Comprehensive theory, key formulas, diagrams, and memory aids for Human Eye and Colourful World.
The previous chapter explored the physical laws of light. But how do we actually perceive this light? The human eye is one of the most valuable and sensitive sense organs. It is essentially a natural optical instrument that uses a lens to form images on a light-sensitive screen. Furthermore, the interaction of light with the atmosphere gives rise to a spectacularly colorful world.
The eye is roughly spherical in shape with a diameter of about 2.3 cm.
The ability of the eye lens to adjust its focal length is called accommodation. * To see distant objects clearly, the ciliary muscles relax, the lens becomes thin, its focal length increases, and the image converges on the retina. * To see nearby objects clearly, the ciliary muscles contract, the lens becomes thicker, its focal length decreases, and the image converges on the retina. * Near Point (Least distance of distinct vision): The minimum distance at which an object can be seen most distinctly without strain. For a normal young adult, it is about 25 cm. * Far Point: The maximum distance up to which a normal eye can see things clearly. It is infinity.
Sometimes, the eye gradually loses its power of accommodation, leading to blurred vision.
A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. * Cause: Excessive curvature of the eye lens or elongation of the eyeball. The image of a distant object is formed in front of the retina. * Correction: Using a concave lens (diverging lens) of appropriate power. It diverges the rays before they enter the eye, pushing the image back onto the retina.
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. Their near point recedes farther than 25 cm. * Cause: The focal length of the eye lens is too long, or the eyeball has become too small. The image of a nearby object is formed behind the retina. * Correction: Using a convex lens (converging lens) of appropriate power. It provides additional converging power to bring the image forward onto the retina.
The power of accommodation of the eye usually decreases with aging. Many people find their near point gradually recedes away. They find it difficult to read without corrective eyeglasses. This defect is called presbyopia. * Cause: Gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. * Correction: Convex lenses, or sometimes bi-focal lenses (upper portion concave for distant vision, lower portion convex for near vision) if the person suffers from both myopia and hypermetropia.
A glass prism has two triangular bases and three rectangular lateral surfaces. The angle between its two lateral faces is called the angle of the prism. When light passes through a prism, it bends towards the base of the prism. The angle between the incident ray and the emergent ray is called the angle of deviation.
In 1665, Isaac Newton used a glass prism to obtain the spectrum of sunlight. * Dispersion: The splitting of light into its component colors. * When a beam of white light (like sunlight) passes through a prism, it splits into a band of seven colors: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). * Why? 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. * Recombination: Newton showed that placing a second identical prism in an inverted position with respect to the first prism recombines the spectrum back into white light. * Rainbow: A natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, which act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.
The earth's atmosphere is not uniform. The physical conditions of the refracting medium (air) are not stationary; hotter air is optically rarer than cooler air. This causes light to refract as it passes through the atmosphere. * Twinkling of Stars: Starlight, on entering the earth's atmosphere, undergoes continuous refraction before it reaches the earth. Since the atmospheric conditions keep changing, the apparent position of the star fluctuates, and the amount of starlight entering the eye flickers. The star sometimes appears brighter and sometimes fainter, which is the twinkling effect. (Planets don't twinkle because they are much closer to earth and act as extended sources of light). * Advance Sunrise and Delayed Sunset: The sun is visible to us about 2 minutes before actual sunrise, and about 2 minutes after actual sunset, because of atmospheric refraction. The apparent position of the sun is slightly higher than its actual position.
When a beam of light interacts with particles (like dust, smoke, water droplets, air molecules) present in the atmosphere, it gets redirected in different directions. This is called scattering.
The human eye is an optical marvel, utilizing a flexible convex lens and a responsive aperture (pupil) to project the world onto a biological screen (retina). While it can suffer from defects like myopia and hypermetropia, understanding refraction allows us to correct these with lenses. Moving beyond the eye, the interaction of light with prisms reveals the spectrum hidden within white light (dispersion). Finally, the Earth's atmosphere acts as a vast optical medium, creating phenomena like the twinkling of stars (atmospheric refraction) and painting the sky blue or red (scattering of light) depending on the sun's position.