CLASS 10 SCIENCE NOTES CHAPTER: HUMAN EYE AND THE COLOURFUL WORLD

THE HUMAN EYE & THE COLOURFUL WORLD

The Human Eye

• One of the most sensitive and important sense organs.
• Gives us sight and the ability to see different colors.

Advantages of Having Eyes in Front of the Face (Binocular Vision)

• It gives a wider field of view (approx. 180°).
• It provides a three-dimensional view (depth perception), helping to judge distances accurately.

Parts of the Eye & Their Functions

Part of the Eye	What It Does (Simple Description)
Cornea	The clear, curved front window. It lets light in and does most of the light bending (refraction).
Eyeball	The main body of the eye, spherical shaped, about 2.3 cm in diameter.
Iris	The colored part. It controls the size of the pupil to regulate light entry.
Pupil	The black opening in the center of the iris. It controls the amount of light entering the eye.
Lens	A clear lens that sits behind the iris. It fine-tunes the focus to make a clear image on the retina.
Retina	The light-sensitive screen at the back. It turns light into electrical signals.
Rods	Cells for seeing in dim light (black-and-white vision).
Cones	Cells for seeing in bright light and responsible for color vision.
Optic Nerve	The cable that carries visual signals from the retina to the brain.
Ciliary Muscles	Muscles that change the shape of the lens (and thus its focal length) for focusing.

💡 How Vision Works & Accommodation

Steps for Image Formation

1. Light enters through the Cornea (maximum refraction).
2. It passes through the Pupil and the Lens (fine-tuning focus).
3. A real and inverted image is formed on the Retina.
4. The retina converts light into electrical signals, which the Optic Nerve sends to the brain.
5. The Brain processes the signals and interprets the image as upright.

Power of Accommodation

Accommodation is the ability of the eye lens to adjust its focal length by changing its curvature (thickness) to focus on objects at different distances.

To View	Ciliary Muscles	Lens Shape	Focal Length
Closer Objects	Contract (Squeeze)	Thicker (More curved)	Decreases (Power increases)
Distant Objects	Relax (Loosen)	Thinner (Less curved)	Increases (Power decreases)

Defects of Vision and Correction

1. Myopia (Near-sightedness)

• Defect: Clear near vision; blurry distant vision. Far Point is less than infinity.
• Cause: Image forms in front of the retina because the eyeball is too long or the lens is too curved.
• Correction: Concave Lens (Diverging Lens) to spread light rays out and shift focus onto the retina.

2. Hypermetropia (Far-sightedness)

• Defect: Clear distant vision; blurry near vision. Near Point is further than 25 cm.
• Cause: Image forms behind the retina because the eyeball is too short or the lens has a focal length that is too long.
• Correction: Convex Lens (Converging Lens) to add converging power and shift focus forward onto the retina.

3. Presbyopia

• Defect: Difficulty focusing on nearby objects (similar to hypermetropia), mainly due to age.
• Cause: Weakening of ciliary muscles and decreasing flexibility of the lens, reducing accommodation power.
• Correction: Convex Lens, Often requires Bifocal Lenses (concave top, convex bottom) if the person also has myopia.

4. Cataract

• A condition in which crystalline lens of eye becomes milky and cloudy due to growth of membrane over it.
• It generally occurs in older adults.
• This causes partial or complete loss of vision.
• Vision can be restored through cataract surgery.

Prism

• Prism is a transparent refracting medium bounded by at least two lateral surfaces, inclined at each other at a certain angle.
• It consists of two triangular bases and three rectangular lateral surfaces.

Refraction of light through prism

• The angle between the two lateral faces of the prism is known as the angle of the prism (A).
• The angle of deviation is the angle between the direction of the incident ray and the direction of the emergent ray after light passes through a prism.

• Refraction in Prism occurs at two surfaces:

1. From air to glass → bends towards normal
2. From glass to air → bends away from normal

Dispersion through prism

The Colourful World: Light Phenomena

Dispersion of White Light & Rainbow

• Dispersion: The phenomenon of splitting of white light into its seven constituent colors (VIBGYOR) when it passes through a transparent medium (like a prism). 

• Cause of Dispersion: In a vacuum and air, light rays of all colors travel at the same speed.
• However, in other mediums, they travel at different speeds and bend at different angles, causing light to disperse into its constituent colors.
• Different colors have different wavelengths and thus bend (refract) by different amounts. Violet bends the most, and Red bends the least.

                                            Wavelength ∝ Velocity ∝ 1/Deviation

NEWTON’S PRISM EXPERIMENT

Sir Isaac Newton showed that sunlight is made of seven colours by passing it through a prism to form a spectrum. He then used a second inverted prism to recombine the colours back into white light, proving that white light is a mixture of all seven colours.

• Rainbow Formation: A rainbow is a natural spectrum formed due to the occurence of Refraction, Dispersion, and Internal Reflection of sunlight by water droplets in the atmosphere.

Tiny water droplets in the atmosphere act like small prisms. These droplets:

1.Refract sunlight when it enters,

2.Disperse it into seven colours,

3.Internally reflect the light inside the droplet,

4.Then refract again when light exits the droplet.

Atmospheric Refraction

This is the bending of light caused by the passage through Earth’s atmosphere, which has layers of continuously varying refractive index (density).

Phenomena Caused by Atmospheric Refraction:

• Twinkling of Stars: The light from a distant star travels through many layers of atmosphere, which are constantly moving and changing density. This causes the light path to vary continuously, making the stars appear to flicker or twinkle. Planets do not twinkle because they are much closer and appear as extended sources.

• Advanced Sunrise and Delayed Sunset: The atmosphere refracts the light from the Sun towards the horizon. Due to this bending, the Sun appears to rise 2 minutes early and set 2 minutes late.

☁️ Scattering of Light (Tyndall Effect)

Scattering is the phenomenon where light is deflected (bounced off) in various directions when it strikes tiny particles (atoms, molecules, or dust) in a medium.

Tyndall Effect

• The phenomenon where the path of a beam of light becomes visible when passing through a medium containing tiny particles (like smoke or a colloidal solution).

Color of the Sky & Clouds

• Blue Sky: According to Rayleigh's criterion, the amount of scattering is inversely proportional to the fourth power of the wavelength. Since blue light has a shorter wavelength than red light, it is scattered much more strongly by the fine air molecules in the atmosphere, making the sky appear blue.
• White Clouds: Clouds are composed of larger water droplets. These large particles scatter all wavelengths (all colors) of light almost equally. The combination of all colors of scattered light makes the clouds appear white.
• Red/Orange Sun at Sunrise/Sunset: At sunrise and sunset, light from the Sun travels the maximum distance through the atmosphere. The shorter-wavelength blue light is scattered away, leaving the longer-wavelength red and orange light to reach our eyes.