You've already studied how light refracts through lenses and how lenses form images of different sizes, positions, and types. These ideas help us understand the structure and function of the human eye, which has its own lens that lets us see objects around us by bending light.

In this chapter, we’ll connect these ideas to understand:

1. The Role of the Eye's Lens – How it helps us focus light to see clearly.
2. Vision Correction with Lenses – How lenses in glasses correct eyesight issues.
3. Natural Optical Phenomena – Such as rainbows, the splitting of light into colors, and why the sky appears blue.

Let’s dive into these topics using what you know about light and lenses to explore the science behind how we see and the fascinating effects of light in nature.

The human eye is one of our most important sense organs. It allows us to see the world around us and enjoy the colors of everything we observe. While we can use our other senses, like touch, smell, taste, or hearing, to identify objects, only our eyes can truly help us see colors.

The Eye as a Camera
The human eye functions like a camera. It has a lens system that focuses an image onto a light-sensitive surface called the retina.

How Light Enters the Eye
Light enters through a clear, thin membrane called the cornea. The cornea forms a transparent bulge on the eye’s surface and is responsible for most of the light bending, or refraction, as it enters the eye.

The Structure of the Eye

• The eyeball is round and measures about 2.3 cm in diameter.
• Behind the cornea lies the iris, a dark, muscular structure. It controls the size of the pupil, the small opening that lets light enter the eye.
• The pupil adjusts to control the amount of light: it expands in low light and contracts in bright light.

Focusing Light on the Retina
The eye lens further adjusts the focus to ensure a clear image of objects at different distances. The lens creates an upside-down, real image on the retina.

The Retina and Vision
The retina is a delicate layer with many light-sensitive cells. When light hits these cells, they create electrical signals. These signals travel to the brain through the optic nerve. The brain then interprets the signals, flipping the image so we see objects as they are.

This complex and precise system is what enables us to see the beautiful and colorful world around us.

The eye lens is made of a jelly-like material and can change its shape slightly, thanks to the ciliary muscles around it. This flexibility allows the eye to adjust its focus for objects at different distances. This ability to adjust the focal length of the eye lens is called accommodation.

• Seeing Distant Objects: When the ciliary muscles are relaxed, the lens becomes thinner, increasing its focal length. This allows us to see distant objects clearly.
• Seeing Nearby Objects: When focusing on closer objects, the ciliary muscles contract, making the lens thicker and decreasing its focal length, allowing us to see nearby objects more clearly.

Least Distance of Distinct Vision
If you hold an object too close, the eye feels strain, and the image may appear blurry. The minimum distance at which we can see objects clearly and without strain is called the least distance of distinct vision or the near point. For a young adult with normal vision, this distance is about 25 cm.

Far Point of the Eye
The far point is the farthest distance at which an object can be seen clearly by the eye. For a normal eye, this point is considered to be infinity. Thus, a person with normal vision can see objects clearly from 25 cm to infinity.

Cataract
In older adults, the eye’s crystalline lens may turn cloudy, a condition known as cataract. This cloudiness can partially or completely obstruct vision, but it can usually be corrected through cataract surgery to restore sight.

Introduction to Dispersion

• The stunning colors of a rainbow are produced by the dispersion of white light from the Sun.
• Dispersion is the process by which white light is separated into its component colors when it passes through a medium like a prism.

Activity 10.2: Observing Dispersion with a Prism

To observe how a prism disperses white light:

1. Setup:
   • Take a thick cardboard sheet and create a small hole or narrow slit in the middle.
   • Position the cardboard to allow sunlight to pass through the slit, forming a narrow beam of white light.
2. Using the Prism:
   • Place a glass prism in the path of the beam so that the light strikes one of its faces.
   • Slowly turn the prism until the light that emerges appears on a nearby screen.
3. Observations:
   • You will see a band of colors on the screen, indicating that the prism has dispersed the white light.
   • The sequence of colors observed will be: Violet, Indigo, Blue, Green, Yellow, Orange, and Red.
   • The acronym VIBGYOR can help remember this sequence.
4. Understanding the Spectrum:
   • The band of colors produced by the prism is known as its spectrum.
   • Although not all colors may be individually distinct, the colors present are clearly recognizable.

---

Why Dispersion Occurs

• Different colors of light bend at different angles when passing through the prism due to their varying wavelengths.
• Red light bends the least, while violet light bends the most. This variation allows each color to emerge along separate paths, creating a spectrum.

Historical Context

• Isaac Newton was the first to use a prism to obtain the spectrum of sunlight.
• He attempted to further split the colors of the spectrum using a second prism but found no additional colors.
• By inverting the second prism, he allowed all colors of the spectrum to pass through, resulting in a beam of white light emerging from the other side.
• This experiment led him to conclude that sunlight consists of seven distinct colors.

Natural Phenomenon: The Rainbow

• A rainbow is a natural spectrum seen in the sky, formed by the dispersion of sunlight through tiny water droplets after rain.
• The water droplets function like small prisms, refracting and dispersing sunlight, reflecting it internally, and refracting it again upon exiting the droplet.
• A rainbow is always formed in a direction opposite to that of the Sun.
• Rainbows can also be observed on sunny days when looking through a waterfall or water fountain with the Sun behind you.

---

Key Definitions and Concepts:

1. Dispersion:
   • The splitting of white light into its component colors when passing through a medium such as a prism.
2. Spectrum:
   • The band of colors produced when white light is dispersed, consisting of the sequence VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
3. Angle of Refraction:
   • The angle at which different colors of light bend when passing through a prism, affecting the resulting spectrum.
4. Rainbow:
   • A natural spectrum appearing in the sky, caused by the dispersion of sunlight through water droplets, often observed after rain.

---

Questions and Answers:

1. What is dispersion?
   A: Dispersion is the process of splitting white light into its component colors when passing through a medium like a prism.
2. What is the sequence of colors observed in a spectrum produced by a prism?
   A: The sequence of colors is Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
3. How did Isaac Newton contribute to our understanding of light dispersion?
   A: Isaac Newton was the first to use a prism to obtain the spectrum of sunlight and demonstrated that sunlight is made up of seven colors.
4. How is a rainbow formed?
   A: A rainbow is formed by the dispersion of sunlight through tiny water droplets, which act like prisms, refracting and reflecting the light to produce a spectrum in the sky.

Definition: Atmospheric refraction is the bending of light rays as they pass through the Earth’s atmosphere, which has varying density and refractive index.

Definition of Scattering of Light:
Scattering of light is the process by which small particles or molecules present in a medium cause light waves to spread out in different directions. This phenomenon is responsible for several natural occurrences, such as the blue color of the sky and the reddening of the sun during sunrise and sunset.

Explanation and Examples:

• Blue Color of the Sky: This is due to the scattering of shorter wavelengths of light, predominantly blue, by particles in the atmosphere.
• Color of Water in Deep Sea: Light that penetrates the water scatters, giving it a blue tint as red wavelengths are absorbed at deeper levels.
• Reddening of the Sun at Sunrise and Sunset: The sun appears red due to the scattering of shorter wavelengths, allowing only the red wavelengths to reach our eyes as the sun is lower on the horizon.

Question:
Why does the path of a beam of light become visible in a colloidal solution but not in a true solution?
Answer:
In a colloidal solution, particles are larger than those in a true solution. This size difference allows the colloidal particles to scatter light, making the path of the beam visible, while in a true solution, the particles are too small to scatter light effectively.

---

10.6.1 - Tyndall Effect

Definition of Tyndall Effect:
The Tyndall Effect is the scattering of light by particles in a colloid or in a very fine suspension, making the path of the light visible.

Explanation:

• Atmosphere as a Mixture: The earth’s atmosphere is filled with particles such as smoke, dust, and water droplets. When a beam of light strikes these particles, light scatters, causing the path to become visible.
• Examples of Tyndall Effect:
  • In a Smoke-filled Room: A beam of sunlight entering through a small hole in a smoke-filled room becomes visible due to scattering by smoke particles.
  • In a Dense Forest Canopy: Sunlight passing through a canopy creates visible beams as water droplets in mist scatter light.

Question:
What type of particles are most effective in scattering blue light, and why?
Answer:
Very fine particles, smaller than the wavelength of visible light, are most effective in scattering blue light. This is because shorter wavelengths, such as blue, are scattered more strongly by small particles.

Question:
Why does the scattered light sometimes appear white?
Answer:
If the size of the scattering particles is large, they scatter light of all wavelengths equally, causing the scattered light to appear white.

---

10.6.2 - Why is the Colour of the Clear Sky Blue?

Explanation of the Blue Color of the Sky:

• Size of Atmospheric Particles: Particles in the atmosphere are smaller than the wavelength of visible light, making them efficient at scattering shorter (blue) wavelengths more than longer (red) wavelengths.
• Wavelength Comparison: Red light has a wavelength about 1.8 times longer than blue light. As a result, blue light is scattered more extensively in the atmosphere, giving the sky its blue color.

Question:
What would the sky look like if Earth had no atmosphere?
Answer:
If there were no atmosphere, there would be no particles to scatter light, so the sky would appear dark, as it does at high altitudes where scattering is minimal.

Additional Information:

• Red Colour for ‘Danger’ Signals: Red light is used in danger signals because it has a longer wavelength and is scattered the least by fog or smoke, making it visible from a greater distance.

Question:
Why do passengers flying at high altitudes see a dark sky?
Answer:
At high altitudes, there are fewer atmospheric particles to scatter light, making the sky appear dark to passengers.