The Human Eye and the Colorful World
In the previous chapter, we learned about lenses and refraction. Now, we will look at the most amazing optical instrument in existence: The Human Eye. We will also explore why the sky is blue, why stars twinkle, and how rainbows are formed.
Part 1: The Human Eye
The eye acts like a camera. It has a lens system that forms an image on a light-sensitive screen called the Retina.
1. Key Parts of the Eye
- Cornea: The transparent front part. Most of the refraction of light happens here.
- Iris & Pupil: The Iris is the dark muscular diaphragm that controls the size of the Pupil. The Pupil regulates the amount of light entering the eye.
- Crystalline Lens: A convex lens made of jelly-like material. It fine-tunes the focus.
- Ciliary Muscles: These hold the lens and modify its curvature (shape) to see near or far objects clearly.
- Retina: The screen at the back of the eye containing light-sensitive cells:
- Rods: Respond to intensity (dim/bright light).
- Cones: Respond to color.
2. Power of Accommodation
The ability of the eye lens to adjust its focal length is called Accommodation.
- Distant Objects: Ciliary muscles relax → Lens becomes thin → Focal length increases.
- Nearby Objects: Ciliary muscles contract → Lens becomes thick → Focal length decreases.
- Near Point: The minimum distance to see clearly without strain (25 cm for adults).
Part 2: Defects of Vision
Sometimes the eye loses its power of accommodation, resulting in refractive defects.
| Defect | Myopia (Near-sightedness) | Hypermetropia (Far-sightedness) |
|---|---|---|
| What happens? | Can see near objects clearly, but distant objects are blurry. | Can see distant objects clearly, but nearby objects are blurry. |
| Cause | Image forms in front of the retina. (Eyeball is too long). | Image forms behind the retina. (Eyeball is too short). |
| Correction | Concave Lens (Diverging lens). | Convex Lens (Converging lens). |
Presbyopia (Old Age Hypermetropia)
With age, the ciliary muscles weaken and the lens loses flexibility. The person cannot read comfortably. This is corrected using Bifocal lenses (Upper part Concave for distance, lower part Convex for reading).
Part 3: Refraction through a Prism
A triangular glass prism has two triangular bases and three rectangular lateral surfaces.
Dispersion of White Light
When white light passes through a prism, it splits into its seven constituent colors. This band of colors is called a Spectrum.
* Red: Bends the least (Longest wavelength).
* Violet: Bends the most (Shortest wavelength).
Part 4: Atmospheric Refraction
The earth's atmosphere is not uniform. Hot air is lighter (rarer) than cold air (denser). This changing density causes light to bend continuously.
1. Twinkling of Stars
Starlight enters the atmosphere and undergoes continuous refraction. Since the physical conditions of the air keep changing, the apparent position of the star fluctuates, and the amount of light entering our eye flickers. This creates the "twinkling" effect.
(Note: Planets are closer and appear as extended sources, so they don't twinkle.)
2. Advanced Sunrise and Delayed Sunset
Due to atmospheric refraction, the sun is visible to us about 2 minutes before actual sunrise and 2 minutes after actual sunset.
Part 5: Scattering of Light
When light hits small particles (like dust or gas molecules) in the atmosphere, it gets scattered.
1. Why is the Sky Blue? (Tyndall Effect)
The molecules of air are smaller than the wavelength of visible light. They scatter shorter wavelengths (Blue/Violet) more effectively than longer wavelengths (Red). This scattered blue light enters our eyes.
(Astronauts see the sky as black because there is no atmosphere in space to scatter light.)
2. Why is the Sun Red at Sunrise/Sunset?
During sunrise or sunset, sunlight travels a much longer distance through the atmosphere to reach our eyes. The blue light gets scattered away completely, leaving only the longer wavelength Red light to reach us.
📷 Click here to View Diagram: Scattering (Blue Sky vs Red Sunset)🧬 Looking for Biology Notes?
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