Chapter 10: The Human Eye and the Colourful World
Introduction
The human eye is a vital organ that enables us to see the world around us. It functions like a camera, forming images on the retina. The eye has various parts that work together to regulate light entry, focus objects, and perceive colors. In addition, phenomena like dispersion, scattering, and atmospheric refraction explain the various natural optical effects we observe.
Structure of the Human Eye
The human eye is a spherical structure enclosed in a bony socket for protection. It consists of the following parts:
A. Important Parts of the Eye
- Cornea – Transparent outer covering that refracts most of the light entering the eye.
- Iris – A colored ring that controls the amount of light entering the eye.
- Pupil – An adjustable opening that regulates light entry.
- Lens – A transparent, flexible structure that focuses light on the retina.
- Retina – The light-sensitive layer where images are formed.
- Optic Nerve – Carries visual information from the retina to the brain.
Diagram: Structure of the Human Eye
Power of Accommodation
The eye’s ability to adjust the focal length of its lens to focus objects at different distances is called accommodation.
- Near Point: Closest distance (about 25 cm) at which an eye can focus comfortably.
- Far Point: The farthest distance at which an eye can see clearly (infinity).
Limitations of Accommodation:
- Cannot focus on very close objects for long.
- Difficulty in focusing on distant objects due to defects in vision.
Defects of Vision and Their Correction
A. Myopia (Short-Sightedness)
- Cause: The eyeball is too long, or the lens is too curved.
- Effect: Distant objects appear blurry.
- Correction: Concave lenses are used to diverge incoming light rays before they reach the eye.
B. Hypermetropia (Long-Sightedness)
- Cause: The eyeball is too short, or the lens is too flat.
- Effect: Nearby objects appear blurry.
- Correction: Convex lenses are used to converge light rays before they reach the eye.
C. Presbyopia
- Cause: Aging causes the lens to lose flexibility.
- Effect: Difficulty in seeing nearby objects.
- Correction: Bifocal lenses with both concave and convex lenses.
D. Astigmatism
- Cause: Irregular shape of the cornea or lens.
- Effect: Blurred or distorted vision at all distances.
- Correction: Cylindrical lenses.
Diagram: Types of Vision Defects and Their Corrections
Refraction of Light Through a Prism
When light passes through a glass prism, it bends due to refraction and splits into different colors. This phenomenon is called dispersion.
- White light splits into seven colors (VIBGYOR).
- Reason: Different colors have different refractive indices.
Diagram: Dispersion of Light Through a Prism
Atmospheric Refraction and Optical Phenomena
A. Twinkling of Stars
- Reason: Due to the refraction of light through varying layers of the atmosphere.
- The density and temperature variations cause the star’s light to bend.
B. Apparent Position of Stars
- Due to atmospheric refraction, stars appear higher than their actual position.
C. Advanced Sunrise and Delayed Sunset
- The sun appears 2 minutes earlier than actual sunrise and sets 2 minutes later than actual sunset due to refraction by the Earth’s atmosphere.
Diagram: Atmospheric Refraction and Apparent Star Position
Scattering of Light
Scattering occurs when light interacts with atmospheric particles.
A. Tyndall Effect
- The scattering of light by particles in a medium.
- Visible in forests when sunlight filters through mist and dust.
B. Why is the Sky Blue?
- Shorter wavelengths (blue light) scatter more than longer wavelengths (red light).
- Our eyes perceive the scattered blue light.
C. Why are Sunrises and Sunsets Red?
- During sunrise and sunset, light travels a longer path.
- Shorter wavelengths scatter away, and only red light, which has a longer wavelength, reaches our eyes.