The Human Eye: Structure and Function

• The human eye allows sight of the colorful world
• Light is required to see and appreciate the colors of objects
• A lens system in the human eye works like a camera, forming an image on the retina, a light-sensitive screen

Parts of the Human Eye and Their Functions

• Cornea: Refracts light and aids focusing as the transparent front of the eye, covering the iris, pupil, and anterior chamber
• Iris: By adjusting pupil size, this dark muscular diaphragm regulates the light entering the eye
• Pupil: The iris's aperture regulates how much light enters the eye
• Crystalline Lens: Focuses objects at varying distances on the retina with fine focal length adjustments
• Ciliary Muscles: Enable eye focus on near or distant objects by changing the curvature of the eye lens
• Retina: Houses many light-sensitive cells in a delicate membrane
• Optic Nerve: This transmits visual data from the retina to the brain

Accommodation

• Accommodation: The eye lens' capacity to change focal length, enabling clear vision of both near and far objects
• Ciliary muscles adjust the eye lens' focal length
• Ciliary muscles relax to focus on distant objects, thinning the eye lens and lengthening the focal length
• Ciliary muscles contract to focus on close objects, thickening the eye lens and shortening the focal length

Near Point and Far Point

• Near point: About 25 cm for a young adult with normal vision, this is the closest distance at which an object can be clearly seen
• Far point: The farthest an object can be clearly seen. Infinity for a normal eye

Defects of Vision and Their Correction

• Myopia (Nearsightedness): Close objects are clear, distant objects are blurry
• Myopia Cause: Elongated eyeball or excessive curvature of the eye lens
• Myopia Correction: Concave lens of appropriate power corrects nearsightedness
• Hypermetropia (Farsightedness): Distant objects are clear, close objects are blurry
• Hypermetropia Cause: The eyeball is too short, or the eye lens has insufficient curvature
• Hypermetropia Correction: Convex lens of suitable power corrects farsightedness
• Presbyopia: Usually occurring in old age, ciliary muscles weaken, reducing accommodation
• Presbyopia Correction: Bifocal lenses
• Cataract: The clouding of the eye lens causes blurred vision and potential blindness
• Cataract Correction: Replacing the cataractous lens with a new artificial lens

Refraction of Light Through a Prism

• Prism: Light is refracted by this transparent optical element, which has flat, polished surfaces
• Light bends at both prism surfaces
• Angle of Deviation: The angle formed between the incident and emergent rays
• Dispersion: White light splits into its component colors
• Each color bends differently, resulting in white light splitting into a spectrum
• The colors are violet, indigo, blue, green, yellow, orange, and red (VIBGYOR)
• Violet has the most deviation, red has the least

Atmospheric Refraction

• Atmospheric refraction: Light bends as it passes through Earth's atmosphere
• Atmospheric refractive index varies with temperature and density
• Twinkling Stars: Atmospheric refraction makes stars twinkle; fluctuations in the apparent position of the star happen when light passes through atmospheric layers with varying refractive indices
• Advanced Sunrise and Delayed Sunset: Atmospheric refraction makes the sun visible for about two minutes before actual sunrise and after actual sunset

Scattering of Light

• Scattering: Particles in a medium deflect light rays from their straight path
• The size of the scattering particles determines the color of scattered light
• Tyndall Effect: Light scatters when a beam of light passes through a colloid, making the light's path visible
• Color of the Sky: Blue light is scattered more effectively by small particles in the atmosphere, making the sky appear blue
• Color of Sun at Sunrise and Sunset: The sun appears reddish as most of the blue light is scattered away, leaving mostly red light to reach our eyes