The Human Eye and the Colourful World

Cheatsheet Content

The Human Eye Structure: Cornea: Transparent front part, refracts light. Iris: Controls pupil size, regulates light entry. Pupil: Opening in the iris, allows light to enter. Lens: Crystalline, convex, focuses light on retina. Its focal length can be adjusted. Ciliary Muscles: Change lens curvature, thus its focal length. Retina: Light-sensitive screen, contains rods (for dim light) and cones (for bright light and colour). Optic Nerve: Transmits electrical signals from retina to brain. Working: Light enters through cornea, pupil, lens. Lens focuses light onto retina, forming a real, inverted image. Photoreceptors convert light into electrical signals, sent to brain via optic nerve, where image is interpreted. Power of Accommodation: The ability of the eye lens to adjust its focal length to focus objects at different distances on the retina. Near Point (Punctum Proximum): Closest point at which objects can be seen clearly (approx. 25 cm for a young adult). Far Point (Punctum Remotum): Farthest point at which objects can be seen clearly (infinity for a normal eye). Defects of Vision and Their Correction Myopia (Nearsightedness): Cause: Eye lens too convergent or eyeball too long. Image forms in front of the retina. Symptoms: Distant objects appear blurred. Correction: Concave lens (diverging lens) of appropriate power. Hypermetropia (Farsightedness): Cause: Eye lens too divergent or eyeball too short. Image forms behind the retina. Symptoms: Near objects appear blurred. Correction: Convex lens (converging lens) of appropriate power. Presbyopia: Cause: Weakening of ciliary muscles and decreased flexibility of the eye lens with age. Symptoms: Difficulty seeing near objects (like hypermetropia). Correction: Bifocal lenses (convex for near, concave for distant vision). Astigmatism: Cause: Irregular curvature of cornea or lens. Symptoms: Blurred vision at all distances, distorted images. Correction: Cylindrical lenses. Cataract: Cause: Clouding of eye lens due to age or injury. Symptoms: Partial or complete loss of vision. Correction: Surgical removal of opaque lens and implantation of an artificial intraocular lens. Refraction of Light Through a Prism Prism: A transparent optical element with flat, polished surfaces that refract light. Angle of Prism (A): Angle between two refracting surfaces. Angle of Deviation ($\delta$): Angle between the incident ray and the emergent ray. Dispersion of Light: The splitting of white light into its constituent colours (VIBGYOR) when passing through a prism. Cause: Different colours (wavelengths) of light travel at different speeds in glass, hence refract at different angles. Violet deviates the most, red the least. Spectrum: The band of colours produced by the dispersion of white light. Atmospheric Refraction Definition: Refraction of light by the Earth's atmosphere due to varying optical densities. Twinkling of Stars: Due to continuous change in refractive index of atmospheric layers, light from stars (point sources) appears to fluctuate in intensity. Advance Sunrise & Delayed Sunset: Sun appears about 2 minutes before actual sunrise and stays visible about 2 minutes after actual sunset due to atmospheric refraction bending sunlight towards the observer. Planets Don't Twinkle: Planets are relatively closer and appear as extended sources. The total amount of light entering our eye from a planet remains constant, nullifying twinkling effect. Scattering of Light Tyndall Effect: Scattering of light by colloidal particles or very fine suspended particles in a medium. E.g., Path of light visible in a smoky room or through a forest canopy. Dependence on Wavelength: Amount of scattering is inversely proportional to the fourth power of the wavelength ($\text{Scattering} \propto 1/\lambda^4$). Shorter wavelengths (blue) scatter more than longer wavelengths (red). Why Sky Appears Blue: Blue light (shorter wavelength) from the sun is scattered much more effectively by the fine particles in the atmosphere than red light (longer wavelength). Why Clouds Appear White: Clouds consist of larger water droplets/ice crystals, which scatter all wavelengths of light almost equally, making them appear white. Colour of Sun at Sunrise/Sunset: At sunrise/sunset, sunlight travels a longer distance through the atmosphere. Most of the blue light and shorter wavelengths are scattered away, leaving primarily red and orange light to reach our eyes. Danger Signals are Red: Red light scatters the least, allowing it to travel farthest through fog, smoke, or rain, making it visible from a distance.