Human Eye and Colourful World Class 10 Notes explain how the human eye works as a natural optical instrument and how various natural phenomena related to light occur around us.
This chapter from Class 10 Science Syllabus helps students understand vision, defects of vision, dispersion of light, scattering, and atmospheric effects, making it an important part of Class 10 Physics.
Class 10 Science Human Eye and Colourful World Notes
The human eye is a crucial sensory organ. It enables us to see and understand the vibrant world.
Structure of the Human Eye
The human eye is nearly spherical with a diameter of about 2.3 cm. It consists of several key parts:
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Cornea: This is the thin, transparent, bulging front part. It refracts most of the incoming light. The cornea also protects the eye from dust and germs.
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Iris: A dark, muscular diaphragm behind the cornea. It controls the size of the pupil.
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Pupil: A small opening in the centre of the iris. It regulates the amount of light entering the eye.
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Crystalline Lens: This is a transparent, flexible, convex lens. It forms real and inverted images on the retina.
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Ciliary Muscles: These muscles adjust the focal length of the eye lens. They help in focusing on objects at different distances.
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Retina: A light-sensitive screen at the back of the eye. It contains rods and cones. Rods help in dim light vision, while cones detect colour and bright light.
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Optic Nerve: This nerve transmits electrical signals from the retina to the brain. The brain then interprets these signals into images.
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Aqueous Humour: A water-like fluid found between the cornea and the lens. It provides nutrition to the eye.
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Vitreous Humour: A gel-like substance located between the lens and the retina. It helps maintain the eye's shape.
Working of the Human Eye
Light enters the eye through the cornea and pupil. The crystalline lens focuses this light onto the retina. The retina converts light into electrical signals. The optic nerve carries these signals to the brain. The brain then processes these signals, allowing us to see objects.
Power of Accommodation
This is the eye lens's ability to adjust its focal length. For near objects, ciliary muscles contract, making the lens thicker and decreasing focal length. For distant objects, muscles relax, making the lens thinner and increasing focal length. This ensures images always focus sharply on the retina.
Near Point and Far Point
The near point is the minimum distance for clear vision. For a normal young adult, it is 25 cm. The far point is the maximum distance for clear vision. For a normal eye, the far point is at infinity.
Defects of Vision and Correction
Several common eye defects can affect vision.
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Myopia (Nearsightedness): A person sees nearby objects clearly but distant objects blur. This happens when the image forms in front of the retina. Causes include excessive lens curvature or an elongated eyeball. It is corrected using a concave lens.
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Hypermetropia (Farsightedness): A person sees distant objects clearly but nearby objects blur. The image forms behind the retina. Causes include too long focal length of the lens or a shortened eyeball. It is corrected using a convex lens.
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Presbyopia: This age-related defect makes it difficult to see nearby objects. It results from weakening ciliary muscles and reduced flexibility of the eye lens. Bifocal lenses often correct this condition.
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Cataract: The crystalline lens becomes cloudy or opaque, causing partial or complete vision loss. It is treated with surgery to replace the clouded lens.
Dispersion of Light by a Prism
Dispersion is the splitting of white light into its component colours. A glass prism causes this phenomenon. Different colours of light bend at different angles when passing through a prism. Violet light bends the most, while red light bends the least. This creates a spectrum of colours, like a rainbow.
Atmospheric Refraction
Atmospheric Refraction is the bending of light as it travels through different layers of the Earth’s atmosphere. Since the atmosphere is made up of layers with varying temperature and density, the refractive index changes continuously from higher layers to lower layers. As a result, light rays do not travel in a straight line but bend gradually towards the Earth.
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Twinkling of Stars:
Stars appear to twinkle because the light coming from them undergoes continuous atmospheric refraction due to constantly changing air currents. These changes cause fluctuations in the apparent position and brightness of stars. Since stars are very far away and act as point sources of light, even small changes in refraction make them appear to twinkle.
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Advance Sunrise and Delayed Sunset:
Atmospheric refraction causes the Sun to appear above the horizon even when it is actually below it. Due to this effect, we can see the Sun about 2 minutes before actual sunrise and about 2 minutes after actual sunset. This happens because sunlight bends while passing through the dense layers of the atmosphere near the Earth’s surface.
Scattering of Light
Scattering of Light is the phenomenon in which light rays deviate from their straight path when they strike small particles present in a medium such as air, dust, smoke, or water droplets. This occurs because light interacts with these particles and gets redirected in different directions.
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Tyndall Effect:
The Tyndall effect refers to the scattering of light by colloidal particles, making the path of a light beam visible. This effect is commonly observed when a beam of sunlight passes through a dusty or smoky room, or when light passes through fog or mist. It helps distinguish between true solutions and colloids.
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Blue Colour of the Sky:
The sky appears blue because air molecules scatter light of shorter wavelengths, such as blue and violet, much more effectively than red light. Although violet light scatters the most, our eyes are more sensitive to blue light, and some violet light is absorbed by the upper atmosphere. As a result, the scattered blue light reaches our eyes from all directions, making the sky appear blue. -
Reddening of the Sun at Sunrise and Sunset:
At sunrise and sunset, sunlight travels a longer distance through the atmosphere to reach the observer. During this long path, most of the blue and shorter wavelength light is scattered away, leaving mainly the red and orange wavelengths to reach our eyes. This causes the Sun to appear reddish at these times. - Danger Signals: These are red because red light is least scattered by fog or smoke, making it visible from a distance.
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