The Human Eye and the Colourful World - Notes

The Human Eye and the Colourful World

Introduction

The chapter discusses the human eye, its functions, defects of vision, and optical phenomena like rainbow formation, splitting of white light, and the blue color of the sky.

10.1 The Human Eye

The human eye is a valuable and sensitive sense organ that enables us to see the world and colors around us.
The human eye is like a camera, forming an image on the retina, a light-sensitive screen.
Light enters through the cornea, a thin membrane that forms a transparent bulge on the front surface of the eyeball.
The eyeball is approximately spherical, with a diameter of about 2.3 cm.
Most refraction occurs at the cornea's outer surface, while the crystalline lens provides finer focal length adjustments.
The iris, a dark muscular diaphragm behind the cornea, controls the size of the pupil.
The pupil regulates the amount of light entering the eye.
The eye lens forms an inverted real image on the retina.
Light-sensitive cells on the retina activate upon illumination and generate electrical signals.
These signals are sent to the brain via optic nerves, which interprets the signals and processes the information.

10.1.1 Power of Accommodation

The eye lens is composed of a fibrous, jelly-like material, and its curvature is modified by ciliary muscles.
Changing the curvature adjusts the focal length.
When muscles are relaxed, the lens thins, increasing focal length for clear viewing of distant objects.
When viewing closer objects, ciliary muscles contract, increasing lens curvature and decreasing focal length.
Accommodation is the eye lens's ability to adjust its focal length.
The focal length cannot decrease below a certain limit.
The least distance of distinct vision (near point) is about 25 cm for a young adult with normal vision.
The farthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for a normal eye.
A normal eye can see objects clearly between 25 cm and infinity.
Cataract: the crystalline lens becomes milky or cloudy with old age, causing vision loss. Vision can be restored through surgery.

10.2 Defects of Vision and Their Correction

The eye may gradually lose its power of accommodation, leading to blurred vision due to refractive defects.
Three common refractive defects: myopia, hypermetropia, and presbyopia.
These defects can be corrected using suitable spherical lenses.

(a) Myopia

Myopia (near-sightedness): nearby objects are seen clearly, but distant objects are not.
The far point is nearer than infinity.
The image of a distant object forms in front of the retina.
Causes: excessive curvature of the eye lens or elongation of the eyeball.
Correction: using a concave lens of suitable power to bring the image back onto the retina.

(b) Hypermetropia

Hypermetropia (far-sightedness): distant objects are seen clearly, but nearby objects are not.
The near point is farther away than the normal near point (25 cm).
Light rays from a closeby object are focused at a point behind the retina.
Causes: focal length of the eye lens is too long, or the eyeball is too small.
Correction: using a convex lens of appropriate power to provide additional focusing power to form the image on the retina.

(c) Presbyopia

Presbyopia: the power of accommodation decreases with ageing, making it difficult to see nearby objects clearly.
It arises due to weakening of the ciliary muscles and diminishing flexibility of the eye lens.
Correction: often requires bi-focal lenses (both concave and convex lenses).
The upper portion of bi-focal lenses consists of a concave lens for distant vision, and the lower part is a convex lens for near vision.
Refractive defects can also be corrected with contact lenses or surgical interventions.

Eye Donation

Eyes can be donated after death to light the life of a blind person.
Eye donors can be of any age group or sex. People who use spectacles, or those operated for cataract, can still donate the eyes.
People who are diabetic, have hypertension, asthma patients and those without communicable diseases can also donate eyes.
Eyes must be removed within 4-6 hours after death. Inform the nearest eye bank immediately.
The eye bank team will remove the eyes at the home of the deceased or at a hospital.
Eye removal takes only 10-15 minutes. It is a simple process and does not lead to any disfigurement.
Persons who were infected with or died because of AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate eyes.
One pair of eyes gives vision to up to FOUR CORNEAL BLIND PEOPLE.

10.3 Refraction of Light Through a Prism

Light refracts through a rectangular glass slab, the emergent ray is parallel to the incident ray with slight lateral displacement.
A triangular glass prism has two triangular bases and three rectangular lateral surfaces inclined to each other.
The angle between its two lateral faces is called the angle of the prism.
When light passes from air to glass, it bends toward the normal; when passing from glass to air, it bends away from the normal.
The prism's shape causes the emergent ray to bend at an angle to the incident ray. This angle is called the angle of deviation (\angle D).

10.4 Dispersion of White Light by a Glass Prism

White light from the sun can be split into various colors of the rainbow through dispersion.
The inclined refracting surfaces of a glass prism cause this phenomenon.
The prism splits white light into a band of colors: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
The band of colored components of a light beam is called its spectrum.
Dispersion is the splitting of light into its component colors.
Different colors of light bend through different angles when passing through a prism.
Red light bends the least, while violet light bends the most.
Isaac Newton used a glass prism to obtain the spectrum of sunlight and recombined the spectrum using another prism to form white light.
A rainbow is a natural spectrum appearing in the sky after a rain shower, caused by dispersion of sunlight by tiny water droplets.
Water droplets act like small prisms, refracting, dispersing, and internally reflecting sunlight.

10.5 Atmospheric Refraction

Atmospheric refraction is the refraction of light by the earth’s atmosphere. It causes phenomena like the apparent wavering of objects seen through turbulent hot air.

Twinkling of Stars

The twinkling of stars is due to atmospheric refraction of starlight.
Starlight undergoes continuous refraction as it enters the Earth's atmosphere, which has a gradually changing refractive index.
The apparent position of a star is slightly different from its actual position, appearing higher than its actual position near the horizon.
The apparent position fluctuates due to changing physical conditions in the atmosphere.
Stars are point-sized sources of light, so the amount of starlight entering the eye flickers, causing the twinkling effect.

Planets Don't Twinkle

Planets are much closer to Earth and are seen as extended sources.
The total variation in the amount of light entering our eye from all the individual point-sized sources averages out to zero, nullifying the twinkling effect.

Advance Sunrise and Delayed Sunset

The Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset due to atmospheric refraction.
The apparent flattening of the Sun’s disc at sunrise and sunset is also due to this phenomenon.

10.6 Scattering of Light

The interplay of light with objects around us gives rise to several spectacular phenomena in nature, such as the blue color of the sky and the reddening of the sun at sunrise and sunset.

10.6.1 Tyndall Effect

The Earth’s atmosphere is a heterogeneous mixture of minute particles, including smoke, tiny water droplets, suspended particles of dust, and molecules of air.
When a beam of light strikes such fine particles, the path of the beam becomes visible.
The phenomenon of scattering of light by colloidal particles is called Tyndall effect.
The color of the scattered light depends on the size of the scattering particles.
Very fine particles scatter mainly blue light, while larger particles scatter light of longer wavelengths.
If the size of the scattering particles is large enough, then the scattered light may even appear white.

10.6.2 Why is the Colour of the clear Sky Blue?

Molecules of air and other fine particles in the atmosphere have sizes smaller than the wavelength of visible light.
These particles are more effective in scattering light of shorter wavelengths (blue) than light of longer wavelengths (red).
Red light has a wavelength about 1.8 times greater than blue light.
When sunlight passes through the atmosphere, the fine particles in the air scatter the blue color more strongly than red.
The scattered blue light enters our eyes, making the sky appear blue.
If the Earth had no atmosphere, there would be no scattering, and the sky would appear dark.
The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.
Danger signal lights are red because red is least scattered by fog or smoke and can be seen at a distance.