Class 10 Chapter : Human Eye and Colorful world

 

                 

Class 10 Science – Chapter 11: The Human Eye and the Colourful World

The Human Eye and the Colourful World is Chapter 11 of Class 10 Science (CBSE). In this chapter, students learn about the structure of the eye, its working, common defects like myopia and hypermetropia, and natural phenomena like rainbow and scattering of light...

1. The Human Eye – Structure and Working

The human eye is one of the most sensitive and important sensory organs. It functions like a camera, helping us to see the world by receiving light rays, focusing them, and sending signals to the brain.

Major Parts and Their Functions

Part	Function
Cornea	Transparent outer layer; refracts most of the incoming light.
Aqueous Humour	Fluid between cornea and lens; maintains pressure and provides nutrients.
Iris	Coloured part; controls size of pupil.
Pupil	Allows light to enter; black spot in the center.
Ciliary Muscles	Change curvature of lens for focusing.
Eye Lens	Convex lens focusing light on the retina.
Vitreous Humour	Gel-like substance maintaining eye shape.
Retina	Light-sensitive screen with rods and cones.
Optic Nerve	Carries signals to brain.
Blind Spot	No image formation as no receptors.
Fovea	Sharpest vision area with dense cones.

2. Accommodation of Eye

The ability of the eye lens to change its focal length to see distant and nearby objects clearly is called power of accommodation. Ciliary muscles adjust the curvature of the lens.

Important Points:

Feature	Value
Near Point	25 cm (normal eye)
Far Point	Infinity
Power of accommodation	Eye adjusts focal length between ∞ and 25 cm

3. Defects of Vision and Their Correction

Myopia (Short-sightedness)

• Can see nearby objects clearly, but distant objects appear blurry.
• Caused by eye-ball being too long or lens being too convex.
• Image is formed in front of the retina.
• Corrected using a concave lens.

Hypermetropia (Long-sightedness)

• Can see distant objects clearly, but nearby objects appear blurry.
• Caused by eye-ball being too short or lens less convex.
• Image is formed behind the retina.
• Corrected using a convex lens.

Presbyopia

• Old-age defect where near and distant vision both become difficult.
• Caused due to loss of flexibility in lens and weakening of ciliary muscles.
• Corrected using bifocal or progressive lenses.

4. Refraction of Light Through a Prism

When white light passes through a glass prism, it splits into 7 colors – VIBGYOR. This is called dispersion of light. Different colors refract differently due to varying wavelengths.

5. Atmospheric Refraction Phenomena

Twinkling of Stars

Light from stars undergoes continuous refraction while passing through various layers of Earth's atmosphere. Due to changing densities, the apparent position of the star shifts, making it appear to twinkle.

Advanced Sunrise and Delayed Sunset

Due to atmospheric refraction, we see the Sun about 2 minutes before it actually rises and 2 minutes after it has actually set. The light bends, making the Sun visible even when it is below the horizon.

Why the Sky Appears Blue

Shorter wavelengths (blue light) are scattered more than longer wavelengths (red light) by the atmospheric particles. Hence, the sky appears blue.

Why the Sun Appears Red at Sunrise and Sunset

During sunrise and sunset, sunlight travels a longer path through the atmosphere. Shorter wavelengths scatter out, and mostly red light reaches our eyes, making the Sun appear red.

6. Rainbow Formation

A rainbow is formed due to the dispersion, internal reflection, and refraction of sunlight inside raindrops. It appears as a spectrum of seven colours in the sky opposite to the Sun after rainfall.
Steps involved:
1. Refraction – Light enters the raindrop and bends.
2. Dispersion – It splits into seven colours (VIBGYOR).
3. Internal Reflection – Light reflects inside the drop.
4. Refraction Again – Light exits and reaches our eyes.

7. Scattering of Light

Scattering is the process of light getting deflected in many directions by small particles. Rayleigh scattering explains that shorter wavelengths (like blue) scatter more than longer ones (like red).

8. Important Facts and Values (for Revision)

Topic	Value
Minimum distance of clear vision (normal eye)	25 cm
Maximum distance of clear vision	Infinity
No. of colours in visible spectrum	7
Cause of rainbow	Dispersion + Total internal reflection
Red light	Least scattered
Violet light	Most scattered
Old age eye defect	Presbyopia
Defect for blurry near vision	Hypermetropia
Correction for Myopia	Concave lens