Chapter-11 Human Eye and Colourful World

Explanation:

The power of accommodation of the eye refers to its ability to change the shape of the lens and adjust its focal length in order to focus on objects at different distances. This is a key component of the eye's ability to form clear images on the retina, which is essential for clear vision.

The power of accommodation allows the eye to focus on objects at varying distances by adjusting the shape of the lens. When an object is far away, the lens becomes thinner and flatter, reducing its refractive power. When an object is closer, the lens becomes thicker and more curved, increasing its refractive power.

Explanation:

A person with myopia (nearsightedness) has difficulty seeing distant objects clearly because the light entering the eye is focused in front of the retina rather than on it. To correct this, a concave (or diverging) lens is needed to spread out the light rays before they enter the eye, allowing for proper focus on the retina.

The power of the concave lens needed to correct myopia is determined by the degree of the person's myopia, which is measured in diopters (D). The greater the myopia, the stronger the concave lens required.

In the given scenario, the person cannot see objects beyond 1.2 m distinctly, indicating a myopia of -0.83 D. To correct this, a concave lens with a power of +0.83 D (or -0.83 D, depending on the sign convention) should be used. This lens will diverge the incoming light rays by just the right amount so that they focus correctly on the retina, allowing the person to see distant objects clearly.

Explanation:

The far point and near point of the human eye with normal vision depend on the age of the person. For a young adult with normal vision, the far point is at infinity and the near point is typically around 25 centimeters.

The far point is the farthest distance from the eye at which an object can be seen clearly without accommodation, meaning that the lens is in its most relaxed state. For a normal eye, this point is at infinity, since the lens is able to focus parallel rays of light on the retina without any effort.

The near point, on the other hand, is the closest distance at which an object can be seen clearly with maximum accommodation, meaning that the lens is in its most curved state. For a young adult with normal vision, the near point is typically around 25 centimeters, as this is the closest distance at which the lens can focus light rays on the retina.

As a person ages, the lens becomes less flexible and loses some of its ability to change shape, causing a shift in both the near and far points. This leads to a condition called presbyopia, which is a gradual loss of the ability to focus on nearby objects.

Explanation:

If a student is having difficulty reading the blackboard while sitting in the last row, the child may be suffering from myopia (nearsightedness). This is a common refractive error in which the eye is able to focus on near objects, but has difficulty focusing on distant objects.

To correct myopia, the student would need a concave (or diverging) lens to help the eye focus light correctly onto the retina. The power of the lens needed to correct the myopia depends on the degree of the myopia and can be determined by an eye doctor during an eye exam.

Once the appropriate lens prescription is determined, the student can get glasses or contact lenses to correct the myopia. Wearing corrective lenses can help the student see distant objects clearly, including the blackboard while sitting in the last row.

It's important for the student to have regular eye exams to ensure that the corrective lenses remain up to date and that any changes in vision are detected and treated promptly.

Explanation:

(b) accommodation.

The ability of the human eye to focus on objects at different distances is due to accommodation, which is the adjustment of the focal length of the eye lens. The lens of the eye changes shape to focus light rays from objects at varying distances onto the retina. This process is controlled by the ciliary muscles in the eye, which adjust the shape of the lens to change its focal length.

Presbyopia is a condition in which the lens of the eye becomes less flexible with age, leading to difficulty in focusing on near objects. Near-sightedness (myopia) is a condition in which the eye is able to focus on near objects, but has difficulty focusing on distant objects. Far-sightedness (hyperopia) is a condition in which the eye is able to focus on distant objects, but has difficulty focusing on near objects.

Explanation:

(d) retina.

The human eye forms an image of an object on the retina, which is a layer of light-sensitive cells at the back of the eye. When light rays from an object enter the eye, they pass through the cornea, the pupil, and the lens, which help to focus the light onto the retina. The retina contains photoreceptor cells (rods and cones) that convert the light energy into electrical signals, which are transmitted to the brain via the optic nerve. The brain then processes these signals to form a visual image of the object that was viewed.

Explanation:

(c) 25 cm.

The least distance of distinct vision, also known as the near point, is the closest distance at which an object can be seen clearly with maximum accommodation (the lens is in its most curved state). For a young adult with normal vision, the near point is typically around 25 centimeters, which means that any object closer than 25 cm will appear blurred. This distance can vary depending on factors such as age and the individual's eye health.

Explanation:

(c) ciliary muscles.

The change in focal length of an eye lens is caused by the action of the ciliary muscles. The ciliary muscles are located within the eye and are attached to the lens by a suspensory ligament. When the ciliary muscles contract, they cause the suspensory ligament to relax, which allows the lens to become more curved and increases its refractive power. This allows the eye to focus on nearby objects.

Explanation:

We can use the following formula to relate the power of a lens to its focal length:

Power of lens = 1 / focal length of lens (in meters)

To convert dioptres to meters, we can use the conversion factor of 1 dioptre = 1 / 1 meter.

(i) For correcting distant vision:

Power of lens = -5.5 dioptres

Focal length of lens = 1 / (-5.5) = -0.182 m

The negative sign indicates that the lens is a concave lens, which is used to correct myopia (nearsightedness).

(ii) For correcting near vision:

Power of lens = +1.5 dioptres

Focal length of lens = 1 / (+1.5) = +0.67 m

The positive sign indicates that the lens is a convex lens, which is used to correct presbyopia (age-related farsightedness).

Explanation:

If the far point of a myopic person is 80 cm in front of the eye, it means that the person cannot see objects beyond 80 cm clearly. This indicates that the person has a refractive error known as myopia, or nearsightedness, which is caused by the eyeball being too long or the cornea being too curved.

To correct myopia, a concave lens is needed, which diverges light rays before they enter the eye and helps to bring the image into focus on the retina. The power of the lens required to correct myopia can be calculated using the formula:

Power of lens = -1 / focal length of lens (in meters)

where the negative sign indicates that the lens is a concave lens.

The focal length of the lens can be calculated using the formula:

1 / focal length of lens = 1 / distance of far point + 1 / distance of lens from the eye

where the distance of the far point is given as 80 cm or 0.8 m, and the distance of the lens from the eye is assumed to be negligible (i.e., close to the eye).

Substituting the given values into the formula, we get:

1 / focal length of lens = 1 / 0.8

focal length of lens = 1.25 meters

Therefore, the power of the concave lens required to correct the myopic person's vision is:

Power of lens = -1 / 1.25 = -0.8 diopters

The lens needed has a power of -0.8 diopters and is a concave lens.

Explanation:

In hypermetropia, the eyeball is too short or the cornea is not curved enough, causing light to focus behind the retina instead of on it. To correct hypermetropia, a convex lens is needed, which converges light rays before they enter the eye and helps to bring the image into focus on the retina.

To calculate the power of the lens required to correct hypermetropia, we can use the formula:

Power of lens = 1 / focal length of lens (in meters)

where the positive sign indicates that the lens is a convex lens.

The near point of the hypermetropic eye is given as 1 m or 100 cm, while the near point of the normal eye is given as 25 cm. The difference between these two values represents the amount of hypermetropia present in the eye, which is:

Amount of hypermetropia = 100 cm - 25 cm = 75 cm

This means that the hypermetropic eye cannot see objects clearly at distances closer than 75 cm. To correct this, a convex lens is needed that will provide additional convergence of light rays so that they focus correctly on the retina.

Using the formula above, we can calculate the power of the lens required as follows:

Power of lens = 1 / focal length of lens = 1 / (distance of near point + amount of hypermetropia)

Substituting the given values, we get:

Power of lens = 1 / (0.25 m + 0.75 m) = 1 / 1.00 m = +1.00 diopters

Therefore, a convex lens with a power of +1.00 diopters is required to correct the hypermetropic eye.

Explanation:

A normal eye has a certain range of accommodation or the ability to adjust its focus to see objects at different distances. When we try to focus on an object placed too close to our eyes, the ciliary muscles present in the eye contract to increase the curvature of the lens, thereby increasing its refractive power. This allows the light to converge properly and form a sharp image on the retina.

However, there is a limit to how much the lens can curve, and this limit is reached when we try to focus on objects placed too close to our eyes. At this point, the ciliary muscles cannot contract any further, and the lens cannot increase its refractive power any more. As a result, the light rays entering the eye are not refracted enough, and the image formed on the retina is blurred.

The minimum distance at which the eye can see clearly is called the near point, and for a normal eye, it is typically around 25 cm. Beyond this distance, the ciliary muscles can relax and the lens can flatten out, decreasing its refractive power and allowing the eye to focus on objects at greater distances.

Therefore, a normal eye cannot see objects placed closer than 25 cm clearly because the lens is unable to adjust its refractive power enough to focus the light rays properly on the retina.

Explanation:

When we increase the distance of an object from the eye, the image distance in the eye also increases. This is because the eye has to adjust its focal length to focus on the object at the new distance.

The human eye works like a camera, with a lens that refracts light rays and forms an image on the retina. The distance between the lens and the retina is fixed, so the eye adjusts its focal length by changing the shape of the lens through the action of the ciliary muscles.

When we look at an object that is far away, the ciliary muscles relax, causing the lens to flatten out and decrease its refractive power. This results in the light rays entering the eye being less refracted, and the image being formed at a greater distance from the lens, which is the retina.

In contrast, when we look at an object that is close to the eye, the ciliary muscles contract, causing the lens to become more convex and increase its refractive power. This allows the light rays entering the eye to be more refracted, and the image to be formed at a closer distance from the lens, which is again the retina.

Thus, as we increase the distance of an object from the eye, the eye adjusts its focal length by decreasing its refractive power, and the image distance in the eye increases accordingly.

Explanation:

Stars appear to twinkle because of the effect of the Earth's atmosphere on the light passing through it. The atmosphere is not uniform, and it has many layers with different temperatures, pressures, and densities. When light from a star enters the Earth's atmosphere, it passes through these layers and gets refracted, or bent, in different directions.

As the light passes through these different layers, its path is constantly changing, causing the apparent position of the star to shift slightly. This creates the twinkling effect that we see. The amount of twinkling depends on the amount of turbulence in the atmosphere and the angle at which the star's light passes through it. When the atmosphere is more turbulent, the stars appear to twinkle more.

The twinkling effect is more pronounced for stars that are closer to the horizon, as their light has to pass through more of the Earth's atmosphere than the stars that are overhead. This is why stars close to the horizon appear to twinkle more than stars that are higher in the sky.

To astronomers, the twinkling effect is a problem because it distorts the light coming from stars and makes them more difficult to observe and study. To minimize the effect of atmospheric turbulence, observatories are often built at high altitudes or in areas with very stable atmospheric conditions. Additionally, telescopes and other astronomical instruments are sometimes equipped with adaptive optics systems that can correct for the distortions caused by atmospheric turbulence in real-time.

Explanation:

Unlike stars, planets do not twinkle in the night sky. This is because planets are much closer to the Earth than stars and they appear as small disks rather than point sources of light. When the light from a planet enters the Earth's atmosphere, it does not get refracted and scattered as much as the light from a star, and hence, the effect of atmospheric turbulence is much less pronounced.

Another reason why planets do not twinkle is that they do not emit their own light like stars. Instead, they reflect the light of the Sun. The light reflected by planets is much weaker than the light emitted by stars, and hence, it is less affected by atmospheric turbulence.

Finally, planets are much closer to the Earth than stars and their light has to pass through a much smaller column of the Earth's atmosphere, which reduces the amount of refraction and scattering. This means that the light from planets is less affected by the variations in air density and temperature, which are responsible for the twinkling effect.

Overall, the combination of the small size of the planet's disk, the weak reflected light, and the small column of atmosphere that the light has to pass through all contribute to the fact that planets do not twinkle.

Explanation:

Because of a process known as air scattering, the Sun appears reddish early in the morning or late in the afternoon. The Earth's atmosphere is thicker when the Sun is low on the horizon, making it more difficult for sunlight to reach our eyes. Blue and green light, which have shorter wavelengths than red and orange light, is scattered more by the Earth's atmosphere than red and orange light, which have longer wavelengths. Because of this, when the Sun is low on the horizon, we can notice a reddish or orange glow.

The daytime blueness of the sky is a result of an effect comparable to this one. Blue light scatters more than other colours of light throughout the day, therefore it is more difficult to see.

Explanation:

The sky appears dark instead of blue to an astronaut in space because there is no atmosphere to scatter sunlight and create the blue color. On Earth, the gases and particles in the atmosphere scatter sunlight in all directions, and blue light is scattered more than other colors of light. This is why the sky appears blue to us.

However, in space, there is no atmosphere to scatter the sunlight, so the sky appears dark. This is because the sunlight travels in a straight line and does not get scattered by any particles or gases. As a result, the light from the stars and other celestial objects is not washed out by scattered sunlight, and the night sky appears much darker than it does on Earth.