Human Eye and Colourful World: Class 10 NCERT Science Guide

The Human Eye: A Natural Camera

The human eye is one of the most valuable and sensitive sense organs. It acts like a camera, enabling us to see the beautiful, colourful world around us. Its main parts work together to focus light and send signals to the brain.

1. Cornea: This is the transparent outer layer at the front of the eye. Light enters the eye through the cornea, which performs most of the refraction (bending) of light.
2. Iris and Pupil: Behind the cornea is the iris, a dark muscular diaphragm that controls the size of the pupil. The pupil is the opening in the center of the iris. The iris adjusts the pupil's size to regulate the amount of light entering the eye. In bright light, the iris contracts the pupil, and in dim light, it dilates the pupil.
3. Eye Lens: This is a convex lens made of a transparent, fibrous, jelly-like material. Its curvature can be adjusted by the ciliary muscles.
4. Ciliary Muscles: These muscles hold the lens in place and control its focal length. When they relax, the lens becomes thinner, increasing its focal length for viewing distant objects. When they contract, the lens becomes thicker, decreasing its focal length for viewing nearby objects.
5. Retina: This is a delicate membrane at the back of the eye, which acts like the screen of a camera. It contains a vast number of light-sensitive cells: rods (sensitive to light intensity) and cones (sensitive to colour). When light is focused on the retina, these cells get activated and generate electrical signals.
6. Optic Nerve: These electrical signals are transmitted to the brain through the optic nerve. The brain interprets these signals, and we perceive the object as it is.

Focusing, Vision Defects and Their Correction

1. Power of Accommodation — The ability of the eye lens to adjust its focal length to see both distant and nearby objects clearly is called the power of accommodation. For a young adult with normal vision, the near point is about 25 cm, and the far point is infinity.
2. Myopia (Near-sightedness) — A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. This defect occurs because the image of a distant object is formed in front of the retina. Causes: (i) Excessive curvature of the eye lens (lens is too thick), or (ii) Elongation of the eyeball. Correction: This defect is corrected by using a concave lens of suitable power. The concave lens diverges the incoming light rays so that they appear to come from a farther point, allowing the eye lens to focus the image correctly on the retina.
3. Hypermetropia (Far-sightedness) — A person with hypermetropia can see distant objects clearly but finds it difficult to see nearby objects. This happens because the image of a nearby object is formed behind the retina. Causes: (i) The focal length of the eye lens is too long (lens is too thin), or (ii) The eyeball has become too short. Correction: This defect is corrected by using a convex lens of appropriate power. The convex lens provides the additional converging power required to form the image on the retina.
4. Presbyopia — This is the defect of vision due to ageing. With age, the ciliary muscles weaken, and the eye lens loses its flexibility. This diminishes the power of accommodation. As a result, the person finds it difficult to see nearby objects comfortably and distinctly. It is often corrected using bifocal lenses, which have both concave (for distant vision) and convex (for near vision) parts.

Optical Phenomena in Nature

• Twinkling of Stars: Stars are very far away and act as point sources of light. As starlight enters the Earth's atmosphere, it undergoes continuous refraction due to the changing refractive index of the different atmospheric layers. This random bending of light causes the apparent position of the star to fluctuate, and the amount of starlight entering the eye flickers. This flickering effect is what we perceive as the twinkling of stars.
• Why the Sky is Blue: The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. They are more effective in scattering light of shorter wavelengths (like blue and violet) than light of longer wavelengths (like red and yellow). This is called Rayleigh scattering. When sunlight passes through the atmosphere, the blue light is scattered in all directions. When this scattered blue light enters our eyes, the sky appears blue to us.
• Why the Sun Appears Reddish at Sunrise/Sunset: During sunrise and sunset, the sun is near the horizon. Sunlight has to travel through a thicker layer of the atmosphere to reach our eyes. Most of the shorter wavelength blue light gets scattered away by the particles in the atmosphere. The light that reaches us is predominantly of longer wavelengths, like red and orange. Therefore, the sun and the sky around it appear reddish.

Common Mistakes & Exam Tips

1. Ray Diagrams are Crucial: Practice drawing neat, labelled ray diagrams for myopia and hypermetropia, showing both the defective eye and the corrected eye. Use a ruler and pencil. Arrows showing the path of light are mandatory and carry marks.

2. Lens Confusion: Students often get confused between the lenses used for correction. Remember: Myopia (Near-sightedness) is corrected with a Concave lens. Think of it as needing to 'push' the image back onto the retina. Hypermetropia (Far-sightedness) is corrected with a Convex lens to 'pull' the image forward onto the retina.

3. Scattering vs. Refraction: Be very clear about the phenomena. Twinkling of stars is due to atmospheric refraction. Blue colour of the sky and reddish sunsets are due to scattering of light. Do not mix them up in your answers. When explaining scattering, always mention that shorter wavelengths (blue) are scattered more than longer wavelengths (red).

4. Power Calculation Sign: When calculating the power of a corrective lens, remember that for a concave lens (myopia), the focal length (f) is negative, so the power (P = 1/f) will also be negative. For a convex lens (hypermetropia), the focal length and power are positive.

Practice Questions with Solutions

• Q: A person needs a lens of power –5.5 dioptres for correcting their distant vision. For correcting their near vision, they need a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision? A: Step 1: Identify the given powers for distant and near vision. Power for distant vision, P1 = –5.5 D. Power for near vision, P2 = +1.5 D. Step 2: Use the formula relating power and focal length, P = 1/f, where f is in meters. Therefore, f = 1/P. Step 3: Calculate the focal length for distant vision (f1). f1 = 1 / P1 = 1 / (–5.5) m = –0.1818 m ≈ –18.2 cm. The negative sign indicates a concave lens, used for correcting myopia. Step 4: Calculate the focal length for near vision (f2). f2 = 1 / P2 = 1 / (+1.5) m = +0.666... m ≈ +66.7 cm. The positive sign indicates a convex lens, used for correcting hypermetropia/presbyopia. Final answer: The focal length for correcting distant vision is -18.2 cm, and for near vision is +66.7 cm.
• Q: Explain why planets do not twinkle but stars do. A: Step 1: Differentiate between stars and planets as sources of light from our perspective. Stars are extremely distant and thus appear as point-sized sources of light. Planets are much closer to Earth and are seen as extended sources (a collection of many point-sized sources). Step 2: Explain the effect of atmospheric refraction on a point-sized source (a star). The light from a star undergoes continuous refraction as it passes through the Earth's atmosphere, which has varying density. This causes the apparent position of the star to fluctuate, and the amount of light entering the eye flickers. This is perceived as twinkling. Step 3: Explain the effect on an extended source (a planet). A planet can be considered a collection of a large number of point sources. The light from all these points also undergoes refraction. However, the random flickering effects from all these points average out. The increase in light from one point is cancelled by the decrease from another, so the overall brightness remains constant. Final answer: Stars twinkle because they are point-sized sources and their light path is easily disturbed by atmospheric refraction. Planets, being extended sources, have their twinkling effect averaged out, so they appear to have a steady brightness.
• Q: Draw a labelled diagram of the human eye and describe the function of the retina. A: Step 1: Draw a neat, diagram of the human eye. The diagram should be roughly spherical and include and label the following parts: Cornea, Iris, Pupil, Crystalline Lens, Ciliary Muscles, Retina, and Optic Nerve. Step 2: Describe the function of the retina. The retina is the light-sensitive screen at the back of the eyeball. It is analogous to the film or sensor in a camera. It contains a large number of photoreceptor cells called rods and cones. Step 3: Explain the roles of rods and cones. Rods are sensitive to the intensity of light (dim or bright). Cones are sensitive to colour. Step 4: Explain the process of image formation and signal transmission. When an image is formed on the retina by the eye lens, these light-sensitive cells get activated and generate electrical signals. These signals are then sent to the brain via the optic nerve. The brain interprets these signals and allows us to perceive the image. Final answer: (Student should present a labelled diagram). The retina acts as a screen where the image is formed. It contains rods and cones that convert light energy into electrical signals, which are sent to the brain via the optic nerve for interpretation.
• Q: What is scattering of light? Use it to explain why the danger signals are made of red colour. A: Step 1: Define scattering of light. Scattering of light is the phenomenon by which a beam of light is redirected in many different directions when it interacts with particles or molecules of a medium. Step 2: State Rayleigh's law of scattering. According to Rayleigh scattering, the intensity of scattered light is inversely proportional to the fourth power of its wavelength (I ∝ 1/λ⁴). This means shorter wavelengths (like blue and violet) are scattered much more strongly than longer wavelengths (like red and orange). Step 3: Apply this principle to the colour red. Red light has the longest wavelength in the visible spectrum. Because of its long wavelength, it is scattered the least by the particles of fog, smoke, or dust in the atmosphere. Step 4: Conclude why red is used for danger signals. Since red light is scattered the least, it can travel the longest distance through the atmosphere without being deviated. This allows danger signals made of red colour to be seen clearly from a large distance, even in foggy or misty conditions, ensuring safety. Final answer: Scattering is the redirection of light by particles. Red light has the longest wavelength and is scattered the least. Therefore, it is used for danger signals as it can travel the farthest through atmospheric particles like fog and be seen from a distance.

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