Chapter: The Human Eye and The Colourful World

Class 10 Science | Comprehensive Notes

1. Introduction

The human eye is a complex and remarkable organ that enables us to perceive the world around us. It functions like a camera, capturing light and forming images on the retina. The ability to see colors and objects is due to the refraction of light. This chapter explores the structure of the human eye, its working mechanism, defects of vision, their corrections, and various optical phenomena like dispersion, scattering, and atmospheric refraction. Understanding these concepts is crucial in learning how light interacts with different media in our daily lives.

2. Key Terms of the Chapter

3. Structure of the Human Eye

The human eye is a spherical organ that acts as a natural optical instrument. It consists of the cornea, iris, pupil, lens, retina, and optic nerve. The cornea is the outermost transparent layer that helps in the initial focusing of light. The iris controls the size of the pupil, which regulates the amount of light entering the eye. The eye lens, which is convex and flexible, focuses light on the retina to form an image. The retina contains rod and cone cells, responsible for detecting light intensity and color. The optic nerve transmits the visual signals to the brain, enabling us to perceive images.

Table: Important Parts of the Human Eye and Their Functions

4. Functioning of the Human Eye

The human eye functions by capturing light and converting it into electrical signals for the brain to interpret. When light enters the eye through the cornea, it gets refracted towards the pupil. The lens further converges the light, focusing it onto the retina, where an inverted and real image is formed. The retina contains rod and cone cells that detect light and color. This information is transmitted to the brain via the optic nerve, where the image is processed and perceived in an upright position. The ciliary muscles adjust the curvature of the lens, allowing us to see objects at different distances, a process known as accommodation.

5. Power of Accommodation

The power of accommodation refers to the ability of the eye lens to adjust its focal length to see objects clearly at different distances. The ciliary muscles play a crucial role in changing the curvature of the lens. When viewing a nearby object, these muscles contract, making the lens thicker and increasing its converging power. Conversely, when looking at a distant object, the muscles relax, making the lens thinner. However, with age, the flexibility of the lens decreases, leading to presbyopia, a condition where both near and distant vision are affected. This is why older individuals often require bifocal lenses.

6. Defects of Vision and Their Correction

(i) Myopia (Short-Sightedness)

Cause: The inability of the eye to focus distant objects clearly occurs when the eyeball is elongated or the lens becomes too curved. As a result, light rays converge before reaching the retina.

Correction: A concave lens of suitable power is used to diverge the incoming light rays so they focus on the retina.

(ii) Hypermetropia (Long-Sightedness)

Cause: The inability to see nearby objects clearly occurs when the eyeball is too short or the lens is too flat, causing light to focus behind the retina.

Correction: A convex lens of suitable power is used to converge the incoming light rays before they enter the eye.

(iii) Presbyopia

Cause: It occurs due to age-related weakening of the ciliary muscles and reduced flexibility of the lens, affecting both near and distant vision.

Correction: Bifocal lenses (having both concave and convex lenses) are used.

7. Refraction Through a Prism & Dispersion of Light

When light passes through a triangular glass prism, it undergoes refraction twice—once upon entering and once upon exiting. Since different colors have different wavelengths, they bend by different amounts. This phenomenon is called dispersion, leading to the formation of a spectrum (VIBGYOR).

Example: The formation of a rainbow is due to dispersion caused by water droplets acting as tiny prisms.

8. Atmospheric Refraction

Atmospheric refraction occurs when light passes through layers of air with varying densities. This causes bending of light, leading to several natural phenomena like:

• Twinkling of Stars: Light from stars bends due to temperature variations in the atmosphere, making them appear to twinkle.
• Apparent Position of the Sun: The Sun appears slightly above the horizon even when it has set due to refraction.

9. Scattering of Light & Tyndall Effect

Scattering occurs when light is deflected by small particles. Rayleigh’s Law states that shorter wavelengths scatter more than longer ones.

Examples:

• Blue Sky: The sky appears blue because blue light is scattered more than other colors.
• Red Sun at Sunrise & Sunset: At low angles, blue light is scattered away, leaving red and orange hues.

Table: Scattering of Light in Nature

10. Summary

• The human eye functions through refraction and accommodation.
• Myopia, hypermetropia, and presbyopia are common vision defects.
• Dispersion causes the splitting of white light into colors.
• Atmospheric refraction and scattering result in natural optical phenomena.