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Chapter 11: The Human Eye And The Colourful World
The human eye is an amazing sense organ that allows us to perceive the world around us in its vibrant colours. This chapter explores the structure and functioning of the human eye, common vision defects and their correction, and some fascinating natural optical phenomena caused by light's interaction with the atmosphere.
The Human Eye
The human eye functions similarly to a camera, forming an image on a light-sensitive screen called the retina.
Structure and Function of Parts of the Human Eye:
- Cornea: The thin, transparent, bulging membrane on the front surface of the eyeball. Most of the refraction of light entering the eye occurs at the outer surface of the cornea.
- Iris: A dark, muscular diaphragm located behind the cornea. It controls the size of the pupil.
- Pupil: An opening in the centre of the iris that regulates and controls the amount of light entering the eye. In bright light, the iris contracts the pupil (smaller aperture); in dim light, the iris expands the pupil (larger aperture).
- Eye Lens: A crystalline, fibrous, jelly-like lens located behind the iris. It provides the finer adjustment of focal length needed to focus light from objects at different distances onto the retina. It forms a real, inverted image on the retina.
- Retina: A delicate membrane at the back of the eye containing a large number of light-sensitive cells (rods and cones). These cells get activated by light and generate electrical signals.
- Optic Nerves: Transmit the electrical signals generated by the retina to the brain.
The brain interprets these electrical signals, processing the information to allow us to perceive objects and colours as they are.
Power Of Accommodation
The eye lens is not rigid; its curvature can be modified by the ciliary muscles, thereby changing its focal length. This ability to adjust focal length to focus on objects at different distances is called accommodation.
- When viewing distant objects, ciliary muscles are relaxed, the lens becomes thin, and its focal length increases, allowing focus on the retina.
- When viewing nearby objects, ciliary muscles contract, the lens becomes thicker (curvature increases), and its focal length decreases, allowing focus on the retina.
The focal length of the eye lens has limitations; it cannot be decreased below a certain minimum. The minimum distance at which objects can be seen most distinctly without strain is the least distance of distinct vision or the near point. For a normal young adult, this is about 25 cm. The farthest point at which the eye can see clearly is the far point. For a normal eye, the far point is infinity.
A normal eye can comfortably see objects located between 25 cm and infinity by adjusting its focal length through accommodation.
Cataract: A condition where the crystalline lens becomes milky and cloudy, often in old age, causing partial or complete loss of vision. Vision can often be restored through cataract surgery.
Defects Of Vision And Their Correction
Sometimes, the eye loses its ability to accommodate effectively, leading to vision defects where objects cannot be seen clearly and comfortably. These refractive defects can often be corrected using appropriate spherical lenses (spectacles).
There are three common refractive defects of vision:
Myopia
Myopia, or near-sightedness, is a defect where a person can see nearby objects clearly but has difficulty seeing distant objects distinctly. The far point for a myopic eye is nearer than infinity.
Causes of Myopia:
- Excessive curvature of the eye lens.
- Elongation of the eyeball.
In a myopic eye, light rays from distant objects are focused in front of the retina instead of on the retina.
Correction: Myopia is corrected by using a concave lens of suitable power. The concave lens diverges the light rays slightly before they enter the eye, causing the image of distant objects to be formed on the retina.
Hypermetropia
Hypermetropia, or far-sightedness, is a defect where a person can see distant objects clearly but has difficulty seeing nearby objects distinctly. The near point for a hypermetropic eye is farther away from the normal near point (25 cm).
Causes of Hypermetropia:
- The focal length of the eye lens is too long.
- The eyeball has become too small.
In a hypermetropic eye, light rays from nearby objects are focused behind the retina instead of on the retina.
Correction: Hypermetropia is corrected by using a convex lens of appropriate power. The convex lens converges the light rays slightly before they enter the eye, providing the additional focusing power needed to form the image on the retina.
Presbyopia
Presbyopia is a vision defect related to age, where the eye gradually loses its power of accommodation. The near point recedes, making it difficult to see nearby objects comfortably.
Causes of Presbyopia:
- Gradual weakening of the ciliary muscles.
- Diminishing flexibility of the eye lens.
Correction: Presbyopia is typically corrected using convex lenses. Sometimes, individuals suffer from both myopia and presbyopia, requiring bi-focal lenses (with a concave lens in the upper part for distant vision and a convex lens in the lower part for near vision).
Contact lenses or surgical interventions can also correct refractive defects.
Refraction Of Light Through A Prism
Light refracts (bends) when passing through transparent materials like a glass prism. Unlike a rectangular glass slab (where the emergent ray is parallel to the incident ray), the inclined surfaces of a prism cause the emergent ray to bend at an angle to the incident ray.
When a ray of light enters a triangular glass prism from air:
- At the first surface (air to glass), the ray bends towards the normal (as it enters a denser medium).
- At the second surface (glass to air), the ray bends away from the normal (as it exits to a rarer medium).
The specific shape of the prism results in the emergent ray being deviated from the original direction of the incident ray. The angle between the direction of the incident ray and the direction of the emergent ray is called the angle of deviation ($\angle D$).
Dispersion Of White Light By A Glass Prism
One of the remarkable phenomena associated with prisms is the splitting of white light into its constituent colours.
When white light (like sunlight) passes through a glass prism, it is split into a band of seven distinct colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). This phenomenon is called dispersion of white light.
The band of coloured components of a light beam is called a spectrum.
Dispersion occurs because different colours of light have different wavelengths, and the refractive index of the prism material is slightly different for different wavelengths. This causes different colours to bend at slightly different angles as they pass through the prism.
- Violet light (shortest visible wavelength) bends the most.
- Red light (longest visible wavelength) bends the least.
This differential bending causes the separation of colours.
Isaac Newton was the first to demonstrate that white light is composed of these seven colours by obtaining a spectrum using a prism. He also showed that these seven colours can be recombined to form white light by passing the spectrum through an identical prism placed in an inverted position.
A rainbow is a natural spectrum formed by the dispersion of sunlight by tiny water droplets (acting as small prisms) in the atmosphere, typically seen after a rain shower. Sunlight entering a raindrop is refracted and dispersed, then reflected internally, and finally refracted again upon exiting the raindrop, producing the visible spectrum of colours. Rainbows are always observed in the direction opposite to the Sun.
Atmospheric Refraction
Atmospheric refraction is the refraction (bending) of light as it passes through the Earth's atmosphere. The Earth's atmosphere has layers with different densities and temperatures, causing the refractive index to vary. This leads to the bending of light from celestial objects.
Phenomena caused by Atmospheric Refraction:
- Apparent wavering/flickering: Objects seen through hot air rising above a fire appear to waver because hot air has a lower refractive index than cooler air, and the movement of air causes the apparent position of the object to fluctuate.
- Twinkling of stars: Starlight undergoes continuous refraction as it passes through the atmosphere with constantly changing refractive index layers. Stars are effectively point sources. The varying path of light causes the amount of starlight reaching the eye to fluctuate (appears brighter or fainter), resulting in twinkling.
- Planets do not twinkle: Planets are much closer than stars and appear as extended sources. Light from different parts of a planet is refracted differently, but the average effect cancels out the twinkling.
- Advance sunrise and delayed sunset: Atmospheric refraction causes sunlight to bend towards the Earth's surface. At sunrise, we see the Sun about 2 minutes before it actually crosses the horizon because its light is refracted upwards. Similarly, at sunset, we continue to see the Sun for about 2 minutes after it has dipped below the horizon.
Scattering Of Light
When light interacts with particles in a medium, it can be scattered in different directions. This phenomenon, called scattering of light, explains several natural optical effects.
The Earth's atmosphere contains a heterogeneous mixture of minute particles, including air molecules, dust, smoke, and water droplets. When light strikes these particles, it is scattered.
Tyndall Effect
The Tyndall effect is the scattering of light by colloidal particles in a medium, making the path of the light beam visible. This is observed when a beam of sunlight enters a dusty or smoky room through a small hole, making the dust/smoke particles visible as they scatter the light. Tiny water droplets in mist in a dense forest canopy can also cause the Tyndall effect.
The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter light of shorter wavelengths (blue end of the spectrum) more effectively. Larger particles scatter light of longer wavelengths or may scatter all wavelengths equally, making the scattered light appear white.
Why Is The Colour Of The Clear Sky Blue?
The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. These particles are more effective at scattering light of shorter wavelengths (blue and violet) compared to longer wavelengths (red and orange).
As sunlight passes through the atmosphere, the fine particles scatter the blue light components much more strongly than the red components. This scattered blue light enters our eyes, making the clear sky appear blue during the day.
If the Earth had no atmosphere, there would be no scattering, and the sky would appear dark, as seen by astronauts in space.
Red light is least scattered by particles like fog or smoke, making it useful for danger signals that need to be visible over long distances.
Colour Of The Sun At Sunrise And Sunset
The Sun often appears reddish-orange during sunrise and sunset.
At sunrise and sunset, sunlight travels a longer distance through thicker layers of the Earth's atmosphere before reaching our eyes. During this longer path, most of the shorter wavelengths (blue and violet light) are scattered away by atmospheric particles.
The light that reaches our eyes after this extensive scattering is predominantly composed of longer wavelengths (red, orange, yellow). This makes the Sun and the surrounding sky appear reddish at sunrise and sunset.
At noon, the Sun is overhead, and sunlight travels a relatively shorter distance through the atmosphere. Less scattering of shorter wavelengths occurs, so the Sun appears white or yellowish.
Intext Questions
Page No. 190
Question 1. What is meant by power of accommodation of the eye?
Answer:
Question 2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Answer:
Question 3. What is the far point and near point of the human eye with normal vision?
Answer:
Question 4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
Answer:
Exercises
Question 1. The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia.
(b) accommodation.
(c) near-sightedness.
(d) far-sightedness.
Answer:
Question 2. The human eye forms the image of an object at its
(a) cornea.
(b) iris.
(c) pupil.
(d) retina.
Answer:
Question 3. The least distance of distinct vision for a young adult with normal vision is about
(a) 25 m.
(b) 2.5 cm.
(c) 25 cm.
(d) 2.5 m.
Answer:
Question 4. The change in focal length of an eye lens is caused by the action of the
(a) pupil.
(b) retina.
(c) ciliary muscles.
(d) iris.
Answer:
Question 5. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs 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?
Answer:
Question 6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer:
Question 7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Answer:
Question 8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer:
Question 9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer:
Question 10. Why do stars twinkle?
Answer:
Question 11. Explain why the planets do not twinkle.
Answer:
Question 12. Why does the Sun appear reddish early in the morning?
Answer:
Question 13. Why does the sky appear dark instead of blue to an astronaut?
Answer: