Human Eye and the Colourful World Class 10 ||Science|| Chapter 10 Notes
Introduction
The human eye is one of the most important sensory organs, allowing us to perceive the world around us. In this chapter, we will study the structure of the eye, how it helps in vision, and various phenomena such as the dispersion of light and the formation of rainbows. We will also discuss defects of vision and how they can be corrected.
1. Structure of the Human Eye
The human eye functions like a camera, enabling us to see the objects around us. Here are the major parts of the eye and their functions:
Cornea:
The transparent, outermost part of the eye. It allows light to enter the eye and helps in focusing the incoming light.Iris:
The colored part of the eye that controls the size of the pupil and thus regulates the amount of light entering the eye.Pupil:
The small, central opening in the iris that controls the amount of light entering the eye.Lens:
A transparent, convex structure that focuses light rays on the retina. The lens changes its shape to focus on near and far objects (a process called accommodation).Retina:
The innermost layer at the back of the eye where the image is formed. It contains two types of light-sensitive cells: rods (for dim light) and cones (for bright light and color).Optic Nerve:
Carries the visual information from the retina to the brain for processing.Ciliary Muscles:
These muscles help change the shape of the lens during accommodation.
2. Functioning of the Eye
Refraction of Light by the Eye:
When light enters the eye through the cornea, it is refracted, focused by the lens, and an image is formed on the retina. The image formed is real, inverted, and smaller than the object.
Accommodation of the Eye:
The lens changes its focal length to focus on both nearby and distant objects. When focusing on near objects, the lens becomes thicker, and when focusing on distant objects, it becomes thinner.
Refraction of Light by the Eye:
When light enters the eye through the cornea, it is refracted, focused by the lens, and an image is formed on the retina. The image formed is real, inverted, and smaller than the object.
Accommodation of the Eye:
The lens changes its focal length to focus on both nearby and distant objects. When focusing on near objects, the lens becomes thicker, and when focusing on distant objects, it becomes thinner.
3. Defects of Vision
There are three common defects of vision:
Myopia (Nearsightedness):
- A person with myopia can see nearby objects clearly, but distant objects appear blurred.
- Cause: The image is formed in front of the retina because the eyeball is elongated or the lens is too curved.
- Correction: A concave (diverging) lens is used to correct myopia, which helps focus the image on the retina.
Hypermetropia (Farsightedness):
- A person with hypermetropia can see distant objects clearly, but nearby objects appear blurred.
- Cause: The image is formed behind the retina because the eyeball is too short or the lens is too flat.
- Correction: A convex (converging) lens is used to correct hypermetropia, which helps focus the image on the retina.
Presbyopia:
- This is an age-related condition where the eye loses its ability to focus on nearby objects.
- Cause: The lens becomes less flexible with age, affecting its ability to accommodate.
- Correction: Bifocal lenses are used, which have both concave and convex sections to correct for both near and distant vision.
4. Refraction of Light Through a Prism
When light passes through a glass prism, it bends or refracts. The light splits into its constituent colors (Violet, Indigo, Blue, Green, Yellow, Orange, Red), forming a spectrum. This phenomenon is called dispersion of light.
Cause of Dispersion:
Different colors of light bend by different amounts when passing through the prism due to their varying wavelengths. Violet bends the most, and red bends the least.
When light passes through a glass prism, it bends or refracts. The light splits into its constituent colors (Violet, Indigo, Blue, Green, Yellow, Orange, Red), forming a spectrum. This phenomenon is called dispersion of light.
Cause of Dispersion:
Different colors of light bend by different amounts when passing through the prism due to their varying wavelengths. Violet bends the most, and red bends the least.
5. Atmospheric Refraction
Twinkling of Stars:
Stars appear to twinkle due to atmospheric refraction. As starlight passes through the Earth’s atmosphere, it gets refracted due to temperature variations and changing densities of air. This causes stars to appear to twinkle.
Apparent Position of Stars:
The atmosphere bends light from stars, making them appear higher than their actual position.
Twinkling of Stars:
Stars appear to twinkle due to atmospheric refraction. As starlight passes through the Earth’s atmosphere, it gets refracted due to temperature variations and changing densities of air. This causes stars to appear to twinkle.
Apparent Position of Stars:
The atmosphere bends light from stars, making them appear higher than their actual position.
6. Scattering of Light
Tyndall Effect:
The scattering of light by particles in a colloid or fine suspension is known as the Tyndall effect. This explains why the sky appears blue and why the Sun appears reddish at sunrise and sunset.
Why is the Sky Blue?:
Blue light has a shorter wavelength and is scattered more by the atmosphere’s molecules compared to other colors. Hence, the sky appears blue to us.
Why does the Sun Appear Red at Sunrise and Sunset?:
During sunrise and sunset, sunlight has to pass through a larger thickness of the Earth’s atmosphere. Most of the blue light is scattered away, and the red light, which has the longest wavelength, reaches our eyes, making the Sun appear reddish.
Tyndall Effect:
The scattering of light by particles in a colloid or fine suspension is known as the Tyndall effect. This explains why the sky appears blue and why the Sun appears reddish at sunrise and sunset.
Why is the Sky Blue?:
Blue light has a shorter wavelength and is scattered more by the atmosphere’s molecules compared to other colors. Hence, the sky appears blue to us.
Why does the Sun Appear Red at Sunrise and Sunset?:
During sunrise and sunset, sunlight has to pass through a larger thickness of the Earth’s atmosphere. Most of the blue light is scattered away, and the red light, which has the longest wavelength, reaches our eyes, making the Sun appear reddish.
7. Formation of a Rainbow
A rainbow is formed due to the dispersion, refraction, and internal reflection of sunlight in water droplets in the atmosphere.
Process:
Sunlight enters a water droplet, gets refracted, and disperses into its constituent colors. The light undergoes internal reflection inside the droplet and is refracted again when it leaves the droplet, creating a rainbow.
A rainbow is formed due to the dispersion, refraction, and internal reflection of sunlight in water droplets in the atmosphere.
Process:
Sunlight enters a water droplet, gets refracted, and disperses into its constituent colors. The light undergoes internal reflection inside the droplet and is refracted again when it leaves the droplet, creating a rainbow.
8. The Eye and Its Care
- To maintain healthy eyes:
- Avoid excessive exposure to bright light or screens.
- Have regular eye check-ups.
- Maintain proper reading habits and a balanced diet rich in vitamins A, C, and E.
- Avoid excessive exposure to bright light or screens.
- Have regular eye check-ups.
- Maintain proper reading habits and a balanced diet rich in vitamins A, C, and E.
Conclusion
The human eye is a complex organ responsible for our ability to see the colorful world. Understanding its structure, functioning, and common defects helps us appreciate how it works. The chapter also explores interesting natural phenomena like atmospheric refraction, scattering of light, and the formation of rainbows, providing insights into the wonders of optics.