The Human Eye and the Colourful World-Notes

The human eye is a spherical, complex organ designed to capture light and convert it into images that the brain can interpret.
Its overall diameter is about 2.3 cm, and it is filled with fluids that maintain its shape and support its functions

Parts of the Eye

• Sclera:
  This is the tough, white outer layer of the eye. It protects the internal parts and helps maintain the shape of the eyeball.
• Cornea:
  At the front of the eye, the cornea is a transparent, dome-shaped surface that covers the iris and pupil. It acts as the primary lens, bending (refracting) incoming light rays to direct them inward
• Iris:
  The iris is the colored part of the eye surrounding the pupil. It consists of muscles that control the size of the pupil. By contracting or expanding, the iris regulates how much light enters the eye to protect the inner structures and optimise vision.
• Pupil:
  The pupil is the opening in the center of the iris. It appears black because it is a hole through which light passes into the inner eye. The pupil size changes dynamically depending on light intensity, controlled by the iris muscles.
• Lens:
  Located just behind the pupil, the lens is a transparent, flexible, biconvex structure. It fine-tunes the focus of light onto the retina by changing shape through the action of ciliary muscles — thicker to focus on nearby objects and thinner for distant ones (accommodation).
• Retina:
  The retina is the innermost, light-sensitive layer lining the back of the eye. It houses two types of photoreceptors: rods and cones. Rods detect low light levels and help with peripheral and night vision. Cones detect colours and fine details in bright light. The retina converts captured light into electrical impulses.
• Optic Nerve:
  The optic nerve transmits these electrical signals from the retina to the brain, where they are processed into visual images.
• Aqueous Humour:
  This clear, watery fluid fills the space between the cornea and the lens. It nourishes these parts and maintains intraocular pressure.
• Vitreous Humour:
  Filling the large cavity between the lens and retina, this gel-like substance helps maintain the eye's spherical shape and cushions internal structures.
• Blind Spot:
  This is the point on the retina where the optic nerve fibres exit the eye; it lacks photoreceptor cells, so no image detection occurs here.
• Extraocular Muscles:
  Six muscles attach the eye to the skull and control its movements for directing gaze in different directions.

Together, these components allow the eye to capture light, focus it on the retina, convert it into nerve signals, and send these signals to the brain. The dynamic adjustment of the pupil and focusing lens enables clear vision across different lighting conditions and distances. This coordinated function of structures enables the rich and detailed sense of sight humans experience.

The power of accommodation is the ability of the human eye to change the shape of its lens so that it can focus on objects at different distances.
When we look at something close, the lens becomes thicker to focus the image clearly on the retina.
This change in lens shape helps us see clearly whether an object is near or far.
The process involves the ciliary muscles adjusting the lens curvature to change its focal length. This flexible focusing ability of the eye is called the power of accommodation.

The least distance of distinct vision

The least distance of distinct vision is the closest distance from the eye at which an object can be seen clearly and comfortably without causing any strain to the eye.
For a normal human eye, this distance is about 25 centimeters.
Objects placed closer than this distance appear blurry because the eye lens cannot adjust enough to focus the image sharply on the retina. This distance is also called the near point of the eye. As a person grows older, the least distance of distinct vision tends to increase because the flexibility of the eye lens reduces, making it harder to focus on very close objects.

The far point

The far point of the eye is the farthest distance at which an object can be seen clearly and distinctly without the eye having to make any effort to focus.
For a normal human eye, this far point is at infinity, meaning the eye can see distant objects clearly without strain.
When objects are beyond this point, the eye lens does not need to change shape to focus the image on the retina because the incoming light rays are almost parallel.

Cataract

A cataract is a condition in the human eye where the normally clear lens becomes cloudy or opaque.
This cloudiness results in blurred or dim vision as it prevents light from passing clearly through the lens to the retina.
Cataracts commonly develop due to ageing but may also be caused by factors such as injury, prolonged exposure to sunlight, certain diseases like diabetes, or genetic factors.
The main symptom is a gradual loss of vision, often accompanied by difficulty seeing at night or sensitivity to bright lights.
Cataract cannot be improved with glasses or medicines; the effective treatment is surgical removal of the cloudy lens, which is then replaced with a clear artificial lens. This restores vision effectively.
Cataract is a common cause of visual impairment and blindness, especially among older individuals.

Defects of vision occur when the eye is unable to focus images properly on the retina, causing blurred or unclear vision either for near objects, distant objects, or both. These defects arise due to changes in the shape of the eyeball, the flexibility of the lens, or the eye’s focusing ability. The main defects of vision are:

Myopia (Near-sightedness)

Myopia (Near-sightedness): A person with this condition can see nearby objects clearly, but distant objects appear blurry.
This happens because the image is focused in front of the retina instead of directly on it, often due to an elongated eyeball or an excessively curved lens.

Hypermetropia (Far-sightedness)

Hypermetropia (Far-sightedness): Here, distant objects are seen clearly, but nearby objects appear blurred.
The image forms behind the retina due to a shortened eyeball or a less curved lens.

Presbyopia

Presbyopia: This age-related condition occurs when the eye loses its ability to adjust the lens shape due to weakening of the ciliary muscles and loss of lens elasticity.
It results in difficulty seeing nearby objects clearly. Presbyopia is corrected with convex lenses or bifocal glasses, which helps focus light on the retina.

Refraction of light through a prism occurs when a ray of light passes from one medium into another medium with a different optical density, causing the light to change direction.

A glass prism typically has two triangular bases and three rectangular lateral surfaces inclined at an angle called the angle of the prism.
When a ray of light enters the prism from air (a rarer medium) to glass (a denser medium), it bends towards the normal at the first surface due to slowing down.
This refracted ray inside the prism travels to the second surface, where it moves from glass to air. Here, it bends away from the normal because it speeds up, emerging out of the prism.
The path of the ray is thus deviated from its original direction by an angle called the angle of deviation.

Note:

• The incident ray is the incoming light ray striking the prism.
• The refracted ray is the ray inside the prism after bending towards the normal.
• The emergent ray is the ray leaving the prism, bending away from the normal.
• The angle of incidence is the angle between the incident ray and the normal to the surface.
• The angle of emergence is the angle between the emergent ray and the normal at the second surface.
• The angle of deviation is the angle between the incident and the emergent rays.

Because the two refracting surfaces of a prism are inclined (not parallel), the emergent ray deviates from the original path. The deviation angle depends on the angle of incidence and the refractive index of the prism.

Dispersion of white light by a glass prism is the phenomenon where white light splits into its constituent colours when it passes through the prism. White light is composed of multiple colours, each having a different wavelength. When this light enters the glass prism, it refracts —bends—due to the change in medium from air to glass. However, because each colour of light travels at a different speed inside the prism, they bend by different amounts. This variation in bending is due to the differing refractive indices for each colour in the glass.

Violet light, which has the shortest wavelength, bends the most, while red light, with the longest wavelength, bends the least. As a result, the white light spreads out into a continuous spectrum of seven colours— violet, indigo, blue, green, yellow, orange, and red—often remembered by the acronym VIBGYOR. This entire process is called dispersion.

When the separated colours emerge from the second surface of the prism, they are refracted again, which further increases the separation between them. Hence, the prism not only refracts the light twice— once upon entry and once upon exit—but also causes the spatial separation of the colours, forming a visible spectrum.

This behaviour of light through a prism was first studied by Isaac Newton, who showed that white light is a mixture of different colours.
Dispersion is fundamental in understanding optical phenomena like rainbows and is used in devices like spectrometers to analyse light composition.

Atmospheric refraction is the bending of light as it travels through Earth’s atmosphere, which is composed of layers of air with varying densities and refractive indices.
Because air near the Earth’s surface is denser (optically denser) than air higher up, light rays coming from celestial objects like stars or the Sun bend continuously as they pass through these layers.

Twinkling of Stars

Stars appear to twinkle because their light passes through varying atmospheric layers with constantly changing optical density.
This causes the apparent brightness and position of stars to fluctuate as the light refracts differently at different moments.

Apparent Position of Stars

Stars appear slightly higher than their actual position in the sky due to the bending of their light towards the Earth’s surface by the atmosphere.

Advanced Sunrise and Delayed Sunset:

Because of atmospheric refraction, the Sun is visible to us even before it actually rises above the horizon and remains visible slightly after it has set. The light bends downward, making the Sun appear earlier at sunrise and stay longer at sunset.

Flattening of the Sun’s Disc:

The Sun appears slightly flattened near the horizon because its lower edge is refracted more than the upper edge, due to passing through more atmosphere.

These phenomena occur because different layers of air act like a medium of gradually varying refractive indices, causing light to follow a curved path rather than a straight line. This bending of light by Earth’s atmosphere is called atmospheric refraction.

Scattering of light is the phenomenon where light rays deviate from their straight path due to particles or irregularities in the medium through which the light travels. When light encounters particles or molecules in a medium such as air, water, or dust, portions of the light are absorbed and then re-radiated in different directions.
This process depends on the size of the particles and the wavelength of light, with shorter wavelengths (like blue) scatter more than longer wavelengths (like red)

Types of Scattering

• Rayleigh scattering:
  It occurs with particles much smaller than the wavelength of light and explains why the sky is blue due to more scattering of shorter blue wavelengths.
• Mie scattering:
  involves larger particles similar in size to the wavelength of light and explains phenomena such as the white appearance of clouds.
• Tyndall effect:
  It describes light scattering by colloidal particles, which makes visible the path of a beam of light in certain solutions, like milk in water.