Human Eye and The Colourful World Complete - CUET Biology

Human Eye and the Colourful World

Structure and Functions of Human Eye

The human eye is approximately spherical with a diameter of about 2.3 cm. It consists of several specialized parts, each with distinct functions essential for vision. The main components include the cornea, iris, pupil, crystalline lens, ciliary muscles, retina, optic nerve, aqueous humor, and vitreous humor.

• Cornea: Thin, transparent, bulged outermost layer; refracts most of the incoming light and protects the eye from dust and germs.
• Iris: Colored muscular part surrounding the pupil; acts as a shutter to control the size of the pupil.…

Human Eye and the Colourful World

Structure and Functions of Human Eye

The human eye is approximately spherical with a diameter of about 2.3 cm. It consists of several specialized parts, each with distinct functions essential for vision. The main components include the cornea, iris, pupil, crystalline lens, ciliary muscles, retina, optic nerve, aqueous humor, and vitreous humor.

• Cornea: Thin, transparent, bulged outermost layer; refracts most of the incoming light and protects the eye from dust and germs.
• Iris: Colored muscular part surrounding the pupil; acts as a shutter to control the size of the pupil.
• Pupil: Central dark circular opening; regulates and controls the amount of light entering the eye.
• Crystalline Lens: Convex, flexible lens; focuses light onto the retina and can change its focal length.
• Ciliary Muscles: Surround the lens; contract or relax to alter lens thickness and focal length.
• Retina: Light-sensitive screen at the back; forms real and inverted images, contains photoreceptor cells (rods and cones).
• Optic Nerve: Bundle of nerve fibers; transmits electrical signals from retina to the brain.
• Aqueous Humor: Watery fluid between cornea and lens; maintains shape and provides nutrition.
• Vitreous Humor: Gel-like substance between lens and retina; supports the eye and maintains its shape.

Image Formation and Accommodation

The eye forms real and inverted images on the retina, which are then interpreted upright by the brain. The crystalline lens, with variable focal length, enables the eye to focus on objects at different distances—a property called the power of accommodation.

• Accommodation: The ability of the eye to change its focal length using ciliary muscles.
  • For near objects: Ciliary muscles contract, lens becomes thicker, focal length decreases, power increases.
  • For distant objects: Ciliary muscles relax, lens becomes thinner and longer, focal length increases, power decreases.
• Power of Accommodation: The range over which the eye can adjust its focal length to focus on objects.

Near Point, Far Point, and Image Properties

The least distance at which the eye can see objects clearly is called the near point or least distance of distinct vision, typically 25 cm for a normal eye. The farthest point visible is the far point, considered infinity for a normal eye.

• Near Point: Minimum distance for clear vision (25 cm for normal eye).
• Far Point: Maximum distance for clear vision (infinity for normal eye).
• Image on Retina: Always real and inverted; brain processes it to appear upright.

Defects of Vision and Their Correction

Common defects include myopia (near-sightedness), hypermetropia (far-sightedness), presbyopia, and cataract. Each defect has specific causes and corrections.

• Myopia (Near-sightedness):
  • Can see near objects clearly, distant objects appear blurred.
  • Image of distant objects forms before the retina.
  • Causes: Excessive curvature of lens (thick lens, high power, low focal length) or elongated eyeball.
  • Correction: Use of concave (diverging) lens with negative power.
• Hypermetropia (Far-sightedness):
  • Can see distant objects clearly, near objects appear blurred.
  • Image of near objects forms behind the retina.
  • Causes: Thin lens (low power, high focal length) or shortened eyeball.
  • Correction: Use of convex (converging) lens with positive power.
• Presbyopia:
  • Difficulty in seeing near objects with age.
  • Causes: Weakening of ciliary muscles or loss of lens flexibility.
  • Correction: Use of bifocal lenses (upper part concave for distance, lower part convex for reading).
• Cataract:
  • Lens becomes opaque due to formation of a milky or cloudy layer.
  • Correction: Surgical removal of the opaque lens.

Refraction Through a Prism and Dispersion

A prism is a transparent refracting medium with two triangular and three rectangular surfaces. When light passes through a prism, it bends towards the base due to refraction.

• Refraction in Prism:
  • Incident ray bends towards the normal when entering denser medium (glass).
  • Emergent ray bends away from the normal when exiting to rarer medium (air).
  • The overall deviation is towards the base of the prism.
• Key Angles:
  • Angle of incidence: Between incident ray and normal.
  • Angle of refraction: Between refracted ray and normal.
  • Angle of emergence: Between emergent ray and normal.
  • Angle of deviation: Angle between the direction of incident ray and emergent ray.
• Dispersion: Splitting of white light into its constituent seven colors (VIBGYOR) when passing through a prism due to different angles of deviation for different colors.
  • Violet bends the most; red bends the least.

Spectrum, Newton’s Experiment, and Rainbow Formation

The band of seven colors formed on a screen after dispersion is called the spectrum. Newton’s experiment with two identical prisms proved that white light consists of seven colors, not that the prism creates colors.

• Spectrum: The band of seven colors (violet, indigo, blue, green, yellow, orange, red) obtained after dispersion.
• Newton’s Experiment:
  • White light passed through a prism splits into seven colors.
  • Passing the spectrum through a second, inverted prism recombines the colors into white light.
• Rainbow Formation:
  • Involves dispersion, refraction, and internal reflection of sunlight in raindrops.
  • Sunlight enters a raindrop, disperses into colors, reflects internally, and emerges to form a rainbow.
  • Red appears at the top, violet at the bottom.
  • Sun must be behind the observer and rain in front.

Atmospheric Refraction and Related Phenomena

Atmospheric refraction occurs due to varying air density at different altitudes, causing light to bend as it passes through the atmosphere.

• Apparent Position of Stars: Stars appear higher than their actual position due to refraction.
• Advance Sunrise and Delayed Sunset: Sun appears above the horizon before actual sunrise and after actual sunset due to atmospheric refraction.
• Twinkling of Stars: Caused by continuous changes in atmospheric density, altering the path and intensity of starlight entering the eye.
• Planets Do Not Twinkle: Planets are extended sources; variations in light from different points average out, so no twinkling is observed.

Scattering of Light and Tyndall Effect

Scattering of light by small particles in the atmosphere is responsible for various optical phenomena. The Tyndall effect is the scattering of light by colloidal particles.

• Tyndall Effect: Scattering of light by colloidal particles, making the path of light visible in colloidal solutions (e.g., milk, smoke, dust in air).
• Color of Scattered Light: Depends on particle size.
  • Very small particles scatter blue light most (shortest wavelength).
  • Large particles scatter all colors equally, resulting in white light.
• Examples:
  • Sunlight beams visible in a dusty room or through mist in a forest.

Color of the Sky and Redness of Sun

The blue color of the sky and the red color of the sun at sunrise and sunset are due to scattering.

• Blue Sky: Air molecules and fine particles scatter short-wavelength blue light more than red, making the sky appear blue.
• Red Sun at Sunrise/Sunset: Sunlight travels a longer path through the atmosphere; blue light is scattered out, leaving predominantly red and orange hues.
• If No Atmosphere: No scattering would occur; the sky would appear dark (as seen by astronauts or at high altitudes).
• Red Color for Danger Signals: Red light scatters least and can travel the farthest through fog and smoke, making it visible from a distance.

Key Diagrams and Exam Points

Practice drawing and labeling the human eye, ray diagrams for myopia and hypermetropia corrections, prism refraction and dispersion, Newton’s prism experiment, rainbow formation, and atmospheric refraction effects. Label all parts and indicate the direction of light rays, angles, and color order where relevant.

• Important Exam Points:
  • Functions of each eye part.
  • Mechanism and correction of vision defects.
  • Sequence and explanation of dispersion and spectrum.
  • Atmospheric refraction effects and their consequences.
  • Scattering phenomena and their real-life implications.