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The Vault
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The Vault
CBSE Class 10 Physics · 11 questions · 26 marks
Your eye is a living optical instrument that focuses light using a lens whose shape you control without even trying. This chapter connects that biology to the physics of refraction, explaining common vision defects, why prisms paint rainbows, and why the sky is blue and sunsets are red. It is where ray optics meets everyday wonder.
In a myopic (near-sighted) eye, where does the image of a distant object form?
Answer
In front of the retina — in myopia the eyeball is elongated or the eye lens is too strongly curved, so the image of a distant object forms before it reaches the retina, making distant objects look blurred.
Which type of lens is used in spectacles to correct hypermetropia (far-sightedness)?
Answer
A convex lens — in hypermetropia the image forms behind the retina because the eye lens is too weak or the eyeball too short; a convex (converging) lens adds the extra converging power needed to bring the image forward onto the retina.
A prism splits white sunlight into a band of seven colours. What causes this splitting?
Answer
Different colours of light travel at different speeds inside glass and so bend by different amounts — white light is a mixture of seven colours (VIBGYOR) of different wavelengths; each bends differently on refraction through the prism, with violet bending the most and red the least, spreading the colours into a spectrum.
Why does the clear daytime sky appear blue rather than white or red?
Answer
Blue light, having a shorter wavelength, is scattered by atmospheric molecules far more than red light — this is Rayleigh scattering; the tiny gas molecules of the atmosphere scatter shorter wavelengths much more strongly, and since the eye is quite sensitive to blue, the sky appears blue.
Assertion (A): Stars appear to twinkle in the night sky, but planets do not. Reason (R): Planets are much closer to the Earth than stars and are seen as extended sources, so the fluctuations from the many points of light average out and cancel the twinkling effect.
Answer
Both A and R are true and R is the correct explanation of A — starlight, coming from a point source, undergoes continuous refraction through atmospheric layers of changing density, causing its apparent position and brightness to fluctuate (twinkling); planets are near enough to appear as extended sources, and light from their many points averages out this fluctuation, so they do not twinkle.
What is meant by the power of accommodation of the eye?
Answer
The power of accommodation is the ability of the eye lens to change its focal length by adjusting its curvature, through the action of the ciliary muscles, so that the eye can form clear, focused images of objects placed at different distances — both near and far — on the retina.
A person can see nearby objects clearly but cannot see distant objects clearly. Name this defect of vision, state its cause, and name the type of lens used to correct it.
Answer
This defect is myopia (near-sightedness or short-sightedness). It is caused by excessive curvature of the eye lens or elongation of the eyeball, which makes the image of a distant object form in front of the retina instead of exactly on it. It is corrected by using spectacles fitted with a concave (diverging) lens of suitable focal length, which diverges the incoming light slightly before it enters the eye so the image shifts back onto the retina.
Why does the Sun appear reddish at sunrise and sunset but appear white or yellowish-white at noon?
Answer
At sunrise and sunset, sunlight has to travel a much longer path through the denser lower layers of the atmosphere to reach an observer. Along this long path, most of the shorter-wavelength blue light is scattered away out of the direct line of sight, leaving mainly the longer-wavelength red light to reach the eye, so the Sun looks reddish. At noon, sunlight travels a much shorter path through the atmosphere, so far less scattering occurs and the Sun appears white or yellowish-white.
Explain why the least distance of distinct vision for a normal human eye is taken as 25 cm. Describe what happens to the near point and far point of the eye in (i) myopia and (ii) hypermetropia, and explain the role of the ciliary muscles in accommodation.
Answer
The least distance of distinct vision (near point) for a normal, healthy young adult eye is taken as 25 cm — this is roughly the closest distance at which the eye can focus on an object clearly and comfortably without straining the ciliary muscles excessively; the far point of a normal eye is at infinity, so it can also see very distant objects clearly. Role of ciliary muscles: these muscles control the curvature of the eye lens. They contract to make the lens more curved (increasing its power/decreasing its focal length) to focus on nearby objects, and relax to flatten the lens (decreasing its power/increasing its focal length) to focus on distant objects — this adjustment is called accommodation. (i) In myopia (near-sightedness), the far point shifts from infinity to some finite distance in front of the eye, so distant objects cannot be seen clearly, though the near point may stay at 25 cm or even come closer. (ii) In hypermetropia (far-sightedness), the near point shifts farther away from the eye (to a distance greater than 25 cm), so nearby objects cannot be seen clearly, though the far point usually remains at infinity, as in a normal eye.
Describe, in words, how Newton demonstrated that ordinary white light is made up of seven colours using a glass prism, and explain what happens when the spectrum from one prism is passed through a second, inverted prism placed in its path.
Answer
When a narrow beam of white sunlight passes through a glass prism, it splits into a band of seven colours in the order violet, indigo, blue, green, yellow, orange, red (remembered as VIBGYOR) — this band is called the spectrum, and the splitting is called dispersion of light. This happens because white light is actually a mixture of seven colours of different wavelengths, and each wavelength refracts by a different amount on entering and leaving the glass prism — violet light, with the shortest wavelength, bends the most, while red light, with the longest wavelength, bends the least, so the colours spread apart. To prove that this spread of colours was an inherent property of white light itself (and not something added by the glass), Newton placed a second, identical prism in an inverted orientation right in the path of the spectrum coming from the first prism. This second prism bent each colour back by the same amount it had been bent apart, recombining all seven colours, and a beam of plain white light emerged from it. This showed that white light is genuinely composed of seven colours, and a prism only separates (or recombines) these colours rather than creating new ones.
Read the following and answer the questions that follow: On a foggy winter morning, Meera notices that the headlight beams of an approaching car appear as clearly visible bright cones of light cutting through the mist, even though a torch beam is normally invisible when shone through clear air. Later, her science teacher shines a laser pointer through a glass of plain water, where no beam is visible, and then through a glass of water with a few drops of milk stirred in, where the laser's path suddenly appears as a clear bluish streak. (a) Name the phenomenon responsible for the visible beam in both the foggy air and the milky water. (b) State the essential condition a medium must satisfy for this phenomenon to occur. (c) Explain why plain water fails to show a visible beam while the milky water shows it clearly. (d) Give one more everyday example where this phenomenon can be observed.
Answer
(a) The phenomenon is the Tyndall effect — the scattering of a beam of light by fine suspended particles present in a medium, such as fog droplets in air or milk particles in water, which makes the path of the light beam visible. (b) The medium must contain suspended (colloidal-sized) particles, such as dust, smoke, water droplets, or fine particles, whose size is comparable to the wavelength of light; a medium free of such particles will not scatter light noticeably. (c) Plain water is essentially free of suspended particles, so there is nothing present to scatter light sideways towards the observer, and the beam stays invisible. The milk added to the water introduces countless tiny fat and protein particles that scatter the laser light in all directions, making the beam's path visible. (d) Another example is the visible shaft of sunlight seen when it enters a dusty room through a small gap in curtains or a window.
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