The Human Eye and the Colourful World-QnA

Q1: What is the least distance of distinct vision for a normal human eye?

25 cm

Q2: Name the part of the eye that controls the amount of light entering the eye.

Pupil

Q3: What is persistence of vision?

The image remains on the retina for about 1/16 second after light ceases.

Q4: What causes the twinkling of stars?

Atmospheric refraction of starlight.

Q5: Which lens is used to correct myopia?

Concave lens

Q6: What is the phenomenon responsible for the formation of rainbow?

Dispersion and total internal reflection of sunlight in water droplets.

Q7: Name the defect that occurs due to lack of power of accommodation.

Presbyopia

Q8: What causes the blue color of the sky?

Scattering of sunlight by the atmosphere.

Q9: What is atmospheric refraction?

Bending of light due to varying refractive indices in the atmosphere.

Q10: Which part of the eye focuses image on the retina?

Eye lens

Q11: What are rods and cones?

Light-sensitive cells in the retina; rods detect light intensity, cones detect color.

Q12: What is meant by the term 'accommodation' in the human eye?

The ability of the eye lens to adjust its focal length for distant and near objects.

Q13: Name a common defect of vision in old age.

Presbyopia

Q14: What is the range of wavelength for visible light?

400 nm (violet) to 700 nm (red)

Q15: Why does the Sun appear reddish at sunrise and sunset?

Because blue light is more scattered, leaving mainly red to reach our eyes through longer atmospheric path.

Q1: What is color blindness?

Inability to distinguish some colors, typically due to absence of certain types of cone cells.

Q2: What is meant by dispersion of light?

Splitting up of white light into its constituent colors by a prism.

Q3: Why do we see the Sun for a few minutes even after it has set?

Due to atmospheric refraction, sunlight bends so we see the Sun even after it is below the horizon.

Q4: What are bifocal lenses and where are they used?

They have two parts with different focal lengths, used to correct presbyopia.

Q5: How does a normal eye focus on distant and nearby objects?

By changing the curvature of its lens using ciliary muscles for focusing.

Q6: Why is red light used in danger signals?

Red has longest wavelength and is least scattered, making it visible from a distance.

Q7: What is meant by ‘least distance of distinct vision’?

The minimum distance at which the eye can see objects clearly; for normal eye, it is 25 cm.

Q8: Explain twinkling of stars in your own words.

Starlight bends due to changing refractive layers in the atmosphere, causing its intensity to fluctuate.

Q9: What is the function of the retina?

Retina receives the image and transmits signals to the brain through optic nerves.

Q10: Why does a prism split white light but a glass slab does not?

Because in a prism, sides are inclined and light rays emerge at different angles, enabling dispersion.

Q1: Describe the structure of the human eye.

The human eye is a complex, nearly spherical organ responsible for our sense of sight. Acting much like a camera, the eye is composed of several specialized structures working together to focus light and produce clear images of the world around us. The outermost, transparent layer is the cornea, which allows light to enter and bends it toward the inner parts of the eye. Right behind the cornea is the aqueous humour, a clear liquid that maintains pressure and provides nutrients to surrounding tissues.

The colored part of the eye is called the iris. It controls the size of the pupil, which is the black circular opening in the center of the iris. The pupil regulates the amount of light entering the eye—becoming larger in dim light and smaller in bright light. Behind the pupil lies the lens, a transparent, flexible structure that further focuses the light rays by changing its shape. This process of changing the lens curvature—accommodation—is handled by the ciliary muscles, allowing us to see objects at various distances clearly.

Inside the eye, the light passes through a gel-like substance called the vitreous humour, which helps maintain the spherical shape of the eye. The light eventually reaches the retina, a thin layer of nerve tissue at the back of the eyeball. The retina contains two types of light-sensitive cells: rods, which detect brightness and function well in dim light, and cones, which are responsible for color vision and detailed sight.

Once the retina receives the incoming light and forms an image, it converts this visual information into electrical signals. These signals are then sent to the brain via the optic nerve, enabling us to perceive and interpret what we see. Surrounding and protecting these delicate structures is the tough, white outer layer known as the sclera.

Each part of the human eye plays a critical role in the process of vision, and any defect or damage to these parts can result in vision problems

Q2: Explain, with reason, why planets do not twinkle.

Planets do not twinkle because they are much closer to Earth than stars and appear as tiny discs rather than point sources of light. As the light from a planet passes through different layers of Earth's atmosphere, it gets refracted, but since it comes from different points on the disc, the atmospheric variations average out. Therefore, any small intensity changes get neutralized, and the overall light remains steady. In contrast, the light from a distant star comes as a single point and is more affected by atmospheric turbulence, causing twinkling.

Q3: How does atmospheric refraction cause advanced sunrise and delayed sunset?

Atmospheric refraction causes advanced sunrise and delayed sunset because the Earth's atmosphere bends (refracts) the light rays from the Sun. When the Sun is just below the horizon, its light passes through the layers of the atmosphere and bends towards the ground. As a result, we are able to see the Sun earlier than its actual rise, which is called advanced sunrise. Similarly, after the Sun has set below the horizon, the refracted light still reaches our eyes, so we continue to see the Sun for a short time—this is delayed sunset. Thus, due to atmospheric refraction, the day appears slightly longer.

Q4: Describe the process of formation of a rainbow.

A rainbow is formed by the interaction of sunlight with tiny raindrops in the atmosphere. The process involves three main phenomena:

1. Refraction: When sunlight enters a raindrop, it slows down and bends as it passes from air into water. During this bending, white sunlight splits into its constituent colors (dispersion).
2. Internal Reflection: The refracted and dispersed light strikes the inner surface of the raindrop and reflects back inside the drop.
3. Refraction Again: As the light exits the raindrop, it bends again. Each color emerges at a different angle, with red deviating least and violet most.

Q5: How are defects of vision corrected? Explain with examples.

Defects of vision occur when the eye cannot properly focus light on the retina, leading to blurred or unclear vision. These defects can be corrected using suitable lenses.

1. Myopia (Near-Sightedness): In this defect, a person can see nearby objects clearly but distant objects appear blurred. This happens when the eyeball is too long or the eye lens is too powerful, causing the image to form in front of the retina. It is corrected using a concave lens, which diverges the light rays and focuses the image on the retina.
2. Hypermetropia (Far-Sightedness): Here, a person can see distant objects clearly but not nearby ones. It occurs when the eyeball is too short or the lens is too weak, causing the image to form behind the retina. It is corrected using a convex lens, which converges the incoming light rays so they focus properly on the retina.
3. Presbyopia: This defect usually occurs with aging when the eye’s ciliary muscles weaken, reducing the lens’s ability to adjust its focal length. People with presbyopia have difficulty seeing both near and distant objects clearly. It is corrected using bifocal lenses, which combine concave and convex lenses for distance and reading vision respectively.

Q1: What is meant by power of accommodation of the eye?

Power of accommodation of the eye refers to the ability of the eye lens to adjust its focal length so that it can clearly focus both distant and nearby objects onto the retina. This is achieved by the ciliary muscles changing the curvature of the lens to increase or decrease its focal length, allowing the eye to see objects at varying distances sharply.

Q2: 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?

A person with a myopic (near-sighted) eye who cannot see objects clearly beyond 1.2 m should use a concave lens (also called a diverging lens) as the corrective lens.

Concave lenses help diverge the incoming light rays so they can be properly focused on the retina, allowing the person to see distant objects distinctly.

Q3: What is the far point and near point of the human eye with normal vision?

For a human eye with normal vision:

• Near point:
  The minimum distance at which an object can be seen clearly is called the near point. For a normal human eye, the near point is about 25 cm from the eye.
• Far point:
  The maximum distance up to which the eye can see objects clearly is called the far point. For a normal vision, the far point is at infinity.

Q4: 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?

If a student has difficulty reading the blackboard from the last row, the child is likely suffering from myopia (near-sightedness). This means the student can see nearby objects clearly but has trouble seeing distant objects.

Correction:
Myopia is corrected using concave lenses (diverging lenses), which help focus distant objects onto the retina so the student can see the blackboard clearly.