The Human Eye and the Colourful World

1. The Human Eye

• Structure and Function: The human eye functions like a camera, forming an image on a light-sensitive screen called the retina.
• Cornea: Light enters through a thin membrane called the cornea, where most of the refraction occurs.
• Lens: The crystalline lens provides fine adjustment of focal length to focus objects at varying distances on the retina. It forms a real, inverted image.
• Iris and Pupil: The iris is a dark muscular diaphragm behind the cornea that controls the size of the pupil. The pupil regulates the amount of light entering the eye.
• Retina: Contains enormous numbers of light-sensitive cells that generate electrical signals upon illumination. These signals are sent to the brain via the optic nerves.
• Power of Accommodation: The ability of the eye lens to adjust its focal length is called accommodation.
  • To see distant objects, ciliary muscles relax, the lens becomes thin, and focal length increases.
  • To see nearby objects, ciliary muscles contract, the lens becomes thick, and focal length decreases.
• Near and Far Points:
  • The Near Point (least distance of distinct vision) is the minimum distance at which objects can be seen distinctly without strain. For a normal young adult, it is about 25 cm.
  • The Far Point is the farthest point up to which the eye can see clearly. For a normal eye, it is infinity.
• Cataract: A condition in old age where the crystalline lens becomes milky and cloudy, causing vision loss. It can be corrected via surgery.

2. Defects of Vision and Their Correction

• Myopia (Near-sightedness):
  • A person can see nearby objects clearly but cannot see distant objects distinctly.
  • The image of a distant object forms in front of the retina.
  • Causes: Excessive curvature of the eye lens or elongation of the eyeball.
  • Correction: Concave lens of suitable power.
• Hypermetropia (Far-sightedness):
  • A person can see distant objects clearly but cannot see nearby objects distinctly. The near point is farther away than the normal 25 cm.
  • Light rays from close objects focus behind the retina.
  • Causes: Focal length of the eye lens is too long or the eyeball is too small.
  • Correction: Convex lens of appropriate power.
• Presbyopia:
  • A defect arising with aging where the power of accommodation decreases.
  • The near point recedes, making it difficult to read or see nearby objects.
  • Causes: Gradual weakening of ciliary muscles and diminishing flexibility of the eye lens.
  • Correction: Often requires bi-focal lenses (upper portion concave for distance, lower portion convex for reading).

3. Refraction of Light Through a Prism

• A triangular glass prism has two triangular bases and three rectangular lateral surfaces.
• Angle of Prism: The angle between two lateral faces.
• Angle of Deviation: The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation.

4. Dispersion of White Light

• Dispersion: The splitting of light into its component colors is called dispersion.
• Spectrum: The band of colored components of a light beam is called its spectrum. The sequence is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
• Cause: Different colors of light bend through different angles with respect to the incident ray. Red bends the least, and violet bends the most.
• Recombination: Isaac Newton demonstrated that passing a spectrum through a second identical but inverted prism recombines the colors back into white light.
• Rainbow: A natural spectrum appearing in the sky after rain.
  • Caused by dispersion of sunlight by tiny water droplets.
  • Water droplets act like small prisms; they refract and disperse the incident sunlight, then reflect it internally, and refract it again when it emerges.
  • Always formed in the direction opposite to the Sun.

5. Atmospheric Refraction

• Atmospheric refraction is the refraction of light by the earth's atmosphere due to gradually changing refractive indices of air layers.
• Twinkling of Stars:
  • Starlight undergoes continuous refraction before reaching earth.
  • Because physical conditions of the atmosphere are not stationary, the apparent position of the star fluctuates and the amount of light entering the eye flickers.
  • Stars are considered point-sized sources, making the effect noticeable.
• Why Planets Do Not Twinkle: Planets are closer and act as extended sources (collections of point sources). The total variation in light averages out to zero.
• Advance Sunrise and Delayed Sunset: Due to atmospheric refraction, the Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset.

6. Scattering of Light

• Tyndall Effect: The phenomenon of scattering of light by colloidal particles (like smoke, water droplets, or dust) makes the path of a light beam visible.
• Color of Scattered Light: Depends on the size of scattering particles. Very fine particles scatter mainly blue light, while larger particles scatter light of longer wavelengths (like red).
• Blue Colour of the Sky:
  • Air molecules and fine particles are smaller than the wavelength of visible light.
  • They scatter shorter wavelengths (blue) more strongly than longer wavelengths (red). This scattered blue light enters our eyes.
  • If there were no atmosphere, the sky would appear dark (as it does to astronauts).
• Danger Signal Lights: These are red because red light is scattered the least by fog or smoke, allowing it to be seen from a long distance.

Summary based on Chapter 10: The Human Eye and the Colourful World