Class 10 Notes

Class 10 Science Notes Chapter 11 (Chapter 11) – Examplar Problems (English) Book

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Philoid

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Examplar Problems (English)
Detailed Notes with MCQs of Chapter 11, 'The Human Eye and the Colourful World'. This is a crucial chapter, not just for your board exams but also frequently tested in various government exams due to its practical applications and fundamental physics concepts. Pay close attention to the details.

Chapter 11: The Human Eye and the Colourful World - Detailed Notes for Exam Preparation

1. The Human Eye: Structure and Function

Think of the eye as a natural optical instrument, like a camera.

  • Key Components & Roles:

    • Cornea: The transparent, bulging front surface. It's the primary refracting surface, bending most of the incoming light.
    • Aqueous Humour: A watery fluid between the cornea and the lens, maintaining pressure and providing nutrients.
    • Iris: The coloured diaphragm (gives the eye its colour). It controls the pupil size.
    • Pupil: The adjustable opening in the centre of the iris. It regulates the amount of light entering the eye – contracting in bright light, expanding (dilating) in dim light.
    • Eye Lens (Crystalline Lens): A transparent, flexible, biconvex lens situated behind the iris. It provides the adjustable fine-tuning of focus for objects at different distances. Made of fibrous, jelly-like material.
    • Ciliary Muscles: These muscles hold the lens in place and control its shape (curvature). Contraction makes the lens thicker (shorter focal length) for near vision; relaxation makes the lens thinner (longer focal length) for distant vision.
    • Suspensory Ligaments: Connect the ciliary muscles to the lens.
    • Vitreous Humour: A transparent, jelly-like substance filling the large space behind the lens, maintaining the shape of the eyeball.
    • Retina: The light-sensitive screen at the back of the eye, equivalent to the film or sensor in a camera. It contains specialized photoreceptor cells:
      • Rods: Highly sensitive to light intensity (brightness). Responsible for vision in dim light (scotopic vision). Do not detect colour.
      • Cones: Sensitive to colour and detail. Function best in bright light (photopic vision). There are typically three types of cones sensitive to red, green, and blue light.
    • Optic Nerve: A bundle of nerve fibres that transmits the electrical signals generated by the rods and cones to the brain for interpretation as images. The point where the optic nerve leaves the eye has no photoreceptors and is called the blind spot.

  • Working & Image Formation: Light enters through the cornea, passes through the pupil and lens, and is focused onto the retina. The lens system forms a real, inverted image on the retina. The brain interprets this inverted image as upright.

  • Power of Accommodation: The unique ability of the eye lens to adjust its focal length (by changing its shape via ciliary muscles) to see objects clearly at varying distances.

    • Far Point: The maximum distance at which the eye can see objects clearly without strain. For a normal eye, it is infinity. (Lens is thinnest, ciliary muscles relaxed).
    • Near Point (Least Distance of Distinct Vision): The minimum distance at which the eye can see objects clearly without strain. For a normal young adult eye, it is about 25 cm. (Lens is thickest, ciliary muscles contracted maximally).
    • The range of vision for a normal eye is from 25 cm to infinity.

  • Persistence of Vision: The image sensation persists on the retina for about 1/16th of a second after the object is removed. This allows us to perceive motion in movies or animations (which are sequences of still images shown rapidly).

2. Defects of Vision and Their Correction

These occur when the eye cannot focus light correctly onto the retina.

  • Myopia (Near-sightedness):

    • Symptom: Can see nearby objects clearly, but distant objects appear blurry.
    • Cause: The image of a distant object is formed in front of the retina. This happens because the eye lens has excessive converging power (too curved) or the eyeball is too long. The far point is closer than infinity.
    • Correction: A concave lens (diverging lens) of appropriate power is used. It diverges the incoming parallel rays slightly before they enter the eye, effectively reducing the overall converging power so the image forms on the retina.

  • Hypermetropia (Far-sightedness):

    • Symptom: Can see distant objects clearly, but finds it difficult to focus on nearby objects.
    • Cause: The image of a nearby object is formed behind the retina. This happens because the eye lens has insufficient converging power (focal length too long) or the eyeball is too short. The near point is further away than 25 cm.
    • Correction: A convex lens (converging lens) of appropriate power is used. It provides the additional converging power needed to focus light from nearby objects onto the retina.

  • Presbyopia:

    • Symptom: Difficulty focusing on near objects, typically occurring with age (often called 'old-age hypermetropia'). Power of accommodation decreases.
    • Cause: Gradual weakening of the ciliary muscles and reduced flexibility of the eye lens. The near point recedes. Often, both near and far vision can be affected.
    • Correction: Often requires bifocal lenses (upper part concave for distance, lower part convex for reading) or progressive lenses. Reading glasses (convex lenses) may suffice if only near vision is affected.

  • Astigmatism:

    • Symptom: Blurred vision caused by an irregularly shaped cornea or lens, preventing light from focusing properly on the retina. Vision may be blurred at all distances.
    • Correction: Cylindrical lenses.

  • Cataract:

    • Symptom: The eye lens becomes progressively cloudy or opaque, leading to blurred or hazy vision and eventual blindness if untreated.
    • Cause: Often related to aging, UV exposure, or medical conditions like diabetes. Proteins in the lens clump together.
    • Correction: Cannot be corrected by spectacles. Requires surgery to remove the clouded lens and replace it with an artificial intraocular lens (IOL).

3. Refraction of Light Through a Prism

  • A prism is a solid, transparent medium bounded by at least two plane surfaces inclined at an angle. The angle between the two refracting surfaces is the Angle of the Prism (A).
  • When light enters a prism, it bends towards the normal; when it exits, it bends away from the normal. The overall effect is that the light ray bends towards the base of the prism.
  • Angle of Deviation (D): The angle between the extended incident ray and the emergent ray. It depends on A, the angle of incidence (i), and the refractive index (n) of the prism material.
  • Minimum Deviation (Dm): The deviation angle is minimum for a specific angle of incidence. In this condition, the angle of incidence equals the angle of emergence (i = e), and the refracted ray inside the prism is parallel to its base.

4. Dispersion of White Light by a Glass Prism

  • Dispersion: The phenomenon of splitting white light into its constituent colours when it passes through a refractive medium like a prism.
  • Spectrum: The band of colours obtained is called a spectrum. The sequence is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red), remembered from bottom to top (most deviated to least deviated).
  • Cause: The refractive index of the prism material (like glass) is slightly different for different wavelengths (colours) of light. It's highest for violet (shortest wavelength) and lowest for red (longest wavelength). Since the angle of deviation depends on the refractive index (D ≈ (n-1)A for small angles), violet light bends the most, and red light bends the least.
  • Recombination: Newton demonstrated that an inverted prism placed after the first prism can recombine the spectrum back into white light.
  • Rainbow: A natural spectrum formed by the dispersion of sunlight by tiny water droplets suspended in the atmosphere after rain. The droplets act like tiny prisms. It involves refraction, dispersion, and total internal reflection inside the droplets. A rainbow is always formed in the direction opposite to the Sun.

5. Atmospheric Refraction

Refraction of light caused by the Earth's atmosphere due to varying optical densities of air layers. Air density generally decreases with altitude, making upper layers optically rarer than lower layers.

  • Phenomena:
    • Apparent Position of Stars: Stars appear slightly higher in the sky than their actual position, especially near the horizon, because starlight bends towards the normal as it enters denser air layers.
    • Twinkling of Stars: Stars are very distant, acting as point sources. Starlight travels through turbulent atmospheric layers with continuously changing refractive indices. This causes the light path to fluctuate slightly, leading to variations in the amount of light entering the eye, perceived as twinkling.
    • Planets Don't Twinkle: Planets are much closer and appear as extended sources (collections of point sources). Light from different points averages out the variations caused by atmospheric refraction, cancelling the twinkling effect.
    • Advance Sunrise and Delayed Sunset: The Sun appears to rise about 2 minutes earlier and set about 2 minutes later than it actually does. This is because even when the Sun is slightly below the horizon, its rays bend due to atmospheric refraction and reach the observer, making it appear above the horizon.
    • Apparent Flattening of the Sun's Disc: At sunrise and sunset, the Sun appears oval or flattened because refraction is greater for rays coming from the lower part of the Sun (passing through denser air near the horizon) than from the upper part.

6. Scattering of Light

The process where light is absorbed and then re-emitted in various directions by particles (like air molecules, dust, water droplets) in its path.

  • Tyndall Effect: The scattering of light by particles in a colloid or a fine suspension, making the path of the light beam visible. The colour of the scattered light depends on the size of the scattering particles.
  • Rayleigh Scattering: Scattering by particles much smaller than the wavelength of light (e.g., N₂, O₂ molecules in the air).
    • Key Principle: Intensity of scattered light is inversely proportional to the fourth power of the wavelength (Intensity ∝ 1/λ⁴).
    • Implication: Shorter wavelengths (blue/violet) are scattered much more strongly than longer wavelengths (red/orange).
  • Phenomena Explained by Scattering:
    • Blue Colour of the Sky: Air molecules scatter sunlight according to Rayleigh's law. Blue/violet light is scattered most effectively in all directions. Our eyes are more sensitive to blue than violet, so the sky appears blue when we look away from the Sun.
    • Reddish Appearance of Sun at Sunrise/Sunset: Light from the Sun near the horizon travels a longer distance through the atmosphere. Most of the shorter wavelengths (blue/violet) are scattered away from the line of sight. The longer wavelengths (red/orange) are scattered less and reach the observer's eyes directly, making the Sun and surrounding clouds appear reddish.
    • White Colour of Clouds: Cloud droplets (water or ice) are much larger than air molecules and larger than the wavelength of visible light. They scatter all wavelengths of light almost equally (Mie scattering). When all colours are scattered equally, the combination appears white.
    • Danger Signals are Red: Red light has the longest wavelength and is scattered the least by atmospheric particles like fog, mist, or smoke. Therefore, it can travel the farthest distance without significant loss, making it easily visible from afar.

Multiple Choice Questions (MCQs)

  1. The human eye forms the image of an object at its:
    (a) Cornea
    (b) Iris
    (c) Pupil
    (d) Retina

  2. The least distance of distinct vision for a young adult with normal vision is about:
    (a) 25 m
    (b) 2.5 cm
    (c) 25 cm
    (d) 2.5 m

  3. A person uses spectacles with concave lenses. This indicates that the person is suffering from:
    (a) Presbyopia
    (b) Hypermetropia
    (c) Cataract
    (d) Myopia

  4. The splitting of white light into its component colours is called:
    (a) Refraction
    (b) Reflection
    (c) Dispersion
    (d) Scattering

  5. Which phenomenon is responsible for the advance sunrise and delayed sunset?
    (a) Atmospheric refraction
    (b) Scattering of light
    (c) Dispersion of light
    (d) Total internal reflection

  6. The blue colour of the clear sky is due to:
    (a) Dispersion of sunlight by air molecules
    (b) Scattering of sunlight by air molecules
    (c) Reflection of blue light from the oceans
    (d) Atmospheric refraction of blue light

  7. Which colour of light undergoes the least deviation when passed through a glass prism?
    (a) Violet
    (b) Blue
    (c) Red
    (d) Green

  8. The ability of the eye lens to adjust its focal length is called:
    (a) Persistence of vision
    (b) Accommodation
    (c) Astigmatism
    (d) Dispersion

  9. Why do planets not twinkle?
    (a) They are too far away.
    (b) They emit their own light.
    (c) They are much closer to Earth and are seen as extended sources.
    (d) Their light does not get refracted by the atmosphere.

  10. The Tyndall effect is observed due to:
    (a) Reflection of light
    (b) Refraction of light
    (c) Dispersion of light
    (d) Scattering of light

Answer Key:

  1. (d) Retina
  2. (c) 25 cm
  3. (d) Myopia
  4. (c) Dispersion
  5. (a) Atmospheric refraction
  6. (b) Scattering of sunlight by air molecules
  7. (c) Red
  8. (b) Accommodation
  9. (c) They are much closer to Earth and are seen as extended sources.
  10. (d) Scattering of light

Study these notes thoroughly. Understand the 'why' behind each phenomenon, especially related to atmospheric effects and scattering – those are favourite areas for competitive exams. Good luck!

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