Class 10 Science Notes Chapter 11 (The human eye and the colourful world) – Science Book

Right then, let's get straight into the key concepts of Chapter 11: The Human Eye and the Colourful World. Pay close attention, as these topics are frequently tested in various government exams.

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

1. The Human Eye: Structure and Function

Think of the human eye as a sophisticated natural optical instrument, much like a camera. Its main parts and functions are:

• Cornea: The transparent, bulging outer surface at the front of the eye. Function: It acts as the eye's primary lens, refracting (bending) most of the light entering the eye. It also protects the inner eye.
• Iris: A flat, coloured, ring-shaped membrane behind the cornea. Function: It controls the size of the pupil, thereby regulating the amount of light entering the eye. It's the part that gives the eye its colour (e.g., blue, brown, green).
• Pupil: The adjustable opening in the centre of the iris. Function: Allows light to enter the inner eye. Its size changes based on light intensity (contracts in bright light, dilates in dim light).
• Crystalline Lens: A transparent, biconvex structure located behind the iris and pupil. It's made of a fibrous, jelly-like material. Function: Provides finer adjustment of focal length to focus light precisely onto the retina for objects at different distances.
• Ciliary Muscles: Muscles attached to the eye lens. Function: They relax or contract to change the curvature (and thus focal length) of the eye lens. This process is called accommodation.
• Retina: A delicate, light-sensitive membrane lining the back of the inner eye. It acts like the screen in a camera. Function: Contains specialized photoreceptor cells - rods (sensitive to light intensity/dim light) and cones (sensitive to colour/bright light). When light falls on the retina, these cells generate electrical signals.
• Optic Nerve: A bundle of nerve fibres connecting the retina to the brain. Function: Transmits the electrical signals generated by the retina to the brain.
• Aqueous Humour: Watery fluid filling the space between the cornea and the lens.
• Vitreous Humour: Jelly-like fluid filling the space between the lens and the retina.

Image Formation: Light enters the eye, gets refracted primarily by the cornea and then by the lens, forming a real, inverted image on the retina. The brain processes the signals received via the optic nerve and interprets the image as erect and of the correct size.

2. Power of Accommodation

• Definition: The ability of the eye lens to adjust its focal length to see objects clearly at varying distances.
• Mechanism:
  • For distant objects (at infinity): Ciliary muscles are relaxed, the lens becomes thinner, its focal length increases, and light is focused on the retina.
  • For nearby objects: Ciliary muscles contract, putting tension on suspensory ligaments, the lens becomes thicker (more curved), its focal length decreases, and light is focused on the retina.
• Limits of Accommodation:
  • Near Point (Least Distance of Distinct Vision - LDDV): The closest distance at which an object can be seen clearly without strain. For a normal young adult eye, it is about 25 cm.
  • Far Point: The farthest distance at which an object can be seen clearly. For a normal eye, it is infinity.

3. Defects of Vision and their Correction

Common refractive defects of vision:

• (a) Myopia (Near-sightedness):
  • Symptom: Can see nearby objects clearly, but distant objects appear blurred.
  • Cause: The image of a distant object is formed in front of the retina. This happens because:
    • The converging power of the eye lens is too high (excessive curvature), OR
    • The eyeball is too long.
  • Far Point: Closer than infinity.
  • Correction: Using a concave lens (diverging lens) of suitable power. The concave lens diverges the incoming light rays before they reach the eye lens, effectively reducing the overall converging power and allowing the image to form on the retina.
• (b) Hypermetropia (Far-sightedness):
  • Symptom: Can see distant objects clearly, but finds it difficult to see nearby objects distinctly.
  • Cause: The image of a nearby object is formed behind the retina. This happens because:
    • The focal length of the eye lens is too long (insufficient converging power), OR
    • The eyeball is too short.
  • Near Point: Further away than 25 cm.
  • Correction: Using a convex lens (converging lens) of suitable power. The convex lens provides additional converging power required to focus the light from nearby objects onto the retina.
• (c) Presbyopia:
  • Symptom: The near point gradually recedes with age, making it difficult to see nearby objects comfortably (age-related far-sightedness).
  • Cause: Gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. The power of accommodation decreases.
  • Correction: Often requires convex lenses for reading. Sometimes, a person might suffer from both myopia and hypermetropia. Such individuals need bifocal lenses, which have both concave (upper portion, for distant vision) and convex (lower portion, for near vision) parts. Nowadays, progressive lenses are also common.
• (d) Cataract:
  • Symptom: The crystalline lens becomes milky and cloudy, leading to partial or complete loss of vision.
  • Cause: Denaturation of proteins in the lens.
  • Correction: Can only be corrected through surgery, where the clouded lens is removed and replaced with an artificial intraocular lens (IOL).

4. Refraction of Light through a Glass Prism

• A prism is a transparent optical element with flat, polished surfaces that refract light. Typically, it has two triangular bases and three rectangular lateral surfaces.
• Angle of the Prism (A): The angle between its two lateral refracting surfaces.
• Path of Light: When a ray of light enters a prism, it bends towards the normal. When it emerges from the prism, it bends away from the normal. For a triangular prism, the emergent ray is deviated from the direction of the incident ray, bending towards the base of the prism.
• Angle of Deviation (D or δ): The angle between the direction of the incident ray (extended forward) and the emergent ray (extended backward). It depends on the angle of incidence, the angle of the prism, and the refractive index of the prism material.

5. 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 glass prism.
• Spectrum: The band of seven colours obtained is called the spectrum. The sequence of colours is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red), remembered from bottom to top (most deviated to least deviated).
• Cause: White light is a mixture of different colours. Each colour has a different wavelength (and frequency). In a medium like glass, different colours travel at slightly different speeds. Since the refractive index depends on the speed of light (n = c/v), the refractive index of glass is slightly different for each colour. Violet light (shortest wavelength) travels slowest and bends the most. Red light (longest wavelength) travels fastest and bends the least.
• Recombination: Isaac Newton showed that if a second identical prism is placed inverted next to the first prism, the colours of the spectrum recombine to produce white light again.
• Rainbow: A natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion, refraction, and total internal reflection (in some cases, just internal reflection) of sunlight by tiny water droplets suspended in the atmosphere. These droplets act like small prisms. A rainbow is always formed in the direction opposite to that of the Sun.

6. Atmospheric Refraction

The bending of light as it passes through the Earth's atmosphere, which has layers of varying optical densities and refractive indices. Generally, air density and refractive index decrease with altitude.

• Apparent Position of Stars: Light from stars undergoes continuous refraction as it enters the Earth's atmosphere, bending towards the normal at each layer. Therefore, the apparent position of a star is slightly higher than its actual position when viewed near the horizon.
• Twinkling of Stars: Stars are very distant, appearing as point sources of light. As starlight travels through the atmosphere, its path continuously changes slightly due to the changing refractive index of air (caused by turbulence, temperature variations). This causes the apparent position of the star to fluctuate, and the amount of starlight entering the eye flickers, resulting in the twinkling effect.
  • Why Planets Don't Twinkle: Planets are much closer to Earth and appear as extended sources (a collection of point sources). Light from all these individual points also flickers, but the overall effect averages out, nullifying the twinkling.
• Advance Sunrise and Delayed Sunset: The Sun appears to rise about 2 minutes earlier than the actual sunrise and set about 2 minutes later than the actual sunset. This is because when the Sun is slightly below the horizon, its light rays travel through the atmosphere and bend downwards due to refraction, reaching the observer's eye. This makes the Sun appear above the horizon. The total duration of daylight increases by about 4 minutes.
• Apparent Flattening of the Sun's Disc: At sunrise and sunset, the Sun appears oval or flattened. This is also due to atmospheric refraction, which is stronger for light rays coming from the lower edge of the Sun (travelling through denser air near the horizon) compared to the upper edge.

7. Scattering of Light

The process by which particles in the path of light (like dust, gas molecules, water droplets) absorb light energy and re-emit it in different directions.

• Tyndall Effect: The scattering of a beam of light by colloidal particles or very fine suspended particles, making the path of the light visible. Examples: Sunlight entering a dusty room through a small hole, sunlight filtering through the canopy of a dense forest. The colour of the scattered light depends on the size of the scattering particles.
  • Very fine particles: Scatter mainly blue light (shorter wavelengths).
  • Larger particles: Scatter light of longer wavelengths or all wavelengths more equally (appearing white or grey, e.g., clouds, fog).
• Why the Sky Appears Blue: Air molecules and other fine particles in the atmosphere are much smaller than the wavelength of visible light. They scatter shorter wavelengths (blue and violet) much more strongly than longer wavelengths (red and orange) – this is known as Rayleigh Scattering (scattering intensity ∝ 1/λ⁴). Our eyes are more sensitive to blue than violet, and the Sun emits slightly less violet. The scattered blue light enters our eyes from all directions, making the sky appear blue.
• Why the Sun Appears Reddish at Sunrise and Sunset: At these times, sunlight has to travel through a greater distance and thicker layers of the atmosphere to reach our eyes. Most of the blue light and shorter wavelengths are scattered away by the particles. The light that reaches us directly is predominantly composed of longer wavelengths (red, orange, yellow). Hence, the Sun and the surrounding sky appear reddish.
  • At Noon: The Sun is overhead, and light travels a relatively shorter distance through the atmosphere. Only a little blue light is scattered, so the Sun appears white (or slightly yellowish).
• Why Danger Signals are Red: Red light has the longest wavelength in the visible spectrum. It is scattered the least by atmospheric particles like fog, smoke, or rain. Therefore, it can travel the longest distance without significant loss of intensity, making it visible from afar even in poor visibility conditions.
• Why Clouds Appear White: Clouds contain water droplets or ice crystals that are much larger than the wavelength of visible light. These larger particles scatter all colours of visible light almost equally. When all colours are scattered equally, the resulting light appears white.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the chapter for your practice:

1. The change in focal length of the human eye lens is caused by the action of the:
   (a) Pupil
   (b) Retina
   (c) Ciliary muscles
   (d) Iris

2. A person cannot see distant objects clearly. He is suffering from a defect of vision called:
   (a) Hypermetropia
   (b) Myopia
   (c) Presbyopia
   (d) Cataract

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

4. The bluish colour of the sky is due to the phenomenon of:
   (a) Reflection of light
   (b) Refraction of light
   (c) Dispersion of light
   (d) Scattering of light

5. For a normal young adult eye, the least distance of distinct vision is about:
   (a) 25 m
   (b) 2.5 cm
   (c) 25 cm
   (d) 2.5 m

6. Twinkling of stars is primarily due to atmospheric:
   (a) Dispersion
   (b) Interference
   (c) Refraction
   (d) Scattering

7. Which colour of light deviates the most when passing through a glass prism?
   (a) Red
   (b) Green
   (c) Violet
   (d) Yellow

8. A person needs a lens of power -2.5 D for correcting his vision. The focal length of the corrective lens is:
   (a) +40 cm
   (b) -40 cm
   (c) +25 cm
   (d) -25 cm

9. The reddish appearance of the sun at sunrise and sunset is due to:
   (a) The Tyndall effect
   (b) Scattering of light
   (c) Dispersion of light
   (d) Total internal reflection

10. Bifocal lenses are required to correct the defect called:
    (a) Astigmatism
    (b) Myopia
    (c) Hypermetropia
    (d) Presbyopia

Answer Key for MCQs:

1. (c) Ciliary muscles
2. (b) Myopia
3. (c) Dispersion
4. (d) Scattering of light
5. (c) 25 cm
6. (c) Refraction
7. (c) Violet
8. (b) -40 cm (Since P = 1/f, f = 1/P = 1/-2.5 m = -0.4 m = -40 cm)
9. (b) Scattering of light
10. (d) Presbyopia (often requiring correction for both near and far vision)

Revise these notes thoroughly. Understanding the 'why' behind each phenomenon is crucial for competitive exams. Good luck!