HUMAN EYE AND THE COLOURFUL WORLD
The Human Eye
The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colors around us.
Parts of Human Eye (NCERT)
- Cornea: The transparent spherical membrane covering the front of the eye.
- Iris: The coloured diaphragm between the cornea and lens that controls the size of pupil.
- Pupil: The small opening in the iris through which light enters the eye.
- Eye lens: A transparent crystalline lens which provides the finer adjustment of focal length.
- Ciliary muscles: Holds the lens in position and helps in modifying the curvature of lens.
- Retina: The light-sensitive surface of eye on which the image is formed.
- Optic nerve: Transmits visual information from retina to brain.
The ability of the eye lens to adjust its focal length is called accommodation. The ciliary muscles help in changing the curvature of the eye lens and thus its focal length.
NCERT Note: The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision. For a normal eye, it is about 25 cm.
Defects of Vision and Their Correction (NCERT)
1. Myopia (Near-sightedness)
A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
Causes (NCERT):
- Excessive curvature of the eye lens
- Elongation of the eyeball
Correction: Using concave lens of appropriate power.
2. Hypermetropia (Far-sightedness)
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
Causes (NCERT):
- Focal length of the eye lens is too long
- Eyeball has become too small
Correction: Using convex lens of appropriate power.
3. Presbyopia
As we grow older, the ciliary muscles weaken and the eye lens becomes inflexible. This reduces the power of accommodation.
Correction (NCERT): Bi-focal lenses with both concave and convex lenses.
Refraction of Light Through a Prism (NCERT)
The angle between the incident ray and emergent ray is called the angle of deviation.
Dispersion of White Light by a Glass Prism (NCERT)
The splitting of white light into its component colors is called dispersion.
The sequence of colors in the spectrum is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
NCERT Explanation: Different colors of light bend through different angles with respect to the incident ray as they pass through a prism. Red light bends the least while violet light bends the most.
Atmospheric Refraction (NCERT)
The refraction of light by the earth's atmosphere is called atmospheric refraction.
1. Twinkling of Stars (NCERT)
The starlight undergoes continuous refraction as it moves through layers of air with different densities. This causes the apparent position of the star to fluctuate slightly, making it appear to twinkle.
NCERT Explanation: Planets don't twinkle because they are closer and appear as extended sources of light - the variations average out.
2. Advance Sunrise and Delayed Sunset (NCERT)
The sun appears about 2 minutes before actual sunrise and remains visible for about 2 minutes after actual sunset due to atmospheric refraction.
Scattering of Light (NCERT)
Scattering is the phenomenon by which light is redirected in many directions by small particles.
Tyndall Effect (NCERT)
The scattering of light by colloidal particles is called Tyndall effect.
Why is the Sky Blue? (NCERT)
The molecules of air and other fine particles in the atmosphere scatter blue light (shorter wavelength) more strongly than red light. This scattered blue light enters our eyes.
Color of the Sun at Sunrise and Sunset (NCERT)
At sunrise and sunset, sunlight travels longer distance through atmosphere. Most of the blue light gets scattered away, leaving the red light to reach our eyes.
NCERT Activity: Danger signals are red because red light is scattered the least and can travel longer distances through fog and mist.