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 molecules of air and fine particles in the atmosphere scatter light. Shorter wavelengths like blue are scattered more strongly than longer wavelengths like red. As blue light is scattered in all directions, it enters our eyes, making the sky appear blue.

(ii) Atmospheric refraction bends sunlight as it passes through Earth's atmosphere. This causes the Sun to appear above the horizon earlier than its actual position. Similarly, the Sun remains visible for about two minutes after it sets, due to the same phenomenon.

(iii) The eye lens cannot decrease its focal length below a minimum limit. When an object is very close, light rays diverge too much for the lens to focus, resulting in a blurred image. The comfortable viewing distance is about 25 cm.

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 is the splitting of white light into its component colors when it passes through a prism or a similar medium. This occurs because different colors of light have different wavelengths and are refracted by different amounts.

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 far-sightedness, is a vision condition where a person can see distant objects clearly but has difficulty seeing nearby objects distinctly.

(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 looks blue on a clear day because the molecules of air and fine particles in the atmosphere scatter sunlight. These particles are more effective at scattering shorter wavelengths of light, like blue, than longer wavelengths like red. Since blue light has a shorter wavelength, it gets scattered more, and this scattered blue light reaches our eyes, making the sky appear blue.

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 far-sightedness, is a vision condition where a person can see distant objects clearly but has difficulty seeing nearby objects distinctly.

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 when passing through a prism because each color has a different wavelength. Red light bends the least, while violet light bends the most. This causes the colors to spread out, creating a distinct spectrum of colors.

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 :

The cause of this dispersion is that different wavelengths of light bend by different amounts when they pass through the prism. Shorter wavelengths, like violet, bend the most, while longer wavelengths, like red, bend the least.

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.

This occurs due to atmospheric refraction of starlight. As starlight passes through Earth's atmosphere, it continuously bends because of the changing refractive index. This causes the star's apparent position to fluctuate. Since stars are distant and act as point sources of light, the varying path of light causes the brightness to flicker, making the star appear brighter or fainter at different times.

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 : The sun appears reddish-orange during sunrise and sunset, while it appears bright yellow or white at noon.

Sunrise/Sunset (Reddish-Orange): During these times, the sun is near the horizon, and its light passes through a thicker layer of the Earth's atmosphere. As the light travels through more atmosphere, shorter wavelengths (like blue and violet) scatter more, leaving longer wavelengths (like red and orange) to dominate the color observed.

Noon (Bright Yellow/White): At noon, the sun is directly overhead, and its light passes through a smaller portion of the atmosphere. Less scattering occurs, so more of the shorter wavelengths (blue and green) reach the observer, making the sun appear brighter and more white or yellowish.

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 : Ciliary muscles are crucial in the human eye for the process of accommodation, which allows the eye to focus on objects at different distances by changing the shape of the lens. The gradual weakening of the ciliary muscles, along with a reduction in lens flexibility, leads to a vision defect called presbyopia. This condition makes it difficult to see nearby objects clearly. To correct this, individuals with presbyopia often require bi-focal lenses, which combine both concave lenses for distant vision and convex lenses for near vision.

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 likely suffering from myopia (nearsightedness), a condition where distant objects appear blurry while close objects can be seen clearly. To correct this defect, concave lenses are used, as they help in diverging light rays to focus properly on the retina for distant vision.

(b) The teacher displayed a sense of responsibility and care for Akash’s well-being by noticing his difficulty in seeing and taking appropriate action to help him. Salman showed kindness and willingness to help Akash by volunteering to exchange seats so that he could see the blackboard clearly.

(c) Akash can express his gratitude by thanking both the teacher and Salman for their support. He could also offer to help others in the future, showing the same kindness and concern that was shown to him.

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 air with varying temperatures and densities. This causes the apparent position of objects to fluctuate, resulting in effects such as the wavering of objects seen above a fire or the twinkling of stars.

(a) Twinkling of stars: The twinkling of stars occurs due to atmospheric refraction. As starlight enters Earth's atmosphere, it undergoes continuous refraction due to the varying refractive index of the air. This causes the star's apparent position to fluctuate, making it appear brighter and fainter as the amount of light entering the eye changes, resulting in the twinkling effect.

(b) Advanced sunrise and delayed sunset: Atmospheric refraction causes the Sun to be visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset. This happens because the light from the Sun bends as it passes through the Earth's atmosphere, making the Sun appear higher than its actual position, extending the daylight hours by a few minutes.

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 : (a) The student is likely suffering from myopia (nearsightedness). This condition occurs when a person can see nearby objects clearly but struggles to see distant objects clearly. In this case, the student cannot see the words on the blackboard placed at a distance of 3 meters.

Possible Causes of Myopia:

(i) Excessive curvature of the eye lens,

(ii) elongation of the eyeball

Correction Method:

Myopia can be corrected with a concave lens (diverging lens). A concave lens spreads out light rays before they enter the eye, which helps in focusing the image directly on the retina. This enables the student to see distant objects clearly.

(b) Stars twinkle due to atmospheric refraction. As starlight enters Earth’s atmosphere, it continuously bends due to varying refractive indices. This causes the apparent position of the star to fluctuate. Since stars are far away and act as point sources of light, the changing path of light rays makes the star appear brighter or fainter, creating the twinkling effect.

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: (a) (i) Cornea: The cornea helps in focusing light entering the eye by bending (refracting) it towards the retina.

(ii) Iris: The iris controls the size of the pupil, regulating the amount of light entering the eye.

(iii) Crystalline lens: The crystalline lens focuses light onto the retina and helps in adjusting the focus for near and distant vision.

(iv) Ciliary muscles: The ciliary muscles adjust the shape of the lens to focus on objects at different distances, aiding in accommodation.

(b) The Sun appears reddish early in the morning due to scattering of light by the Earth's atmosphere. During sunrise, sunlight has to pass through a thicker layer of the atmosphere. Shorter wavelengths (blue and violet) scatter more due to the presence of dust and air particles, leaving the longer wavelengths (red, orange, and yellow) to dominate the sky. This causes the Sun to appear reddish.

An astronaut on the Moon will not observe this phenomenon. The Moon lacks an atmosphere to scatter sunlight, so the Sun would appear white or bright yellow throughout the day, without the reddish hue observed on Earth during sunrise or sunset.

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

Question: 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.

Answer: (i) As a person ages, the ciliary muscles (which help in changing the shape of the eye's lens) become weaker and less flexible. Additionally, the lens of the eye may lose its ability to change shape, making it harder to focus on nearby or distant objects. This results in a reduced ability to focus properly on objects at different distances.

(ii) The defect that arises in this condition is called Presbyopia, which is the gradual loss of the ability to focus on nearby objects.

(iii) The corrective lens used for restoring vision in such cases is a bifocal lens. Bifocal lenses have two parts: the upper part helps with distance vision (for far objects), while the lower part helps with near vision (for close objects).

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.