NCERT Class 10 Science Solutions: The Human Eye and the Colourful World

Question:

The human eye functions like a camera. Light enters the eye through the pupil and is focused by the cornea and lens. This focused light forms an image on a light-sensitive layer at the back of the eye, which then sends signals to the brain for interpretation.


The question asks where the human eye forms the image of an object. Let’s analyze the options:
A. Cornea: The cornea is the transparent outer layer of the eye that refracts (bends) light as it enters. It does not form the final image.
B. Iris: The iris is the colored part of the eye that controls the size of the pupil, regulating the amount of light entering the eye. It does not form the image.
C. Pupil: The pupil is the opening in the center of the iris. Light passes through the pupil, but the image is not formed here.
D. Retina: The retina is the light-sensitive layer at the back of the eye that contains photoreceptor cells (rods and cones). When light is focused by the cornea and lens, it forms a sharp, inverted image on the retina. This is where the process of converting light into electrical signals that are sent to the brain begins.

Therefore, the human eye forms the image of an object at its retina.

The final answer is $\boxed{D}$.

Question:

The question is about the least distance of distinct vision, which is a fundamental concept in optics related to the human eye’s ability to focus on near objects. It’s the closest an object can be to the eye and still be seen clearly. This distance is a standard value for a healthy young adult.


The least distance of distinct vision refers to the minimum distance at which an object can be viewed by the eye without strain, and the image formed is clear. For a normal human eye, this distance is approximately 25 centimeters (cm). This is the point where the eye’s ciliary muscles can still adjust the focal length of the lens sufficiently to form a sharp image on the retina. Distances closer than this lead to blurred vision because the eye cannot accommodate any further. Looking at the options provided:
A. 25 m is far too large, it’s about the length of a small building.
B. 2.5 cm is extremely close, almost touching the eye, which would result in blurred vision.
C. 25 cm is the standard accepted value for the least distance of distinct vision.
D. 2.5 m is also too far to be considered the *least* distance of distinct vision.

Therefore, the correct answer is 25 cm.

Question:

The human eye has a lens whose focal length can be adjusted to focus on objects at different distances. This process is called accommodation. The muscles responsible for changing the shape of the lens and thus its focal length are crucial for clear vision.


The eye lens is a flexible structure. To focus on distant objects, the ciliary muscles relax, which allows the suspensory ligaments to pull the lens, making it thinner and increasing its focal length. To focus on nearby objects, the ciliary muscles contract. This contraction relaxes the tension in the suspensory ligaments, allowing the lens to become thicker and shorter, thus decreasing its focal length. The pupil is the opening that controls the amount of light entering the eye. The iris is the colored part that controls the size of the pupil. The retina is the light-sensitive layer at the back of the eye where the image is formed. Therefore, the change in focal length of the eye lens is caused by the action of the ciliary muscles.

The final answer is $\boxed{C}$.

Question:

The power of a lens is the reciprocal of its focal length in meters. Power (P) in dioptres = 1 / Focal length (f) in meters. This relationship is fundamental to correcting vision defects.


The problem provides the power of lenses required to correct distant and near vision. We need to find the focal length for each case.

Part (i): Distant Vision Correction
The power of the lens for correcting distant vision is given as P_distant = -5.5 dioptres.
The formula relating power (P) and focal length (f) is: P = 1/f.
Therefore, to find the focal length, we rearrange the formula: f = 1/P.
Substitute the given power for distant vision: f_distant = 1 / (-5.5) dioptres.
Calculate the value of f_distant. Remember that the focal length will be in meters.

Part (ii): Near Vision Correction
The power of the lens for correcting near vision is given as P_near = +1.5 dioptres.
Using the same formula f = 1/P:
Substitute the given power for near vision: f_near = 1 / (+1.5) dioptres.
Calculate the value of f_near. The focal length will be in meters.

Final Answer should clearly state the focal length for both distant and near vision. It might be useful to convert the focal length from meters to centimeters for easier understanding. To convert meters to centimeters, multiply by 100.

Question:

The key concept is the scattering of sunlight by the Earth’s atmosphere. Specifically, Rayleigh scattering is responsible for the blue color of the sky.


The sky appears blue to us on Earth because of the Earth’s atmosphere. Sunlight, which is white light, is made up of all the colors of the rainbow. When sunlight enters our atmosphere, it interacts with the tiny gas molecules present (like nitrogen and oxygen). These molecules scatter the sunlight in all directions. Blue light, having a shorter wavelength, is scattered much more effectively than other colors like red or yellow. This scattered blue light reaches our eyes from all parts of the sky, making it appear blue.

An astronaut in space, however, is above the Earth’s atmosphere. In space, there is no atmosphere to scatter the sunlight. Therefore, when an astronaut looks at the sky, they are essentially looking into the vast emptiness of space. Without any atmospheric particles to scatter the light, the sunlight travels in straight lines. The background of space is essentially black because there is nothing to reflect or scatter light towards the astronaut’s eyes. The only light visible is directly from the sun or from celestial bodies. Hence, the sky appears dark or black to an astronaut.

Question:

The human eye acts like a convex lens. The lens formula relates the object distance (u), image distance (v), and focal length (f) of a lens: 1/f = 1/v – 1/u. For the eye, the focal length (f) of the lens system is relatively constant, though it can change slightly for focusing (accommodation). When an object is far away, its image is formed on the retina.


When we increase the distance of an object from the eye, the object distance (u) becomes larger (its magnitude increases, and it’s a negative value in lens formula). To maintain a clear image on the retina, the eye’s lens system adjusts its focal length through accommodation. However, if we consider the eye’s focal length to be effectively constant for a given state of focus, then according to the lens formula (1/f = 1/v – 1/u), as ‘u’ becomes more negative (object moves further away), ‘v’ must also become more negative (image moves closer to the lens) to keep ‘1/v – 1/u’ constant and equal to ‘1/f’. Therefore, the image distance decreases (moves closer to the lens, i.e., the retina). In simpler terms, as the object moves further away, the eye focuses the image closer to the lens.

Question:

The ability of the eye to focus on objects at different distances is called its power of accommodation. This is achieved by changing the focal length of the crystalline lens with the help of ciliary muscles. The eye has a minimum distance of distinct vision, beyond which it cannot focus properly.


A normal eye can see objects clearly when the image is formed on the retina. To focus on an object, the eye’s lens changes its focal length. This process is called accommodation. However, this accommodation has a limit. When an object is placed too close to the eye, say closer than 25 cm, the ciliary muscles need to contract significantly to increase the curvature of the crystalline lens and thus decrease its focal length. Beyond a certain point, the ciliary muscles cannot contract further to achieve the required focal length. Consequently, the eye cannot form a sharp image of the object on the retina, resulting in blurred vision. The minimum distance at which an object can be seen clearly by a normal eye is called the least distance of distinct vision, which is approximately 25 cm.

Question:

Refraction of light. The bending of light as it passes from one medium to another. In this case, light from stars bends as it travels through Earth’s atmosphere.


Stars twinkle because of the refraction of starlight as it passes through Earth’s atmosphere. The atmosphere is not uniform; it has layers of air at different temperatures and densities. As starlight enters the atmosphere, it encounters these varying layers, which causes the light to bend or refract slightly. These atmospheric layers are constantly moving and changing. Therefore, the path of the starlight reaching our eyes is continuously and slightly changing, making the star appear to shift in brightness and position, which we perceive as twinkling. Planets, being closer to Earth, appear as tiny disks rather than point sources of light. This means that light from different parts of a planet is refracted in slightly different ways, and these effects tend to average out, making planets appear to shine steadily and not twinkle.

Question:

The human eye’s ability to focus on objects at different distances is described by its near point and far point. The near point is the closest distance at which an object can be seen clearly, and the far point is the farthest distance at which an object can be seen clearly.


For a human eye with normal vision, the far point is at infinity. This means that a normally sighted eye can see objects that are extremely far away, such as stars or distant mountains, clearly. The near point of a normal human eye is typically considered to be 25 centimeters (or 0.25 meters). This is the closest distance at which an object can be viewed distinctly without strain. Beyond this point, the eye’s focusing power is insufficient to form a clear image on the retina.

Question:

The eye’s ability to focus on objects at different distances. This involves changing the focal length of the eye lens. The ciliary muscles play a crucial role in this process.


The power of accommodation of the eye is its ability to adjust the focal length of the eye lens so as to focus images of objects at varying distances clearly on the retina. This adjustment is brought about by the ciliary muscles. When we look at distant objects, the ciliary muscles relax, making the eye lens thinner and increasing its focal length. When we look at nearby objects, the ciliary muscles contract, making the eye lens thicker and decreasing its focal length. This dynamic change in focal length allows us to see objects both far away and close up with clarity.

Question:

Refraction of light, atmospheric turbulence, point source vs. extended source.


Planets do not twinkle because they are relatively closer to Earth and appear as discs, not point sources of light. While starlight, originating from distant stars, is a point source and its light rays are easily bent and scattered by the Earth’s atmosphere due to turbulence (causing twinkling), the light from planets comes from a larger area. This means that even if individual light rays from different parts of the planet’s disc are refracted and scattered in different directions by the atmosphere, the overall effect is averaged out. Our eyes perceive a steady, continuous light from the planet, preventing the twinkling effect that we observe from stars.