Chapter – 10

The Human Eye and the Colourful World

In this post we have given the detailed notes of class 10 Science Chapter 10 (The Human Eye and the Colourful World) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 10: The 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 of colours around us. The eye acts like a camera, forming an inverted, real image on the light-sensitive screen called the retina.

Parts of the Human Eye and Their Functions 🔬

• Sclera (Sclerotic): The human eye is roughly spherical in shape. Its outermost covering is a tough, white and opaque layer called the sclera. It protects the inner parts of the eye.
• Choroid: A dark coloured membrane lying just inside the sclera. It prevents internal reflection of light inside the eye.
• Cornea: A thin transparent membrane on the front part of the eye. Most of the refraction of light entering the eye occurs at the outer surface of the cornea.
• Eyeball: The eye is nearly spherical in shape with an approximate diameter of 2.3 cm.
• Eye Lens (Crystalline Lens): A convex lens made of a fibrous, jelly-like material that converges light to focus it on the retina. It only has to adjust its focal length by a small amount to focus objects at various distances on the retina.
• Iris: A dark muscular diaphragm situated behind the cornea. It controls the size of the pupil.
• Pupil: Acts like a variable aperture whose size is controlled by the iris. It regulates and controls the amount of light entering the eye.
• Ciliary Muscles: Control the curvature of the eye lens. A change in the curvature of the eye lens changes its focal length, allowing us to see objects clearly at varying distances.
• Retina: A delicate, thin membrane on which a large number of light-sensitive cells are present. On being illuminated, these cells generate electrical signals which are sent to the brain through the optic nerves. The brain interprets these signals and we perceive objects as they are.

Far Point and Near Point

• Far Point: The farthest point up to which the eye can see objects clearly. For a normal eye, it is at infinity.
• Near Point: The minimum distance at which objects can be seen most distinctly without strain.

Power of Accommodation

• The ability of the eye lens to adjust its focal length is called the power of accommodation of the eye.

Least Distance of Distinct Vision

• For a young adult with normal vision, the near point is about 25 cm from the eye. This distance is called the least distance of distinct vision, denoted by D.

Defects of Vision and Their Correction 👓

Cataract

• In some elderly people, the crystalline lens of the eye becomes milky and cloudy. This condition is called cataract, and it causes partial or complete loss of vision.
• Cataract is treated by surgical procedures, after which vision can be restored.

Myopia (Near-sightedness)

A person with this defect can see nearby objects clearly but cannot see distant objects distinctly. The far point of such a defective eye is nearer than infinity.

Causes:

• Excessive curvature of the eye lens.
• Elongation of the eyeball.

Correction: This defect can be corrected by using a concave lens (diverging lens) of suitable power. A concave lens of appropriate power brings the image back onto the retina, correcting the defect.

Hypermetropia (Far-sightedness)

A person with this defect can see distant objects clearly but cannot see nearby objects distinctly. The near point of such a person is farther away than the normal near point (25 cm).

Causes:

• Excessive focal length of the eye lens (eye lens becomes too flat).
• Shortening of the eyeball.

Correction: This defect can be corrected by using a convex lens (converging lens) of suitable power. Convex lenses provide the additional focusing power required to focus the image on the retina.

Presbyopia

With ageing, the power of accommodation of the human eye decreases. For most people the near point gradually recedes. This defect is called presbyopia.

Cause: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the crystalline lens.

Correction of Presbyopia:

• By using a convex lens.
• Sometimes a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses in which the upper part is a concave lens and the lower part is a convex lens.
• Nowadays, contact lenses and surgical interventions are also used to correct vision defects.

Advantage of Having Two Eyes on the Front of the Head:

• It gives us the benefit of stereoscopic (three-dimensional) vision.
• It widens our field of vision.
• It helps us perceive faint objects better.

Refraction of Light Through a Prism 🔺

• A prism has two triangular bases and three rectangular lateral surfaces.
• Angle of Prism: The angle between the two lateral faces of the prism.
• Angle of Deviation: The angle between the incident ray and the emergent ray.

Dispersion of White Light by a Glass Prism 🌈

When white sunlight passes through a glass prism, the prism splits the white light into a band of seven colours. These seven colours are violet, indigo, blue, green, yellow, orange and red (remembered by the acronym VIBGYOR). This band of the component colours of light is called a spectrum. The splitting of light into its component colours is called dispersion.

The Rainbow

• A rainbow is a natural spectrum appearing in the sky after rainfall, caused due to the dispersion of sunlight by tiny water droplets present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun.
• The tiny water droplets act like small prisms. They refract and disperse the incident sunlight, then internally reflect it, and finally refract it again as it comes out of the raindrop. Due to dispersion of light and internal reflection, different colours reach the observer’s eye.
• After passing through a prism, the different colours of light bend at different angles with respect to the incident ray.
• Red light bends the least while violet light bends the most.

Isaac Newton and the Spectrum

• Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight. He tried to split the colours of the spectrum of white light further by using a second identical prism, but could not get any more colours.
• He then placed a second identical prism in an inverted position with respect to the first prism, and observed that it resulted in a beam of white light emerging from the second prism. This led Newton to conclude that sunlight is made up of seven colours.

Atmospheric Refraction 🌌

Refraction of light caused by the earth’s atmosphere due to variations in its physical conditions (density) is called atmospheric refraction.

Effects of Atmospheric Refraction

• Twinkling of stars.
• Advance sunrise and delayed sunset.
• Stars appearing slightly higher than their actual position.
• The apparent change in position of an object when seen through hot air.

1. Twinkling of Stars: A distant star appears to us as a point source of light. Since the physical conditions of the atmosphere are not stationary, the path of light rays coming from the star keeps changing slightly, so the apparent position of the star fluctuates and the amount of starlight entering the eye keeps changing. This makes the star sometimes appear brighter and sometimes fainter, which is the effect of twinkling.

2. Advance Sunrise and Delayed Sunset: Due to atmospheric refraction, the Sun is visible to us about 2 minutes before the actual sunrise, and remains visible for about 2 minutes after the actual sunset.

3. Stars Appearing Higher Than Their Actual Position: As starlight enters the earth’s atmosphere, it undergoes continuous refraction before reaching the earth’s surface. This refraction occurs in a medium of gradually changing refractive index, and bends the starlight continuously towards the normal. Hence, a star appears slightly higher (its apparent position is different) than its actual position when viewed near the horizon.

4. Apparent Change in Position of Objects Viewed Through Hot Air: The air just above a flame or a hot surface becomes hotter than the air further above it. Hot air is less dense than the cooler air above it, and its refractive index is slightly lower. Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of an object seen through hot air keeps changing (this causes the shimmering or flickering effect seen near a flame or on a hot road).

Scattering of Light ✨

Tyndall Effect: When a beam of light strikes fine particles suspended in the atmosphere, such as smoke, tiny water droplets, suspended dust particles and molecules of air, the path of the beam becomes visible. This phenomenon of scattering of light by colloidal particles is called the Tyndall effect.

Examples:

• The Tyndall effect can be observed when a fine beam of sunlight enters a smoke-filled room through a small hole.
• The Tyndall effect can also be observed when sunlight passes through the canopy of a dense forest.

Rayleigh’s Law of Scattering:

• Amount of scattering, a ∝ 1/λ⁴
• λ = wavelength of the light ray

What Does the Colour of Scattered Light Depend On?

The colour of the scattered light depends on the size of the scattering particles. For example:

• Very fine particles predominantly scatter blue light.
• Larger sized particles scatter light of longer wavelengths.
• If the size of the scattering particles is quite large, the scattered light may even appear white.

Why Don’t Planets Twinkle? Planets are much closer to the earth than stars, and hence they can be considered as a source of a bundle of many point sources of light. The average value of the total amount of light entering our eye from all the individual point-sized sources changes marginally, causing almost no change in the brightness of the planet. Hence, planets do not twinkle.

Why Is the Danger Signal Red in Colour?

The light of a danger signal is red in colour. Red light is scattered the least by fog or smoke. Hence, it is visible even from a distance.

Why Is the Colour of the Clear Sky Blue? 🔵

• Molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. These particles are more effective in scattering light of shorter wavelengths compared to light of longer wavelengths.
• The wavelength of red light is about 1.8 times more than the wavelength of blue light.
• Therefore, when sunlight passes through the atmosphere, the fine particles in air scatter blue colour more strongly than red. This scattered blue light enters our eyes, making the sky appear blue.

Why Do Clouds Appear White? Clouds are made up of fine water droplets that are large enough in size compared to the wavelength of visible light. When white light falls on these larger particles, it gets reflected or scattered nearly equally in all directions and in all colours. Since all colours of white light are reflected/scattered almost equally, clouds appear white to us.

Key Points to Remember

• The eye lens is a convex lens that forms a real, inverted image on the retina; the retina converts this into electrical signals sent to the brain via the optic nerve. 👁️
• The normal near point of the eye is 25 cm and the far point is at infinity. 📏
• Myopia (near-sightedness) is corrected using a concave lens; Hypermetropia (far-sightedness) is corrected using a convex lens; Presbyopia is corrected using bifocal lenses. 👓
• Dispersion is the splitting of white light into its seven component colours (VIBGYOR) by a prism, forming a spectrum. 🌈
• A rainbow is a natural spectrum formed by the dispersion of sunlight through raindrops, always seen opposite to the Sun. 🌦️
• Atmospheric refraction causes twinkling of stars, advance sunrise, delayed sunset and the apparent higher position of stars. ✨
• Scattering of light by colloidal particles is called the Tyndall effect; the scattering intensity is inversely proportional to the fourth power of wavelength (Rayleigh’s law). 🔬
• The sky appears blue because air molecules scatter blue light more than red light; clouds appear white because large water droplets scatter all colours almost equally. ☁️
• Danger signals are red because red light is scattered the least and is visible from the farthest distance. 🚨
• Planets do not twinkle because they are extended sources of light, unlike stars which are point sources. 🪐

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