📘Class 10 Science – Chapter: The Human Eye and the Colourful World Notes

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👁️ The Human Eye

• The human eye is a sensory organ that enables us to perceive light and interpret visual information.

Structure of the Eye:

Part	Function
Cornea	Transparent front layer; refracts light entering the eye
Iris	Colored part; controls pupil size and thus the amount of light
Pupil	Opening in the iris; regulates the light entering the eye
Lens	Transparent, convex; focuses light onto retina
Ciliary muscles	Hold the lens; change its shape to adjust focus (accommodation)
Retina	Layer with light-sensitive cells (rods and cones); forms the image
Optic Nerve	Sends visual signals from retina to the brain

Working of the Eye:

• Light enters through the cornea, passes through the aqueous humor, pupil, and is focused by the lens on the retina.

• The retina converts light into electrical signals sent to the brain via the optic nerve.

Accommodation:

• The ability of the eye lens to adjust its focal length to focus on near and distant objects by changing its shape using ciliary muscles.

Range of Vision:

• Near Point: 25 cm (minimum distance to see clearly)

• Far Point: Infinity

Example: When reading a book, the lens bulges to decrease focal length and focus the text.

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👓 Defects of Vision and Their Correction

Defect	Cause	Correction
Myopia (Nearsightedness)	Eyeball too long or lens too curved	Concave lens
Hypermetropia (Farsightedness)	Eyeball too short or weak lens	Convex lens
Presbyopia	Age-related loss of accommodation	Bifocal lenses
Cataract	Clouding of the eye lens	Surgical removal and lens implant

Example: A person with myopia cannot see a distant tree clearly. A concave lens will help them focus the image on the retina.

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🌈 Refraction of Light Through a Prism

• A prism is a transparent object with two triangular faces and three rectangular faces.

Refraction Through Prism:

• When light passes through a prism, it bends twice—once while entering and once while exiting.

• The overall bending is called the angle of deviation.

Example: White light incident on a glass prism splits into seven colors. Red bends the least, violet the most.

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🌈 Dispersion of Light

• Dispersion is the splitting of white light into its seven constituent colors (VIBGYOR) due to different refractive indices for different wavelengths.

Cause:

• Different colors of light travel at different speeds in a prism, leading to angular separation.

Order of Colors:

• Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR)

Example: Rainbow formation in the sky.

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🌦️ Atmospheric Refraction

• Bending of light as it passes through layers of air with varying densities.

Phenomena Due to Atmospheric Refraction:

1. Apparent Star Position: Stars appear slightly shifted from their actual position.

2. Twinkling of Stars: Due to continuous variation in refractive index of the atmosphere.

3. Advanced Sunrise and Delayed Sunset: The sun appears earlier and sets later due to refraction.

Example: Sun visible 2 minutes before it actually rises.

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🌈 Scattering of Light

• Scattering is the deviation of light from its path due to particles in the atmosphere.

Rayleigh Scattering:

• Intensity of scattered light ∝ 1/λ⁴ (λ = wavelength)

• Shorter wavelengths (blue) scatter more than longer wavelengths (red)

Applications:

Phenomenon	Explanation
Blue Sky	Shorter blue wavelengths scatter more
Red Sun at Sunrise/Sunset	Red light (longer wavelength) scatters less and travels longer

Example: The orange hue of the sky during sunset.

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📘 Key Equations and Facts

• No major equations, but remember the relationship for scattering:

  Scattering ∝ 1 / λ⁴

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