Chapter 10: Human Eye and Colourful World  

Multiple Choose Question and Answer :

Question: The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to

(a) presbyopia.

(b) accommodation.

(c) near-sightedness.

(d) far-sightedness.

Answer: (b) accommodation.

[ Accommodation is the ability of the eye lens to adjust its focal length so that objects at different distances can be focused clearly on the retina.]

Question: Which of the following statements is not true for scattering of light ? [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 .

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

(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 statement is not true because scattering of light is mainly due to the interaction of light with particles in the atmosphere, not because different colours travel at different speeds in air.]

Question: The human eye forms the image of an object at its

(a) cornea. (b) iris. (c) pupil. (d) retina.

Answer: (d) retina.

[ The human eye forms the image of an object on the retina, where it is then converted into nerve signals and sent to the brain. ]

Question: The least distance of distinct vision for a young adult with normal vision is about

(a) 25 m. (b) 2.5 cm. (c) 25 cm. (d) 2.5 m.

Answer: (c) 25 cm.

[ For a young adult with normal vision, the least distance of distinct vision (near point) is about 25 cm.]

Question: The change in focal length of an eye lens is caused by the action of the

(a) pupil.       (b) retina.      (c) ciliary muscles.    (d) iris.

Answer: (c) ciliary muscles.

[ The ciliary muscles control the shape of the eye lens, thereby changing its focal length to help focus on objects at different distances. ]

Short Question and Answer :

Question: What is meant by power of accommodation of the eye? [1M]

Answer: The power of accommodation of the eye is the ability to adjust the focus and maintain clear vision when viewing objects at different distances.

Question: 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? [1M]

Answer: A person with myopia (nearsightedness) who cannot see objects beyond 1.2 meters distinctly would require a corrective lens that diverges light rays before they enter the eye. This type of lens is concave or negative in shape. Therefore, a concave lens would be used to restore proper vision for someone with myopia.

Question: Why do stars twinkle?  [1M]

Answers: Stars twinkle due to the phenomenon called atmospheric scintillation. As starlight passes through the Earth's atmosphere, it gets refracted and distorted by the varying density of air, causing the apparent brightness of stars to fluctuate or twinkle.

Question: What is the far point and near point of the human eye with normal vision? [2M]

Answer: Near point (least distance of distinct vision) is the closest distance at which an object can be seen clearly without strain. For a normal young adult, it is about 25 cm. The far point is the farthest distance up to which the eye can see objects clearly; for a normal eye, the far point is at infinity.

Question: A star at times appears bright and at times fainter . What is this effect called ? State the reason for Answer : [2012 2M]

Answer: This effect is called twinkling of stars.

The effect of a star appearing brighter and fainter at times is called twinkling. 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.

Question: 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? [2M]

Answer: The student is most likely suffering from myopia (short-sightedness). In myopia, a person can see nearby objects clearly but has difficulty seeing distant objects like the blackboard from the last row.
This defect can be corrected by using concave lenses (diverging lenses) in spectacles. These lenses help to focus the image of distant objects properly on the retina, allowing clear vision.

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 . [2012 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: Explain why the planets do not twinkle. [2M]

Answers: Planets do not twinkle because they are much closer to Earth and appear as extended sources of light rather than point sources. The variations in atmospheric refraction affect different parts of the planet’s light differently, which average out. Hence, their brightness remains steady instead of fluctuating like stars.

Question: Why does the Sun appear reddish early in the morning? [2M]

Answers: The Sun appears reddish early in the morning due to a phenomenon called atmospheric scattering. When the Sun is near the horizon, its light has to pass through a thicker layer of the Earth's atmosphere, causing shorter blue and green wavelengths to scatter more, while longer red wavelengths dominate the visible spectrum.  

Question:  Why does the sky appear dark instead of blue to an astronaut? [2M]

Answers: The sky appears dark instead of blue to an astronaut in space because there is no atmosphere to scatter sunlight. On Earth, the atmosphere scatters short-wavelength blue light, giving the sky its blue color. Without this scattering, the sky appears dark in space.

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  . [2010 2M]

Answer: We are able to see the Sun about two minutes before it actually rises above the horizon due to atmospheric refraction.

As sunlight enters the Earth’s atmosphere, it passes through layers of air with increasing density. These layers bend (refract) the light rays towards the normal. Because of this bending, the apparent position of the Sun appears higher than its actual position below the horizon. Hence, we see the Sun earlier than its actual rise time.

Question: A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision? [3M]

Answer:  (i) For distant vision correction : Here,  and  

We have,

 

The focal length for correcting distant vision is approximately  – 0.18 meters .

(ii) For near vision correction : Here,  and

We have, 

 

 

The focal length for correcting near vision is + 0.67 meters.

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 is a defect of vision in which a person can see distant objects clearly but cannot see nearby objects distinctly. In this case, the image of a nearby object is formed behind the retina.

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) Hypermetropic eye (image formed behind the retina):

(ii) Correction using a suitable lens (convex lens):

A convex lens is used to correct hypermetropia.

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 ? [2015 3M]

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 .

(c) In your option , in what way can Akash express his gratitude towards the teacher and Salman ? [2015 3M]

Answer:  (a) Akash is suffering from myopia (short-sightedness). This defect is corrected by using concave lenses (diverging lenses) in spectacles.

(b) The teacher shows care, concern, and responsibility towards the student’s well-being. Salman displays kindness, helpfulness, and a cooperative attitude by willingly helping his classmate.

(c) Akash can express his gratitude by thanking the teacher and Salman sincerely, and by taking proper treatment and wearing the prescribed spectacles regularly so that he can perform better in class.

Question: Why is Tyndall effect show by colloidal particles ? State four instances of observing the Tyndall effect .[2020 3M]

Answer:  Colloidal particles are large enough to scatter the path of light passing through them. This scattering makes the beam of light visible, which is called the Tyndall effect. True solutions do not show this effect because their particles are too small to scatter light.

Four instances of observing Tyndall effect:

(i) Sunlight entering a dusty room through a window appears as a visible beam.

(ii) Sunlight passing through a canopy of trees in a forest.

(iii) Light from car headlights or a torch visible in fog or mist.

(iv) A projector beam visible in a cinema hall filled with dust or smoke particles.

Long Question and Answer :

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 . [2008 5M]

Answer: (i) Colour of the clear sky is blue:
The sunlight gets scattered by air molecules in the atmosphere. Shorter wavelengths (blue light) are scattered more than other colours, so the sky appears blue.

(ii) The Sun can be seen about two minutes before actual sunrise:
This happens due to atmospheric refraction. Sunlight bends as it passes through layers of air, making the Sun appear slightly above the horizon before it actually rises.

(iii) We cannot see an object clearly if it is placed very close to the eyes:
Because the eye has a minimum distance of distinct vision (about 25 cm). If an object is closer than this, the eye cannot focus the image properly on the retina, so it appears blurred.

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 . [2008 5M]

Answer: (a) Dispersion is the splitting of white light into its constituent colours (VIBGYOR) when it passes through a prism or water droplets. This happens because different colours of light bend by different amounts.
A rainbow is formed when sunlight interacts with tiny water droplets present in the atmosphere. When sunlight enters a raindrop, it undergoes refraction, then dispersion into different colours, followed by internal reflection, and finally refraction again while coming out. This process produces a circular arc of seven colours, with red on the outer side and violet on the inner side.

 

(b) Hypermetropia is a defect of vision in which a person can see distant objects clearly but cannot see nearby objects distinctly. In this case, the image of a nearby object is formed behind the retina.

(i) Hypermetropic eye (image formed behind the retina):

(ii) Correction using a suitable lens (convex lens):

A convex lens is used to correct hypermetropia. It converges the light rays before they enter the eye so that the image is formed on the retina.

Question: 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 . [2017 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:  1. Elongation of the Eyeball  2. Increased Curvature of the Eye Lens 

(i) In myopia, the image of distant objects forms in front of the retina instead of on it. So, when the object is beyond the far point (5 m in this case), the eye lens cannot reduce its converging power enough, and the rays focus before reaching the retina, making the image blurred.

Ray diagram for Myopia:

(ii)  A concave lens (diverging lens) is used to correct myopia. It diverges incoming parallel rays so that the eye lens can focus them properly on the retina.

Ray diagram (with correction):

(b)  Given,  (negative for concave lens)

 We have, 

  

The power of the lens is -0.2 diopters.

Question: (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 .[2018 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.