The Human Eye and Vision Correction

• The human eye uses light to enable vision through a lens
• This section addresses how lenses in spectacles correct vision defects, also covering rainbow formation, white light splitting, and the sky's blue color.

The Human Eye

• The human eye is a highly valuable and sensitive sense organ
• It allows us to perceive the world and its colors
• While other senses can identify objects without sight, color identification requires open eyes
• Eye is like a camera, forming images on the retina
• Light enters through the cornea, a transparent bulge on the eyeball's front surface
• Eyeball shape is approximately spherical, with about 2.3 cm diameter
• Most light refraction occurs at the cornea's outer surface
• The crystalline lens provides fine focal length adjustments
• The iris is a dark muscular diaphragm behind the cornea that controls pupil size and regulates light entering the eye
• The eye lens forms an inverted, real image on the retina, a light-sensitive membrane
• Light-sensitive cells activate upon illumination, generating electrical signals sent to the brain via optic nerves
• The brain interprets these signals, enabling object perception

Power of Accommodation

• The eye lens consists of a fibrous, jelly-like material that can be modified by ciliary muscles
• Changing lens curvature adjusts focal length
• Relaxed muscles make the lens thin, increasing focal length for distant vision
• Contracted ciliary muscles thicken the lens, decreasing focal length for close objects
• Accommodation is the eye lens's ability to adjust focal length
• Focal length cannot decrease below a minimum limit
• Holding a printed page too close results in a blurred image or eye strain
• Distinct vision requires holding objects about 25 cm from the eyes
• The least distance of distinct vision, or near point, is the closest distance for clear, strain-free vision
• A young adult with normal vision has a near point of about 25 cm
• The farthest point of clear vision is the far point, at infinity for a normal eye
• Normal eyes can clearly see objects between 25 cm and infinity
• Cataract, a condition where the crystalline lens becomes milky and cloudy, causing vision loss, can be corrected with surgery

Defects of Vision and Their Correction

• Vision defects cause blurred vision because the eye gradually loses its power of accommodation
• Common refractive defects: myopia (near-sightedness), hypermetropia (far-sightedness), and presbyopia
• These defects can be corrected using spherical lenses

Myopia

• Myopia is also known as near-sightedness
• Nearby objects are clear, but distant objects are blurry
• The far point is nearer than infinity
• A myopic person can see clearly up to a few meters
• The distant object is formed in front of the retina, not at the retina itself
• Excessive curvature of the eye lens or elongation of the eyeball might cause this
• Using a concave lens of suitable power can correct it

Hypermetropia

• Hypermetropia is also known as far-sightedness
• Distant objects are clear, but nearby objects are not
• The near point is farther away from the normal near point of 25 cm
• Reading material is kept far beyond 25 cm for comfortable reading
• Light rays from the closeby object are focused at a point behind the retina
• Too long focal length of the eye lens or the eyeball has become too small may cause this
• Using a convex lens of appropriate power can correct this

Presbyopia

• The power of accommodation decreases with ageing
• The near point gradually recedes away
• Corrective eye-glasses are needed to see nearby objects comfortably and distinctly
• Weakening ciliary muscles and diminishing flexibility of the eye lens might cause this
• Bifocal lenses are required when a person may suffer from both myopia and hypermetropia
• Upper part of lens is concave and facilitates distant vision
• Lower part of lens is convex and facilitates near vision
• Refractive defects can be corrected with contact lenses or surgical interventions

Refraction of Light Through a Prism

• A triangular glass prism has two triangular bases and three rectangular lateral surfaces
• The angle between its two lateral faces is called the angle of the prism

Dispersion of White Light by a Glass Prism

• White light splits into a band of colors after refraction
• Violet, Indigo, Blue, Green, Yellow, Orange, and Red colors are seen on the screen
• VIBGYOR will help to remember the sequence of colors
• The band of the colored components of a light beam is called its spectrum
• Dispersion is the splitting of light into its component colors
• Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight

Atmospheric Refraction

• The air just above the fire becomes hotter than the air further up
• Hotter air has a refractive index slightly less than the cooler air
• The twinkling of stars is due to atmospheric refraction of starlight
• Planets don't twinkle becausethey are much closer to the earth, and are thus seen as extended sources
• The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction

Scattering of Light

• The blue colour of the sky, colour of water in deep sea, the reddening of the sun at sunrise and the sunset are spectacular phenomena in nature
• Tyndall Effect is the scattering of light by the colloidal particles
• When a beam of light strikes such fine particles, the path of the beam becomes visible
• The colour of clear sky is blue because The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light
• 'Danger' signal lights are red in colour, becausethe red is least scattered by fog or smoke