Human Eye and Colourful World Class 10 Notes

Detailed Notes on Human Eye and The colourful World Class 10 Physics Ch-10

Introduction to the Human Eye

The human eye is one of the most valuable and sensitive sense organs, enabling us to see the world and its colors. Unlike other senses, vision allows us to perceive colors, making it unique and essential. The human eye is a complex organ that functions like a camera, capturing light and forming images, allowing us to perceive our surroundings through sight.

Structure of the Human Eye

1. Cornea

   • The cornea is a thin, transparent membrane forming the front surface of the eye. It acts as a protective covering and starts the process of light refraction. Most of the light bending occurs at the outer surface of the cornea, allowing light to enter the eye.

2. Iris

   • The iris is the colored, muscular part of the eye located behind the cornea. It controls the size of the pupil, which in turn regulates the amount of light entering the eye. The iris adjusts the pupil size based on the light conditions, contracting in bright light and dilating in dim light.

3. Pupil

   • The pupil is the central opening in the iris. It appears black because it allows light to enter the eye, which is then absorbed by the internal parts of the eye. The pupil size changes to control the amount of light that reaches the retina.

4. Lens

   • The lens is a transparent, flexible structure located behind the pupil. It focuses light rays onto the retina by adjusting its shape with the help of ciliary muscles. This adjustment allows the eye to focus on objects at varying distances, a process known as accommodation.

5. Retina

   • The retina is the light-sensitive layer lining the back of the eyeball. It contains millions of photoreceptor cells (rods and cones) that convert light into electrical signals. These signals are transmitted to the brain via the optic nerves. The brain then interprets these signals to create a visual image.

Additional Points

• Eyeball: Approximately spherical with a diameter of about 2.3 cm.
• Image Formation: The lens forms an inverted real image on the retina.
• Optic Nerves: Carry electrical signals from the retina to the brain.
• Brain Interpretation: The brain processes these signals, allowing us to perceive objects accurately and understand the world around us.

Power of Accommodation

The human eye can focus on objects at various distances due to the ability of the lens to change shape, known as accommodation.

1. Eye Lens Composition

   • The eye lens is made of a fibrous, jelly-like material. Its curvature can be modified by ciliary muscles, allowing changes in focal length.

2. Focal Length Adjustment

   • When the ciliary muscles relax, the lens becomes thinner, increasing its focal length and allowing clear vision of distant objects.
   • To focus on nearby objects, the ciliary muscles contract, increasing the curvature and thickness of the lens, thereby decreasing its focal length.

3. Accommodation

   • Accommodation is the eye's ability to adjust its focal length to see objects at different distances clearly. This adjustment is crucial for maintaining clear vision at varying distances.

4. Least Distance of Distinct Vision

   • The least distance of distinct vision, or near point, is the minimum distance at which an object can be seen clearly without eye strain.
   • For a young adult with normal vision, the near point is typically around 25 cm.
   • If an object is closer than the near point, the eye cannot focus on it properly, resulting in a blurred image.

5. Far Point

   • The far point is the maximum distance at which an object can be seen clearly by the eye.
   • For a normal eye, the far point is considered to be at infinity.
   • A normal eye can see objects clearly between the near point (25 cm) and the far point (infinity).

6. Age-related Changes

   • As people age, the flexibility of the lens decreases, reducing the ability to focus on close objects. This condition is known as presbyopia.
   • Additionally, cataracts can develop, causing the lens to become cloudy and impairing vision. Cataract surgery can restore vision.

7. Cataract

   • A cataract is a condition where the crystalline lens becomes milky and cloudy, leading to partial or complete loss of vision. This condition can often be corrected through surgery.

Defects of Vision and Their Correction

Sometimes, the eye may gradually lose its power of accommodation. When this happens, a person cannot see objects distinctly and comfortably. The vision becomes blurred due to refractive defects of the eye. There are three common refractive defects of vision: myopia, hypermetropia, and presbyopia. These defects can be corrected by the use of suitable spherical lenses.

Myopia (Nearsightedness)

Myopia is a refractive error of the eye that affects the ability to see distant objects clearly.

• Symptoms: A person with myopia can see nearby objects clearly but has difficulty focusing on distant objects, which appear blurred.

• Causes: This defect arises due to either an excessive curvature of the eye lens or elongation of the eyeball.

• Mechanism: In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.

• Far Point: A person with myopia has a far point nearer than infinity. Such a person may see clearly up to a distance of a few meters.

Correction of Myopia:

Myopia can be corrected by using concave lenses. These lenses diverge incoming light rays, allowing the image to form on the retina.

Hypermetropia (Farsightedness)

Hypermetropia (also known as farsightedness) is a refractive error where distant objects can be seen more clearly than nearby ones.

• Symptoms: A person with hypermetropia can see distant objects clearly but has difficulty focusing on nearby objects, which appear blurred.

• Causes: This defect arises either because the focal length of the eye lens is too long, or the eyeball is too small.

• Mechanism: In a hypermetropic eye, light rays from a close object are focused at a point behind the retina.

• Near Point: The near point for a person with hypermetropia is farther away from the normal near point (25 cm).

Correction of Hypermetropia:

Hypermetropia can be corrected by using convex lenses. These lenses converge incoming light rays, enabling the eye to focus the image on the retina.

Presbyopia

Presbyopia is an age-related condition where the power of accommodation of the eye decreases over time.

• Symptoms: Most people with presbyopia find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses.

• Causes: This condition arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.

• Mechanism: The near point gradually recedes away, making it harder to focus on nearby objects.

Correction of Presbyopia:

• For people suffering from both myopia and hypermetropia, bifocal lenses are often required. Bifocal lenses consist of both concave and convex lenses: the upper part for distant vision and the lower part for near vision.

• Nowadays, contact lenses or surgical interventions can also correct these defects.

Refraction of Light Through a Prism

A prism is a transparent optical object with flat, polished surfaces that refract light. The most common type of prism is a triangular prism, which has a triangular base and rectangular sides. When light passes through a prism, it undergoes refraction, bending, and dispersion.

Process of Refraction in a Prism

1. Incidence of Light

A beam of white light is incident on one of the prism's surfaces at an angle to the normal, marked as ∠i (the angle of incidence).

2. Refraction at the First Surface

As the light enters the prism, it slows down and bends towards the normal due to the change in medium from air to glass (or another transparent material). This is the first refraction, and the light ray now travels inside the prism. The refracted ray inside the prism is denoted as EF.

3. Traveling Through the Prism

The light travels through the prism, and its path may be affected by the angles and shape of the prism. The shape of the prism ensures that the light ray does not travel parallel to the incident ray but slightly displaced laterally.

4. Refraction at the Second Surface

As the light exits the prism, it speeds up and bends away from the normal, moving from the glass back into the air. This is the second refraction, and the emergent ray is denoted as FS. The angle at which it emerges is called the angle of emergence (∠e).

5. Angle of Deviation

The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation, marked as ∠D in Figure 10.4. This deviation happens because the light changes its direction twice, once at each surface of the prism.

Comparing Glass Slab and Prism

• In a glass slab, the emergent ray is parallel to the incident ray but displaced laterally.

• In a prism, the emergent ray bends at an angle to the incident ray due to the triangular shape, causing a deviation.

Creating a Spectrum

A prism not only bends light but also disperses it. This means it separates white light into its constituent colours – red, orange, yellow, green, blue, indigo, and violet – creating a spectrum.

This phenomenon occurs because different colours of light bend by different amounts (have different refractive indices) as they pass through the prism. Red light bends the least, and violet bends the most.

Dispersion of Light Through Prism

Dispersion of light occurs when a beam of white light passes through a medium, such as a glass prism, and is separated into its constituent colours. This separation happens because different colours of light travel at different speeds in the medium. Let's explore this phenomenon in detail.

Steps of Dispersion

1. Entering the Prism: When a beam of white light enters a dispersive medium like a prism, it consists of various colours, each with a different wavelength.

2. Slowing Down: As the light enters the medium, each colour slows down by a different amount. Shorter wavelengths (such as violet) slow down more than longer wavelengths (such as red).

3. Refraction: This difference in speed causes each color to be refracted by a different angle, leading to the separation of the colors.

4. Exiting the Medium: As the light exits the medium, the colors spread out further, forming a spectrum of colors ranging from violet to red.

5. Spectrum Formation: The various colors seen are Violet, Indigo, Blue, Green, Yellow, Orange, and Red, remembered by the acronym VIBGYOR.

6. Definition of Spectrum: The band of the colored components of a light beam is called its spectrum.

Refraction of Light Through a Prism mind map

Rainbow Formation

Rainbows in the sky are caused by the dispersion of sunlight by tiny water droplets in the atmosphere after a rain shower. The water droplets act like miniature prisms - as sunlight enters a droplet, it is refracted and dispersed into its component colours.

The light is then reflected off the back of the droplet and refracted again as it exits, separating the colours even more. To the observer, this creates the appearance of a colourful arc in the sky, with red on the top and violet on the bottom.

Table given below shows how rainbows are formed.

Atmospheric Refraction

Atmospheric Refraction is the refraction of light by atmosphere.

Twinkling of stars

The mechanism of twinkling is given below

1. As starlight passes through the Earth's atmosphere, it is refracted multiple times due to the varying refractive indices of the different atmospheric layers.
2. The Earth's atmosphere is turbulent, with constant motion and mixing of air masses. This turbulence causes rapid and random changes in the refractive index along the path of the starlight.
3. The continuous changes in the refractive index cause the path of the starlight to bend and shift rapidly. This leads to variations in the apparent brightness and position of the star as seen from the Earth's surface.
4. Stars are so far away that they appear as point sources of light. Even slight changes in the path of their light can lead to noticeable fluctuations in brightness, resulting in the twinkling effect.

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