10. The Human Eye and the Colourful World

CBSE Class 10 Science Chapter 10. The Human Eye and the Colourful World

Chapter 10. The Human Eye and the Colourful World

Class 10 Science Chapter 10. The Human Eye and the Colourful World Intaernal Questions and Answers :

Previous year Question and Answer , 2008

Question (a) Give reasons for the following :

(i) Colour of the clear sky is blue

(ii) The sun can be seen about two minutes before actual sunrise .

(iii) We cannot see an object clearly If it is placed very close to the eyes . [5M]

Answer : (i) The clear sky appears blue due to a phenomenon called Rayleigh scattering. Sunlight consists of various colors corresponding to different wavelengths. Blue light has a shorter wavelength and is scattered in all directions by the gases and particles in the Earth's atmosphere much more than the other colors, such as red and yellow, which have longer wavelengths. Since blue light is scattered more, it reaches our eyes from every direction, making the sky appear blue.

(ii) This phenomenon is due to atmospheric refraction. As light from the Sun passes through the Earth's atmosphere, it bends due to the change in medium (from the vacuum of space to the denser atmosphere). This bending of light causes the Sun’s rays to reach us even when it is just below the horizon, making the Sun appear to rise about two minutes earlier than its actual position. Similarly, atmospheric refraction causes the Sun to appear about two minutes after it has actually set.

(iii) We cannot see an object clearly if it is placed very close to our eyes because of the minimum distance of distinct vision, which is typically around 25 cm for a normal human eye. When an object is too close, the lens of the eye cannot adjust enough to focus the light rays correctly onto the retina. This inability to converge the light rays sharply causes the image to blur, and hence, we cannot see the object clearly. The eye's ciliary muscles cannot contract further to adjust the curvature of the lens sufficiently when the object is too near.

Question : (a) What is meant by dispersion of white light ? Describe the formation of rainbow in the sky with the help of a diagram .

(b) What is hypermetropia ? Draw ray diagrams to show the image formation of an object by : (i) Hypermetropic eye (ii) Correction made with a suitable lens for hypermetric eye . [5M]

Answer : (a) Dispersion of white light is the phenomenon where white light splits into its constituent colors when it passes through a medium like a prism. White light consists of seven colors: violet, indigo, blue, green, yellow, orange, and red (often remembered as VIBGYOR). When white light passes through a prism, each color bends (refracts) by a different amount due to its unique wavelength, with violet bending the most and red the least. This separation of light into its spectrum of colors is called dispersion.

A rainbow forms in the sky after a rain shower due to the dispersion of sunlight by water droplets in the atmosphere. These droplets act as tiny prisms, refracting and dispersing the sunlight, reflecting it internally, and refracting it again as it exits the droplet. This process separates the light into different colors, which are seen as a spectrum in the sky, always opposite to the Sun. The dispersion and internal reflection of light result in the rainbow's colors reaching the observer's eye.

(b) Hypermetropia, also known as farsightedness, is a condition where a person can see distant objects clearly but struggles to see nearby objects distinctly. This occurs because the near point of the eye is farther away than the normal near point of 25 cm, making close objects appear blurry.

(i) Hypermetropic eye :

(ii) Correction made with a suitable lens for hypermetric eye :

Previous year Question and Answer, 2009

Question: Why does sky look blue on a clear day ?

Answer : The sky appears blue on a clear day because the molecules and fine particles in the atmosphere scatter sunlight more effectively at shorter wavelengths, which are in the blue part of the spectrum. As sunlight passes through the atmosphere, the blue light is scattered more than the red light, which has a longer wavelength. This scattered blue light reaches our eyes from all directions, making the sky look blue. Without an atmosphere, there would be no scattering, and the sky would appear dark. At high altitudes, where scattering is less prominent, the sky also appears darker.

Question : What is hypermetropia ? State the two causes of hypermetropia . With the help of ray diagrams, show (i) The eye-defect hypermetropia (ii) Correction of hypermetropia by using a lens . [3M]

Answer : Hypermetropia, also known as farsightedness, is a condition where a person can see distant objects clearly but struggles to see nearby objects distinctly. This occurs because the near point of the eye is farther away than the normal near point of 25 cm, making close objects appear blurry.

Hypermetropia can arise due to two causes:

(i) The focal length of the eye lens is too long.

(ii) The eyeball has become too small.

(i) (i) The eye-defect hypermetropia :

(ii) Correction of hypermetropia by using a lens :

Previous year Question and Answer 2010

Question : A triangular glass prism . After passing through the prism it produces a spectrum XY on a Screen. [2M]

(a) State the colour seen at X and Y .

(b) Why do different colours of white light bend through different angles with respect to the incident beam of light ?

Answer : (a) The colour seen at X is red and the colour seen at Y is violet .

(b) Different colors of white light bend through different angles due to their varying wavelengths. This phenomenon, known as dispersion, occurs because each color of light is refracted by different amounts when it passes through a medium like a prism. Shorter wavelengths (blue and violet) are bent more than longer wavelengths (red and orange) due to their interaction with the medium's refractive index. As a result, the colors spread out into a spectrum, each bending at a unique angle relative to the incident beam of light.

Previous year Question and Answer, 2011

Question : Explain with the help of a diagram , how we are able to observe the sunrise about two minutes before the sun gets above the horizon . [2M]

Answer : We are able to observe the sunrise about two minutes before the Sun actually crosses the horizon due to atmospheric refraction. This phenomenon causes the light from the Sun to bend as it passes through the Earth's atmosphere, making the Sun appear higher in the sky than its true position. Similarly, we see the Sun for about two minutes after it has actually set. This refraction also causes the apparent flattening of the Sun’s disc at sunrise and sunset.

Question : Draw a diagram to show dispersion of white light by a glass prism. What is the cause of this dispersion ? [2M]

Answer : The diagram shows the dispersion of white light by a glass prism :

Dispersion of white light by a glass prism occurs because different colours of light have different wavelengths and thus different refractive indices when passing through the prism. Shorter wavelengths (blue and violet) are refracted more than longer wavelengths (red and orange) due to their higher interaction with the prism’s material. This varying refraction causes the white light to spread out into a spectrum of colours.

Previous year Question and Answer, 2012

Question : When we place a glass prism in the path of a narrow bean of white light , a spectrum is obtained . What happens when a second identical prism is placed in an inverted position with respect to the first prism ? Draw a labeled ray diagram to illustrate it . [2M]

Answer : When a second identical prism is placed in an inverted position relative to the first prism, it recombines the spectrum produced by the first prism. The second prism bends the dispersed colors back together, resulting in a single white light beam emerging, effectively canceling the dispersion caused by the first prism.

Question : A star at times appears bright and at times fainter . What is this effect called ? State the reason for this effect . [2M]

Answer : This effect is called twinkling of stars.

It occurs because the light from the star passes through various layers of Earth's atmosphere, which have different temperatures and densities. These atmospheric layers cause the light to bend (refract) in different directions. As a result, the star's light fluctuates in brightness and position, making it appear as though it is twinkling.

Previous year Question and Answer, 2013

Question : State the difference in colours of the sun observed during sunrise/sunset and noon. Give explaination for each . [3M]

Answer : Difference in Colors of the Sun at Sunrise/Sunset and Noon:

Time	Color of the Sun	Explanation
Sunrise/Sunset	Reddish/Orange	During sunrise and sunset, the sun's light has to pass through a larger thickness of Earth's atmosphere. Shorter wavelengths (blue and violet) are scattered out due to Rayleigh scattering, leaving the longer wavelengths (red and orange) to reach our eyes.
Noon	White/Yellow	At noon, the sun is directly overhead, and its light travels a shorter path through the atmosphere. Less scattering occurs, and all colors of light, including blue, reach us, making the sun appear white or slightly yellow due to the scattering of blue light.

Previous year Question and Answer, 2014

Question : In the following diagram , the path of a ray of light passing through a glass prism is shown :

In this diagram the angle of incidence, the angle of emergence and the angle of deviation respectively are (select the correct option) :

(a) X , R and T (b) Y , Q and T (c) X , Q and P (d) Y , Q and P

Answer : (d) Y , Q and P

Question : Study the following diagrams in which the path of a ray light passing through a glass prism as traced by four students P , Q , R and S is shown :

The student who has traced the path correctly is :

(a) P (b) Q (c) R (d) S

Answer : (b) Q

Question : A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram . [2014] [3M]

Trace it on your answer sheet and show the path of the emergent bean as observed on the screen DF.

(i) Write the name and cause of the phenomenon observed.

(ii) Where else in nature is this phenomenon observed ?

(iii) Based on this observation, state the conclusion which can be drawn about the constituents of white light .

Answer : (i) Dispersion of Light.
Dispersion occurs because different colours (wavelengths) of light refract by different amounts when passing through the prism. This causes white light to split into its constituent colors, creating a spectrum.

(ii) This phenomenon is observed in nature during the formation of a rainbow. Rainbows occur when sunlight passes through raindrops in the atmosphere, acting like tiny prisms that disperse the light into a spectrum of colours.

(iii) The observation concludes that white light is made up of multiple colours, specifically a spectrum that includes red, orange, yellow, green, blue, indigo, and violet. This demonstrates that white light is a combination of all visible wavelengths of light.

Previous year Question and Answer, 2015

Question : With the help of scattering of light , explain the reason for the difference in colours of the sun as it appears during sunset/sunrise and noon.

Question : State the difference in colours of the sun observed during sunrise/sunset and noon. Give explaination for each . [3M]

Answer : Given 2013

Question: Write the importance of ciliary muscles in the human eye . Name the defect of vision that arises due to gradual weakening of the ciliary muscles . What types of lenses are required by the person suffering from this defect to see the objects clearly ?

Answer : Importance of Ciliary Muscles:

Ciliary muscles in the human eye control the shape of the eye lens, allowing it to adjust focus for near and distant objects through a process called accommodation.

Defect Due to Weakening:

The gradual weakening of ciliary muscles leads to a defect called presbyopia, which affects near vision.

Type of Lenses Required:

People with presbyopia require bifocal lenses or convex (converging) lenses to see nearby objects clearly.

Question : Akash, sitting in the last row in his class, could not see clearly the words write on the blackbond . When the teacher noticed it, he announced if any student sitting in the front row could volunteer to exchange his seat with Akash . Salman immediately agreed to exchange his seat with Akash . He could now see the words written on the black board clearly. The teacher thought it fit to send the message to Akash’s parents advising them to get his eyesight checked . In the context of the above event . Answer the following questions :

(a) What detect of vision is Akash suffering from ? Which type of lens is used to correct this defect ?

(b) State the values displayed by the teacher and Salman .

Answer : (a) Akash is suffering from myopia (nearsightedness), a vision defect where distant objects appear blurry. This defect is corrected using concave (diverging) lenses, which help focus light properly onto the retina, allowing clear vision of distant objects.

(b) Teacher: Empathy, awareness, and responsibility for student well-being by identifying the issue and taking action to help Akash.

Salman: Kindness, willingness to help, and cooperation by volunteering to switch seats with Akash without hesitation.

(c) Akash can express his gratitude by thanking his teacher and Salman for their support. He could also show appreciation by working hard in class, participating actively, and being helpful to others when possible, reflecting the kindness he received.

Previous year Question and Answer 2016

Question: In the following ray diagram the correctly marked angles are :

(a) and (b) and

Answer : (a) and (i.e., The angle of incidence = the angle of emergence.)

Question: What is atmospheric refraction ? Use this phenomenon to explain the following natural events :

(a) Twinkling of stars (b) Advanced sunrise and delayed sunset

Draw diagrams to illustrate your answers . [5M]

Answer : Atmospheric refraction is the bending of light as it passes through layers of the Earth's atmosphere with varying densities and temperatures. As light travels from a star or the Sun, it changes direction due to refraction in the atmosphere, making objects appear slightly shifted from their actual position.

(a) The twinkling of stars occurs due to atmospheric refraction. As starlight passes through Earth's atmosphere, it traverses layers of varying densities and temperatures. This causes the light to bend erratically, making the star's apparent position shift slightly. The constant change in the light's path results in rapid variations in brightness, causing stars to appear as if they are twinkling in the night sky.

(b) Advanced Sunrise and Delayed Sunset occur due to atmospheric refraction. As the Sun approaches the horizon, its light bends downward through Earth's atmosphere, allowing us to see it before it has actually risen above the horizon. Similarly, after the Sun has set, its light continues to be bent towards us, making it appear to linger in the sky for a short period. This effect can make sunrise occur up to 2 minutes earlier and sunset up to 2 minutes later.

Previous year Question and Answer, 2017

Q1. Sudy the following ray diagram :

In this diagram, the angle of incidence , the angle of emergence and the angle of deviation respectively have been represented by

(a) y , p , z (b) x , q , z (c) p ,y , z (d) p .z , y

Answer : (d) p , z , y

Q6. A student very cautiously traces the path of a ray through a glass slab for different values of the angle of incidence () . He then measures the corresponding values of the angle of refraction () and the angle of emergence () for every value of the angle of incidence . On analyzing these measurement of angles , his conclusion would be .

(a) (b)

Answer : (b) .

[ When light passes through a glass slab, the angle of incidence (∠i) and the angle of emergence (∠e) are equal because the light exits parallel to the incoming ray (after being refracted inside the slab).

The angle of refraction (∠r) is less than the angle of incidence due to the change in medium, as light bends towards the normal when it enters the denser medium (glass).]

Q22. What is “ dispersion of white light ” ? Draw a labeled diagram to illustrate the recombination of the spectrum of white light . why it is essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other ? [3M]

Answer : Dispersion of white light is the splitting of white light into its constituent colors (spectrum) when it passes through a prism. This occurs because different colors of light have different wavelengths, and they refract (bend) by different amounts while passing through the prism.

The two prisms used should be identical and placed in an inverted position to recombine the dispersed light into white light, canceling out any deviation and ensuring the light emerges as a single beam without spreading.

Q31. A student suffering from myopia is not able to see distinctly the object placed beyond 5 m . List two possible reasons due to which this defect of vision may have arisen . With the help of ray diagrams explain .

(i) why the student is unable to see distinctly the objects placed beyond 5 cm from his eyes .

(ii) The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens .

(b) If in this case , the numerical value of the focal length of the corrective lens is 5 m . Find the power of the lens as per the new Cartesian sign convention . [5M]

Answer : Myopia (short-sightedness) is a common vision defect where a person can see nearby objects clearly but struggles to see distant objects.

Two Possible Reasons for Myopia: (a) Elongation of the Eyeball (b) Increased Curvature of the Eye Lens.

(i) The myopic eye focuses distant objects (placed beyond 5 m) in front of the retina, rather than on it, as shown in the ray diagram below. This results in a blurred image of distant objects.

The light rays from distant objects converge in front of the retina, causing a blurry image.

(ii) A concave (diverging) lens is used to correct myopia. This lens diverges the light rays before they enter the eye, shifting the focal point back onto the retina.

The concave lens spreads out the incoming light rays slightly, allowing them to focus correctly on the retina, restoring clear vision for distant objects.

(b) Given, (negative for concave lens)

We have ,

The power of the lens is -0.2 diopters.

Previous year Question and Answer ,2018

Q32. (a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him . Name the defect of vision of the boy is suffering from . State the possible causes of this detect and explain in the method of correcting it .

(b) Why do stars twinkle ? Explain. [5M]

Answer :

Q33. (a) Write the function of each of the following parts of human eye :

(i) Cornea (ii) Iris (iii) Crystalline lens (iv) Ciliary muscles

(b) Why does the sun appear reddish early in the morning ? Will this phenomenon be observed by an astronaut on the Moon ? Given reason to justify your answer .[5M]

Answer :

Previous year Question and Answer, 2019

Question : Define the term power of accommodation . Write the modification in the curvature of the eye lens which enables us to see the nearby objects clearly ? [2M]

Answer : The power of accommodation is the eye's ability to adjust its focus to see objects clearly at varying distances. This adjustment is achieved by changing the curvature of the eye lens to alter its focal length.

To see nearby objects clearly, the eye lens becomes thicker and more curved. This increased curvature shortens the focal length of the lens, allowing it to focus light from close objects onto the retina. This process is known as accommodation and helps ensure that objects at a close range are seen distinctly.

Question : Trace the sequence of events which occur when a bright light is focused on your eyes . [3M]

Answer : When a bright light is focused on your eyes, the sequence of events begins with the light entering the eye through the cornea, passing through the pupil. The pupil, controlled by the iris, reacts by constricting to reduce the amount of light entering the eye. This constriction helps protect the sensitive cells in the retina from being overwhelmed by too much light. The retina then receives the light and converts it into electrical signals, which are sent to the brain through the optic nerve. The brain processes these signals, allowing you to perceive the bright light and respond accordingly, such as squinting or looking away to protect your eyes.

Question : What is a rainbow ? Draw a labeled diagram to show the formation of a rainbow . [3M]

Answer : A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by 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 water droplets act like small prisms, refracting and dispersing the incident sunlight, reflecting it internally, and finally refracting it again as it exits the raindrop. Due to the dispersion of light and internal reflection, different colors reach the observer’s eye, creating the beautiful arc of a rainbow.

Previous year Question and Answer 2020

Question : Why is Tyndall effect show by colloidal particles ? State four instances of observing the Tyndall effect .[3M]

Answer : The Tyndall effect is shown by colloidal particles because these particles are large enough to scatter light but small enough to remain suspended in the medium. When light passes through a colloidal solution, the scattered light makes the path visible, demonstrating the Tyndall effect.

Four instances of observing the Tyndall effect:

(i) The scattering of light by water droplets or dust particles makes the sunbeams visible.

(ii) Car headlights or flashlights create visible beams of light when passing through fog or mist.

(iii) The scattering of sunlight by atmospheric particles causes the blue appearance of the sky.

(iv) Sunlight entering a dark room through a small window highlights dust particles, making the light path visible.

Previous year Question and Answer 2021

Q7. Which of the following statements is not true for scattering of light ? [1M 2021]

(a) Colour of the scattered light depends on the size of particles of the atmosphere .

(b) Red light is least scattered in the atmosphere .

(d) The fine particles in the atmospheric air scatter the blue light more strongly than red .So the scattered blue light enters our eyes .

Answer : (c) Scattering of light takes place as various colours of white light travel with different speed in air.

[ This is incorrect because all colors of light travel at the same speed in air, but they scatter differently due to their wavelengths. Scattering is not caused by differences in speed, but rather by interactions with particles that vary in size relative to the wavelength of light.]

Q8. In the diagram given below, x and y are the end colours of the spectrum of white light . The colour of y represents the

(a) Colour of sky as seen from earth during the day .

(b) Colour of the sky as seen from the moon

(d) Colour of sun at the time of noon . [1M]

Answer: (c) Colour used to paint the danger signals.

[ The color y in the inverted prism spectrum, being at the end of the visible light spectrum, typically represents red. Red light is used for danger signals because it scatters the least in the atmosphere, making it visible from a distance.]

Previous year Question and Answer, 2023

1. (a) A narrow beam XY of white light is passing through a glass prism ABC as shown in the diagram : [2M]

Photo

Trace it on your answer sheet and show the path of the emergent beam as observed on the screen PQ.

Name the phenomenon observed and its cause .

(b) It is observed that the power of an eye to see nearby objects as well as far off objects diminishes with age . [2M]

(i) Give reason for the above statement .

(ii) Name the defect that is likely to arise in the eyes in such a condition.

(iii) Draw a labeled ray diagram to show the type of corrective lens used for restoring the vision of such an eye.

Question : Give reasons for the following : [3M]

(a) Danger signals installed at airports and at the top of tall buildings are of red colour.

(b) The sky appears dark to the passengers flying at very high altitudes .

Answer : (a) Danger signals at airports and tall buildings are red because red light has the longest wavelength among visible colors, allowing it to scatter the least in the atmosphere and remain visible from a distance.

(b) The sky appears dark to passengers flying at very high altitudes because the atmosphere becomes thinner at higher altitudes, containing fewer air molecules to scatter sunlight. On the Earth's surface, sunlight is scattered by air molecules, creating a blue sky. However, at high altitudes, the lack of scattering causes the sky to appear dark, resembling space.

(c) The path of a light beam is visible in a colloidal solution due to the Tyndall effect, where light scatters off the colloidal particles, making the light beam visible.