Human Eye and Colourful World, Class 10 Notes
Human Eye and Colourful World is an important chapter in the Class 10 Science syllabus. It explains how the human eye functions as a natural optical device and helps students understand how light behaves to create natural phenomena like rainbows and the blue sky. Students learn about the structure of the eye, common vision defects, and the physics behind refraction, dispersion, and scattering of light.
Structure and Function of the Human Eye
The human eye functions like a camera, forming real and inverted images on the retina. It is almost spherical, about 2.3 cm in diameter, and contains several vital parts:
Part of the Eye |
Function |
Cornea |
Transparent membrane that refracts about 80% of incoming light. |
Pupil and Iris |
The pupil allows light to enter; the iris controls its size to regulate light intensity. |
Lens |
A convex, flexible structure that adjusts shape to focus on near or distant objects. |
Ciliary Muscles |
Control lens curvature; contract for near vision, relax for distant vision. |
Retina |
Light-sensitive inner layer with rods (dim light) and cones (colour vision). |
Optic Nerve |
Carries image signals from the retina to the brain for interpretation. |
Aqueous and Vitreous Humors |
Maintain the eye’s shape and provide nutrients. |
Common Defects of Vision and Their Corrections
The chapter discusses four major defects of vision, Myopia, Hypermetropia, Presbyopia, and Cataract, along with their causes and corrections:
Defect of Vision |
Cause |
Effect on Vision |
Correction |
Myopia (Near-sightedness) |
Excessive curvature of the lens or an elongated eyeball |
Distant objects appear blurred; image forms in front of retina |
A concave lens diverges light rays |
Hypermetropia (Far-sightedness) |
Thin lens or short eyeball |
Nearby objects appear blurred; image forms behind retina |
A convex lens converges light rays |
Presbyopia |
Age-related loss of lens flexibility |
Difficulty in seeing nearby objects clearly |
Bifocal lenses with concave and convex sections |
Cataract |
Clouding of the crystalline lens |
Hazy or blurred vision due to blocked light entry |
Surgical removal of the cloudy lens |
Refraction, Dispersion, and Spectrum
Refraction is the bending of light as it passes through different media, such as a glass prism that bends light towards its base. When white light passes through a prism, it splits into seven colours, Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). This process is called dispersion, forming a spectrum. Newton’s prism experiment showed that a prism only separates colours, not creates them.
Rainbow Formation and Atmospheric Phenomena
A rainbow forms when sunlight is refracted, dispersed, and reflected by water droplets in the air, each acting as a tiny prism. Atmospheric refraction also causes natural events like early sunrise, delayed sunset, and stars appearing higher in the sky. The blue colour of the sky and red hues at sunset are due to the scattering of light, shorter blue wavelengths scatter more, while red light dominates at sunset.
