is mainly blue.

Why does the sun appear reddish at sunrise and sunset?

Near the horizon, sunlight travels through a greater thickness of atmosphere, which scatters away most of the blue light, leaving the longer-wavelength red light to reach us.

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Chapter 10: The Human Eye and the Colourful WorldClass 10 Science — summary, notes, extra questions & NCERT solutions

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The human eye forms an image of an object on the:

Summary

The human eye is a natural optical instrument that focuses light onto the retina using the cornea and a flexible lens. The ability of the eye lens to change focal length and see objects at different distances is called accommodation. Common defects of vision — myopia (short-sightedness), hypermetropia (long-sightedness) and presbyopia — arise when images do not form on the retina, and are corrected with suitable concave, convex or bifocal lenses. A glass prism refracts and disperses white light into its component colours, the spectrum, because each colour bends by a different amount. The same dispersion and scattering of light explain the rainbow, the reddish colour of the rising and setting sun, and the blue colour of the clear sky, where shorter blue wavelengths are scattered most by air molecules.

The human eye and accommodationDefects of vision and their correctionRefraction through a prism and dispersionAtmospheric refractionScattering of light

Key terms

Accommodation

The eye lens adjusting its focal length to focus near and far objects.

Myopia / hypermetropia

Short-sightedness / long-sightedness defects of vision.

Dispersion

Splitting of white light into its component colours.

Scattering of light

Redirection of light by particles, e.g. making the sky blue.

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Power of a lens

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The eye lens adjusting its focal length to focus near and far objects.

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