The Human Eye and the Colourful World - Comprehensive Notes

The human eye is a valuable sense organ that enables us to see the world and colors around us.
It functions like a camera, forming an image on the retina.
Light enters through the cornea, a transparent bulge on the front 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.
The crystalline lens provides finer focal length adjustments.
The iris, a dark muscular diaphragm, controls the pupil's size.
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.
The brain interprets these signals, allowing us to perceive objects.

The eye lens, composed of fibrous, jelly-like material, can have its curvature modified by ciliary muscles.
This changes the focal length of the eye lens.
When muscles are relaxed, the lens becomes thin, increasing focal length for distant objects.
When viewing close 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 be decreased below a minimum 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, which is at infinity for a normal eye.
A normal eye can see objects clearly between $25$ cm and infinity.
Cataract: A condition where the crystalline lens becomes milky and cloudy, leading to vision loss; can be corrected with surgery.

The eye may lose its power of accommodation with age, causing blurred vision.
Three common refractive defects:
- Myopia (near-sightedness)
- Hypermetropia (far-sightedness)
- Presbyopia
These defects can be corrected with suitable spherical lenses.

Also known as near-sightedness.
Nearby objects are clear, but distant objects are not.
The far point is nearer than infinity.
The image of a distant object is formed in front of the retina.
Causes:
- Excessive curvature of the eye lens
- Elongation of the eyeball
Correction: Using a concave lens of suitable power to bring the image back onto the retina.

Also known as far-sightedness.
Distant objects are clear, but nearby objects are not.
The near point is farther away than the normal $25$ cm.
Light rays from nearby objects focus behind the retina.
Causes:
- Focal length of the eye lens is too long
- The eyeball has become too small
Correction: Using a convex lens of appropriate power to provide additional focusing power.

The power of accommodation decreases with aging.
The near point recedes away, making it difficult to see nearby objects.
Causes: Weakening of ciliary muscles and diminishing flexibility of the eye lens.
Correction: Often requires bi-focal lenses (both concave and convex lenses).
- Concave lens (upper portion): Facilitates distant vision.
- Convex lens (lower part): Facilitates near vision.
Other correction methods: Contact lenses or surgical interventions.

Eyes can be donated after death to restore sight to blind individuals.
Corneal transplantation can cure corneal blindness.
Eye donors can be of any age group or sex.
People using spectacles or those operated for cataract can donate.
Diabetics, hypertension, and asthma patients without communicable diseases can also donate.
Eyes must be removed within 4-6 hours after death.
Eye removal takes 10-15 minutes and does not cause disfigurement.
People infected with AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis, or encephalitis cannot donate.
One pair of eyes can give vision to up to FOUR corneal blind people.

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.
Activity 10.1 demonstrates the refraction of light through a prism.
$PE$ is the incident ray, $EF$ is the refracted ray, and $FS$ is the emergent ray.
At the first surface, the light ray bends towards the normal.
At the second surface, the light ray bends away from the normal.
The emergent ray bends at an angle to the direction of the incident ray (angle of deviation, $\angle D$ ).

White light is split into its component colors by a prism.
The sequence of colors is Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
The band of colored components is called the spectrum.
Dispersion is the splitting of light into its component colors.
Different colors bend through different angles; red bends the least, and violet bends the most.
Isaac Newton used a glass prism to obtain the spectrum of sunlight.
He recombined the spectrum using a second prism, demonstrating that sunlight is made up of seven colors.
Rainbow: A natural spectrum caused by dispersion of sunlight by water droplets.
Water droplets act like small prisms, refracting, dispersing, and reflecting sunlight.

Atmospheric refraction is the refraction of light by the earth’s atmosphere.
The apparent wavering or flickering of objects seen through turbulent hot air is an example.
Hotter air is less dense and has a lower refractive index.
Twinkling of stars is due to atmospheric refraction of starlight.
Starlight bends towards the normal as it enters the earth's atmosphere.
The apparent position of the star is slightly different from its actual position.
Planets do not twinkle because they are seen as extended sources, and the variations in light average out.
Advance sunrise and delayed sunset occur because of atmospheric refraction.
The Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset.
The apparent flattening of the Sun’s disc at sunrise and sunset is also due to atmospheric refraction.

The interplay of light with objects gives rise to several phenomena, such as the blue sky and red sunsets.
Tyndall Effect: The scattering of light by colloidal particles makes the path of a light beam visible.
The Earth’s atmosphere is a heterogeneous mixture of minute particles.
Scattering of light makes particles visible.
The color of scattered light depends on the size of the scattering particles.
Fine particles scatter mainly blue light, while larger particles scatter longer wavelengths.

Air molecules and fine particles are smaller than the wavelength of visible light.
These particles scatter blue light more effectively than red light.
Red light has a wavelength about $1.8$ times greater than blue light.
If the earth had no atmosphere, the sky would appear dark.
Danger signal lights are red because red light is least scattered by fog or smoke.