Learning Objectives
- Understand the structure and working of the human eye
- Learn about defects of vision and their correction
- Understand atmospheric refraction and its effects
- Learn about dispersion of light and the formation of a rainbow
Key Concepts
The Human Eye
The eye is like a camera. Key parts:
- Cornea: Transparent front covering; refracts most light entering the eye.
- Iris: Coloured part; controls the size of the pupil.
- Pupil: Opening that regulates the amount of light entering.
- Lens: Crystalline, biconvex lens that fine-focuses light onto the retina.
- Retina: Light-sensitive screen containing rods (dim light) and cones (colour vision). Image formed is real and inverted.
- Optic nerve: Transmits visual signals to the brain.
Accommodation: The ability of the eye lens to change its focal length by adjusting its curvature using ciliary muscles. Near point (least distance of distinct vision) = 25 cm. Far point = infinity for a normal eye.
Defects of Vision
- Myopia (Short-sightedness): Can see nearby objects clearly but not distant ones. Image forms before the retina. Corrected using a concave lens.
- Hypermetropia (Long-sightedness): Can see distant objects clearly but not nearby ones. Image forms behind the retina. Corrected using a convex lens.
- Presbyopia: Age-related loss of accommodation. Near point increases. Corrected using bifocal lenses.
Refraction Through a Prism
When white light passes through a glass prism, it splits into seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR). This splitting is called dispersion.
Violet deviates the most (highest refractive index); Red deviates the least.
Atmospheric Refraction
Earth's atmosphere has varying density layers, causing refraction of light. Effects:
- Twinkling of stars: Due to atmospheric refraction of starlight through turbulent layers.
- Advanced sunrise and delayed sunset: Refraction bends sunlight when the Sun is below the horizon, making it visible about 2 minutes early/late.
Scattering of Light
Tyndall effect: Scattering of light by colloidal particles (visible beam in fog, smoke).
Blue colour of sky: Shorter wavelengths (blue) are scattered more than longer wavelengths (Rayleigh scattering).
Red colour of Sun at sunrise/sunset: Blue light is scattered away; red light (longer wavelength) reaches the observer.
Summary
The human eye uses a convex lens to focus light on the retina. Defects like myopia and hypermetropia are corrected with appropriate lenses. White light is composed of seven colours that can be separated by a prism. Atmospheric refraction causes twinkling of stars and apparent shift of celestial objects. Scattering explains the blue sky and red sunsets.
Important Terms
- Accommodation
- The ability of the eye lens to adjust its focal length to focus on objects at varying distances
- Dispersion
- Splitting of white light into its component colours by a prism
- Spectrum
- The band of seven colours obtained by dispersion of white light
- Scattering
- Deflection of light by small particles in the atmosphere
Quick Revision
- Near point of normal eye = 25 cm; Far point = infinity
- Myopia: corrected by concave lens; Hypermetropia: corrected by convex lens
- VIBGYOR: Violet deviates most, Red deviates least
- Blue sky = Rayleigh scattering of shorter wavelengths
- Stars twinkle due to atmospheric refraction; planets do not (larger apparent size)