The Human Eye And The Colourful World Class 10 Notes

Key Points

• The human eye acts like a camera, forming real and inverted images on the retina.
• The power of accommodation is the eye lens's ability to adjust its focal length to see objects at varying distances.
• Myopia (nearsightedness) is corrected by a concave lens, Hypermetropia (farsightedness) by a convex lens, and Presbyopia by bifocal lenses.
• A prism disperses white light into its seven constituent colours (VIBGYOR) due to different speeds of light in the prism.
• Atmospheric refraction causes phenomena like twinkling of stars, advanced sunrise, and delayed sunset.
• Scattering of light explains why the sky appears blue and why the sun looks reddish at sunrise/sunset.
• The amount of scattering depends on the wavelength of light and the size of the scattering particles. Shorter wavelengths scatter more effectively.

Key Definitions

Pupil

The opening in the centre of the iris through which light enters the eye. Its size is regulated by the iris.

Iris

A dark muscular diaphragm that controls the size of the pupil and thus the amount of light entering the eye.

Retina

The light-sensitive screen at the back of the eye on which the image is formed. It contains photoreceptor cells (rods and cones).

Power of Accommodation

The ability of the eye lens to adjust its focal length to clearly focus objects at different distances on the retina.

Dispersion of Light

The phenomenon of splitting of white light into its constituent colours when it passes through a transparent medium like a glass prism.

Atmospheric Refraction

The bending of light caused by the Earth's atmosphere due to varying optical densities of air layers at different altitudes.

Scattering of Light

The phenomenon in which light rays are deviated from their straight path due to interaction with particles of a medium.

Tyndall Effect

The phenomenon of scattering of light by colloidal particles, making the path of the light beam visible.

The Human Eye: Structure and Function

The human eye is an incredibly complex and sensitive natural optical instrument that enables us to see the vibrant world around us. It functions much like a camera, containing a lens system that forms an image on a light-sensitive screen called the retina. Light enters the eye through the cornea, a transparent spherical membrane on the outer surface of the eyeball. Behind the cornea, there is a dark muscular diaphragm called the iris, which controls the size of the pupil, the opening through which light actually passes into the eye. The iris regulates the amount of light entering the eye; in bright light, it contracts the pupil, and in dim light, it expands it.

The crystalline lens of the eye is a convex lens made of a fibrous, jelly-like material. It is held in position by ciliary muscles. These muscles are crucial because they can modify the curvature of the eye lens, thereby changing its focal length. This remarkable ability of the eye lens to adjust its focal length is called power of accommodation. This allows us to focus on both distant and near objects clearly on the retina. For distant objects, the ciliary muscles relax, making the lens thin and its focal length large. For nearby objects, the ciliary muscles contract, making the lens thicker and its focal length smaller. The image formed on the retina is always real and inverted. The retina contains millions of light-sensitive cells called rods (for dim light vision and detecting movement) and cones (for bright light vision and detecting colour). These cells convert light energy into electrical signals, which are then transmitted to the brain via the optic nerve for interpretation, allowing us to perceive the image upright and in colour.

Common Defects of Vision and Their Correction

Dispersion of White Light and Atmospheric Phenomena

When white light, which is composed of seven different colours, passes through a transparent medium like a glass prism, it splits into its constituent colours. This phenomenon is known as dispersion of light. Each colour of light has a different wavelength, and thus travels at a slightly different speed within the prism, causing them to bend at different angles. Violet light has the shortest wavelength and deviates the most, while red light has the longest wavelength and deviates the least. This results in the formation of a spectrum, commonly remembered by the acronym VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).

The Earth's atmosphere is a heterogeneous mixture of gases, dust particles, and water droplets. The varying optical density of these atmospheric layers causes atmospheric refraction. This phenomenon is responsible for several interesting observations:

1. Twinkling of Stars: Starlight enters the Earth's atmosphere and undergoes continuous refraction due to changes in the refractive index of different air layers. As the atmospheric conditions constantly fluctuate, the apparent position of the star changes rapidly, and the amount of light reaching our eyes varies, causing the stars to appear to twinkle.

1. Advanced Sunrise and Delayed Sunset: Due to atmospheric refraction, the Sun is visible to us approximately 2 minutes before the actual sunrise and remains visible for about 2 minutes after the actual sunset. This happens because the light from the Sun, when it is below the horizon, bends around the Earth's curvature towards the observer as it passes through denser air layers, making the Sun appear higher than its actual position.

Scattering of Light: Explaining Colours in Nature

The phenomenon of scattering of light occurs when light rays strike tiny particles or molecules and are redirected in various directions. The colour of the scattered light depends on the size of the scattering particles and the wavelength of the light. 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 intensely than longer wavelengths (like red and orange).

1. Why the Sky Appears Blue: Sunlight, as it passes through the atmosphere, encounters tiny air molecules (mostly nitrogen and oxygen). These molecules are much smaller than the wavelength of visible light. As per Rayleigh scattering, the shorter wavelength blue light is scattered more effectively in all directions than the longer wavelength red light. When we look at the sky, the scattered blue light reaches our eyes from all directions, making the sky appear blue.

1. Colour of the Sun at Sunrise and Sunset: At sunrise or sunset, the Sun's rays have to travel a much longer distance through the Earth's atmosphere to reach our eyes. During this long journey, most of the shorter wavelength blue light and other scattered colours are scattered away from our line of sight. The light that eventually reaches our eyes is predominantly composed of longer wavelength colours like red and orange, which are scattered the least. This is why the Sun and its surroundings appear reddish at dawn and dusk.

Exam Tips and Common Mistakes

1. Ray Diagrams: Practice drawing clear and accurate ray diagrams for defects of vision (myopia, hypermetropia) with and without corrective lenses. Label all parts precisely. Missing arrows on rays is a common error.
2. Terminology: Use precise scientific terms. For instance, distinguish between 'refraction' and 'dispersion'.
3. Cause and Effect: Clearly explain the cause of each visual defect and the reason for its correction. Similarly, for atmospheric phenomena, explain the underlying principle (e.g., varying refractive index for twinkling).
4. VIBGYOR Order: Remember the correct order of colours in the spectrum (VIBGYOR) and their relative deviation (Violet deviates most, Red least).
5. Scattering Explanation: When explaining why the sky is blue or the sun is red, always mention the wavelength dependence and the size of scattering particles.

Quick Revision Check

• Q: What is the function of the iris in the human eye? A: The iris is a dark muscular diaphragm that controls the size of the pupil, thereby regulating the amount of light entering the eye.
• Q: Name the defect of vision in which a person can see distant objects clearly but cannot see near objects distinctly. How is it corrected? A: This defect is Hypermetropia (farsightedness). It is corrected by using a convex lens of appropriate power.
• Q: Why does a glass prism split white light into its constituent colours? A: A glass prism splits white light into its constituent colours (dispersion) because different colours of light travel at different speeds through the prism, causing them to bend by different amounts.
• Q: Explain why the sky appears blue on a clear day. A: The sky appears blue due to the scattering of light by tiny air molecules in the atmosphere. Blue light, having a shorter wavelength, is scattered more intensely in all directions than other colours, making the sky appear blue.

Frequently Asked Questions