Human Eye And Colourful World Class 10 Notes Science Chapter 10

Welcome to the fascinating world of optics and the incredible marvel that is the human eye! In the Science Class 10 curriculum, Chapter 10 “Human Eye and Colourful World” takes us on a captivating journey, unraveling the secrets of how we perceive the world around us through our eyes. From understanding the intricate workings of the human eye to exploring the mesmerizing phenomena of light and color, this chapter holds the key to unlocking the wonders of our visual perception. So, let’s embark on this enlightening adventure and delve into the captivating realm of the “Human Eye and Colourful World.”

Human Eye and Colourful World Class 10 Notes Science Chapter 10

The Human Eye

The human eye is an invaluable and intricate sense organ that enables us to perceive the world around us.
The eye has an approximately spherical shape with a diameter of approximately 2.3 cm.
Functioning like a camera, it possesses a lens system that projects an image onto the retina, a light-sensitive screen at the back of the eye.
The retina is composed of two types of light-sensitive cells: rods and cones. Rods are highly sensitive to light and are responsible for night vision, while cones facilitate colour vision.
The lens is made of a transparent and flexible material, allowing it to adjust its shape to focus light from objects at varying distances.
Light enters the eye through the cornea, a thin, transparent membrane that forms the bulging surface at the front of the eyeball.
Behind the cornea, the pupil, a dark muscular diaphragm, regulates the amount of light entering the eye.
The lens projects an inverted real image of the object onto the retina.
The retina is a delicate membrane with an extensive array of light-sensitive cells.
When exposed to light, these light-sensitive cells activate and generate electrical signals.
These signals are transmitted to the brain through the optic nerves, a bundle of nerve fibres carrying information from the retina.
The brain processes and interprets the signals from the retina, resulting in the creation of the images that we see.

Power of Accommodation

The eye lens is composed of a flexible, jelly-like material that can change its curvature through the action of the ciliary muscles.
When the ciliary muscles contract, the curvature of the eye lens increases, resulting in a shorter focal length. This enables us to see nearby objects clearly.
The ability of the eye lens to adjust its focal length is known as accommodation.
The near point of the eye refers to the minimum distance at which objects can be seen most distinctly without straining the eyes. For a young adult with normal vision, the near point is typically around 25 cm.
In contrast, the far point of the eye is at infinity for a normal eye. It represents the farthest distance at which the eye can see objects clearly.
Unfortunately, with age, the crystalline lens can become milky and cloudy, leading to a condition known as cataract.
Cataracts are caused by the clouding of proteins in the lens.
To treat cataracts, a common approach is cataract surgery. This procedure involves the removal of the cloudy lens and its replacement with a clear artificial lens.
Cataract surgery has proven to be effective in restoring vision, allowing individuals to see clearly once again.

Defects Of Vision And Their Correction

Myopia

Myopia, commonly known as near-sightedness, is a visual condition where a person can see nearby objects clearly but struggles to see distant objects distinctly.
The far point of a myopic eye is closer than infinity, meaning that the eye can focus on nearby objects without difficulty, but distant objects appear blurred.
In a myopic eye, the image of a distant object is formed in front of the retina rather than directly on it.
This condition can be effectively corrected using a concave lens with an appropriate power.
The power of a concave lens is expressed as a negative value, which helps to diverge light rays before they enter the eye, compensating for the excess focusing power of the myopic eye.
The degree of negative power required to correct myopia depends on the distance of the far point from the eye.
Individuals with myopia may experience eyestrain and headaches when attempting to see distant objects due to the eye’s natural tendency to focus in front of the retina.
Myopia can be caused by a combination of factors, including genetic predisposition, environmental factors, and imbalances in the eye muscles.
Regular eye check-ups and appropriate corrective lenses can effectively manage myopia and improve visual clarity for individuals with this condition.

Hypermetropia

Hypermetropia, also known as far-sightedness, is a visual condition where a person can see distant objects clearly but has difficulty focusing on nearby objects.
The near point of a hypermetropic eye is farther away than the normal near point, which is typically around 25 cm.
In a hypermetropic eye, the image of a nearby object is formed behind the retina rather than directly on it.
To correct this condition, a convex lens with the appropriate power is used.
The power of a convex lens is expressed as a positive value. It converges light rays before they reach the eye, compensating for the insufficient focusing power of the hypermetropic eye.
The level of positive power required to correct hypermetropia depends on the distance of the near point from the eye.
People with hypermetropia may experience eyestrain and headaches when attempting to see nearby objects due to the eye’s challenge in focusing.
Hypermetropia can be caused by various factors, including genetic predisposition, environmental influences, and imbalances in the eye muscles.
Regular eye examinations and the appropriate use of corrective lenses can effectively manage hypermetropia, providing clearer vision for individuals with this condition.

Presbyopia

Presbyopia is a vision condition that typically develops in later life, resulting from the loss of the eye lens’ ability to change shape.
This loss of flexibility in the lens makes it challenging to see both near and far objects clearly.
The gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens are the primary causes of presbyopia.
To address presbyopia, bifocal lenses are a common type of corrective eyewear that allows people to see both near and far objects with clarity.
Bifocal lenses incorporate two different powers of magnification, one for distance vision and another for near vision, conveniently combined in a single lens.
Contact lenses also offer a viable option for correcting presbyopia. They come in various designs to accommodate different visual needs.
In addition to corrective lenses, surgical interventions such as LASIK and cataract surgery are available for managing presbyopia.
LASIK surgery reshapes the cornea to improve overall vision, while cataract surgery involves replacing the clouded natural lens with a clear artificial lens.
The choice of correction method for presbyopia depends on individual preferences, lifestyle, and eye health, so it’s essential to consult an eye care professional to determine the most suitable option.

Refraction Of Light Through A Glass Prism

When a ray of light passes through a triangular glass prism, it undergoes two refractions – once at each of the two lateral faces.
The angle of refraction at each lateral face is dependent on both the angle of incidence and the refractive index of the glass.
The resulting angle of deviation refers to the angle between the incident ray and the emergent ray after passing through the prism.
This angle of deviation is influenced by the angle of incidence, the angle of the prism itself, and the refractive index of the glass.
The refractive index of a material is a fundamental property that indicates how much light is bent or refracted when it traverses the material.
The angle of incidence signifies the angle between the incident ray and the normal to the surface at the point of incidence.
The normal, in this context, is an imaginary line that stands perpendicular to the surface at the point of incidence.
Additionally, the angle of refraction pertains to the angle between the refracted ray and the normal to the surface at the same point of incidence.

Refraction of light through a triangular glass prism

Procedure:

Secure a sheet of white paper on a drawing board using drawing pins.
Place a glass prism on the paper, ensuring it rests on its triangular base.
Trace the outline of the prism accurately using a pencil.
Draw a straight line PE inclined to one of the refracting surfaces (e.g., AB) of the prism.
Fix two pins, P and Q, on the line PE as shown in Figure 10.4.
Observe and locate the images of the pins fixed at P and Q when seen through the opposite face AC.
Secure two more pins, R and S, such that the pins at R and S, and the images of the pins at P and Q, align in a straight line.
Remove all the pins and the glass prism from the paper.
The line PE intersects the boundary of the prism at point E. Similarly, extend the lines through points R and S until they meet the boundary at points E and F, respectively. Join points E and F.
Draw perpendicular lines to the refracting surfaces AB and AC of the prism at points E and F, respectively.
Mark the angle of incidence (∠i), the angle of refraction (∠r), and the angle of emergence (∠e) for each respective point (P, E, and F) on the diagram.

By following this procedure, you can observe and study the refraction of light through a triangular glass prism and measure the corresponding angles of incidence, refraction, and emergence.

Observations:

A ray of light, represented by the incident ray EF, enters the triangular glass prism and undergoes refraction at the first surface AB, emerging as ray FS at the second surface AC.
The angle of deviation, labelled as ∠PEFS, is the angle formed between the incident ray PE and the emergent ray FS.
The magnitude of the angle of deviation depends on the angle of incidence, the angle of the prism, and the refractive index of the glass.

Conclusion:

In conclusion, when a ray of light passes through a triangular glass prism, it experiences refraction and deviates from its original path. The angle of deviation (∠PEFS) is determined by three main factors: the angle of incidence (∠PEA), which is the angle between the incident ray and the normal to the first surface AB, the angle of the prism (∠A), and the refractive index of the glass. This phenomenon plays a crucial role in various optical applications and is essential to understand light’s behaviour in different media.

Dispersion Of White Light By A Glass Prism

When white light passes through a prism, it undergoes dispersion and separates into a beautiful band of colours known as a spectrum.
The colours present in the spectrum, in order of increasing wavelength, are violet, indigo, blue, green, yellow, orange, and red.
To remember the sequence of colours in the spectrum, the acronym VIBGYOR is often used.
Each colour of light bends at a different angle as it passes through the prism due to its unique wavelength.
The wavelength of light plays a critical role in determining the degree of bending when passing through a prism.
Violet light has the shortest wavelength, while red light possesses the longest wavelength among visible light.
Consequently, red light bends the least, whereas violet light bends the most.
This phenomenon positions violet light at the top of the spectrum and red light at the bottom.
The spectrum of light forms a continuous band of colours, displaying the full range of visible wavelengths.
However, the human eye can perceive only around 100 distinct colours, despite the continuous nature of the spectrum.
The splitting of light into its component colours through the prism is referred to as dispersion, showcasing the remarkable properties of light and its interaction with different mediums.

Experiment On White Light By Isaac Newton

Isaac Newton is credited with being the first to employ a glass prism to reveal the spectrum of sunlight. By placing a prism in the path of a beam of sunlight, he made a remarkable observation—the light was dispersed into a beautiful band of colours.
Eager to explore further, Newton attempted to split the colours of the spectrum even more using a second prism of the same kind. Surprisingly, he failed to achieve any additional colours.
Curious and determined, he decided to place a second identical prism in an inverted position relative to the first one. This arrangement allowed all the colours of the spectrum to pass through the second prism, and he noticed a beam of white light emerging from the other side.
This revelation sparked a crucial realisation in Newton’s mind—that sunlight is, in fact, composed of seven distinct colours. He bestowed names upon these colours, calling them: violet, indigo, blue, green, yellow, orange, and red.
Consequently, any light that produces a spectrum similar to that of sunlight is commonly referred to as white light.

Newton’s pioneering experiments with prisms significantly advanced our understanding of light and laid the foundation for further studies in the field of optics. His findings continue to be a cornerstone in our comprehension of the properties of light and colour.

Rainbow

A rainbow is a magnificent sight that appears in the sky after it rains.
It happens because sunlight is split into colours when it passes through tiny water droplets in the air.
These droplets work like little prisms, bending and spreading the sunlight, reflecting it inside, and bending it again when it comes out.
This creates a variety of colours that we can see.
The colours in a rainbow are arranged in a special order, with violet at the top and red at the bottom.
Each colour is caused by a different size of light waves.
Violet light has the smallest waves, and red light has the biggest waves.
That’s why violet light is at the top, and red light is at the bottom.
The angle of a rainbow is always about 42 degrees to the line between you and the sun.
You can also see a rainbow when you look at the sky through a waterfall or a water fountain, with the sun behind you, on sunny days.
To remember the order of colours in a rainbow, you can use the acronym VIBGYOR.
The rainbow is a natural wonder that people all over the world can appreciate for its beauty.

Atmospheric Refraction

Atmospheric refraction is a phenomenon where light bends as it passes through the Earth’s atmosphere.
This bending occurs because the air at different heights has different densities.
Hotter air is less dense than cooler air, which results in a lower refractive index for the hotter air.
As a result, light tends to bend towards the hotter air.
This bending can make objects appear to waver or twinkle, and it’s the reason why stars appear to twinkle.
The amount of atmospheric refraction depends on the wavelength of light.
Shorter wavelengths, like blue light, are bent more than longer wavelengths, such as red light.
Hence, stars may appear bluer than they actually are due to this bending.
Atmospheric refraction can also create mirages, which are optical illusions caused by the bending of light in the atmosphere.
In deserts, mirages can be seen because the hot air near the ground has a lower refractive index than the cooler air above it, causing light to bend and create the illusion of water or objects that aren’t really there.

Twinkling Of Stars

The twinkling of stars is caused by atmospheric refraction.
The twinkling effect is more pronounced when stars are near the horizon.
This is because the atmosphere is more turbulent near the ground, leading to more variable refraction.
When stars are near the horizon, their light has to travel through more atmosphere to reach our eyes, intensifying the twinkling effect.
The atmospheric refraction occurs in a medium with a gradually changing refractive index, causing the apparent position of the star to fluctuate.
Since stars are incredibly distant, they appear as point-sized sources of light.
As a result, the amount of starlight entering our eyes flickers, causing the stars to appear to brighten and fade.
In contrast, planets are much closer to Earth and are seen as extended sources of light.
This means that the total variation in the amount of light from all the individual point-sized sources (like stars) averages out to zero, nullifying the twinkling effect on planets.

Advance Sunrise And Delayed Sunset

Due to atmospheric refraction, we can see the Sun approximately 2 minutes before the actual sunrise and about 2 minutes after the actual sunset.
Atmospheric refraction refers to the bending of light as it passes through the Earth’s atmosphere.
This bending causes the Sun to appear higher in the sky than it actually is.
As a result, we can observe the Sun before it has fully risen above the horizon and after it has already set below the horizon.
Additionally, atmospheric refraction contributes to the apparent flattening of the Sun’s disc at sunrise and sunset.
The bending of light by the atmosphere makes the Sun’s disc appear flattened at its edges during these times.

Scattering Of Light

Scattering of light is a fascinating phenomenon where light rays are redirected by particles that are much smaller in size.
When light passes through a true solution, its path remains invisible to our eyes.
However, when light travels through a colloidal solution, which contains relatively larger particles, its path becomes visible.
The scattering of light by these colloidal particles is known as the Tyndall effect, named after the Irish physicist John Tyndall who discovered it in 1869.
The Tyndall effect plays a significant role in the blue colour of the sky.
Blue light, having a shorter wavelength, is scattered more than other colours, which is why we often see a blue sky.
Additionally, the Tyndall effect is responsible for the reddening of the sun during sunrise and sunset.
At these times, sunlight has to travel through more of the atmosphere, and the blue light is scattered away by particles in the atmosphere, causing us to see a red sun.
The Tyndall effect provides us with beautiful blue skies and captivating sunrises and sunsets, enriching our experience of the natural world around us.

Tyndall Effect

The Tyndall effect is a captivating phenomenon where light gets scattered by colloidal particles.
When a beam of light passes through a colloid, its path becomes visible due to the scattering of light in all directions by the colloidal particles.
This effect is named after John Tyndall, an Irish physicist who discovered it in 1869.
The Tyndall effect finds application in various fields, including the study of colloids, detecting smoke and dust particles, and purifying water.
The colour of the scattered light depends on the size of the scattering particles.
Very fine particles primarily scatter blue light, while larger particles scatter light of longer wavelengths.
In some cases, when the scattering particles are large enough, the scattered light may even appear white.
The Tyndall effect provides valuable insights into colloidal systems and proves useful in multiple practical applications, enriching our understanding of light and matter interactions.

The Colour Of The Clear Sky Is Blue

The blue colour of the sky is a result of Rayleigh scattering.
Rayleigh scattering is the phenomenon where light is scattered by particles much smaller than the wavelength of light.
The molecules of air and fine particles in the atmosphere are indeed much smaller than the wavelength of visible light.
These particles have a stronger scattering effect on blue light compared to red light.
The reason is that blue light has a shorter wavelength than red light.
As a result of this scattering, blue light is scattered in different directions and enters our eyes, creating the perception of a blue sky.
At very high altitudes, the scattering effect is not as prominent, which is why the sky appears dark to passengers flying at such heights.
Danger signal lights are red in colour because red light is least scattered by fog or smoke, making it more visible and useful in such conditions.
Understanding Rayleigh scattering helps us appreciate the beauty of the blue sky and the practicality of using red lights in certain situations for better visibility.

As we conclude our exploration of Chapter 10 “Human Eye and Colourful World,” we are left awe-inspired by the incredible abilities of the human eye and the enchanting world of colors that surrounds us. From understanding how light bends and scatters, giving rise to the vibrant hues of the rainbow, to comprehending the complexities of vision and optical illusions, we have gained a profound appreciation for the miracles of nature and the brilliance of science. The knowledge we have acquired here will forever shape our understanding of vision, optics, and the captivating interplay between light and color.

Remember, the human eye is not just a remarkable organ; it is a gateway to experiencing the beauty and diversity of the world. Let us cherish our vision, protect our eyes, and continue to explore the wonders of the “Human Eye and Colourful World” in all its splendor!