LAQ for Human Eye and the Colourful World Class 10 Science NCERT
Important Questions1
(a) Write the function of each of the following parts of human eye: Cornea, iris, crystalline lens, ciliary muscles.
(b) Millions of people of the developing countries of aworld are suffering from corneal blindness. These persons can be cured by replacing the defective cornea with the cornea of a donated eye. A charitable society of your city has organized a campaign in your neighbourhood in order to create awareness about this fact. If you are asked to participate in this mission how would you contribute in this noble cause ?
(i) State the objective of organizing such campaigns.
(ii) List two arguments which you would give to motivate the people to donate their eyes after death
(iii) List two values which are developed in the persons who actively participate and contribute in such programmes.
Answer
(a) (i) Cornea - Refracts the rays of light falling on the eye.
(ii) Iris - Controls the size of the pupil.
(iii) Crystalline lens - Focuses the image of the object on the retina.
(iv) Ciliary muscles - Holds the eye lens and adjusts its focal length
(b) (i) Objectives - To make people aware and realize their duties towards society.
(ii) One person can give sight to two people.
Our eyes can live even after our death.
(iii) Concern for others Responsible behaviour Group work/ or any other.
2
(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 the boy is suffering from State the possible causes of this defect and explain the method of correcting it.
(b) Why do stars twinkle ? Explain.
Answer
(a) Defect of vision - Myopia or short sightedness or near sightedness
Causes of myopia:
(i) Excessive curvature of eye lens/eye lens becomes more converging
(ii) Elongation of eye ball
Methods of correction: By the use of concave lens of suitable power or focal length the defect is corrected./suitable diagrammatic representation.
(b) Due to atmospheric refraction : The density of different layers of air keep on changing due to which the apparent images of the stars keep on changing. This changing positions of stars appears as twinkling of stars.
Detailed Answer :
(a) The defect is Myopia or short-sightedness i.e., inability of an eye in viewing long distance objects. This defect is caused by elongation of eye-ball and excessive curvature of the lens.
The short-sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.
(b) Stars very distant and approximately point-sized sources. Path of star light changes continuously due to gradual changing of refractive index of the layers of earth’s atmosphere. Thus, the apparent positions of the stars fluctuate and the amount of star light entering the eye flicker giving the twinkling effect.
3
(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? Give reason to justify your answer.
Answer
(a) Function of:
• Cornea: Focuses light rays/permits the light to enter eye.
• Iris: Controls amount of light entering the eye./controls the size of pupil.
• Crystalline lens: Converges light rays onto retina.
• Ciliary Muscles : Adjusts focal length of eye lens by contraction and relaxation so that sharp image can be obtained on the retina./helps in accommodation
(b) Early in the morning, the sun is near the horizon, sunlight reaches us after travelling a longer distance through thick layers of atmosphere. Thus, most of the blue light and shorter wavelength are scattered away by the particles in the atmosphere. The light that reaches us is of longer wavelengths giving a reddish appearance.
This phenomenon will not be observed by an astronaut on the moon, because of the absence of atmosphere on the moon.
4
(a) What is meant by the term ‘power of accommodation’ ? Name the component of eye that is responsible for the power of accommodation.
(b) A student sitting at the back bench in a class has difficulty in reading. What could be his defect of vision? Draw ray diagrams to illustrate the image formation of the blackboard when he is seated at the
(i) Back seat
(ii) front seats. State two possible causes of this defect. Explain the method of correcting this defect with the help of a ray diagram.
Answer
(a) The ability or the property of the eye lens to adjust its focal length in order to focus both near and distant objects is known as the power of accommodation.
The component of eye that is responsible for the power of accommodation is ciliary muscles because they are responsible for change in its focal length.
(b) The defect in the student is Myopia:
Causes for Myopia:
(i) Excessive curvature of the eye lens.
(ii) Elongation of eyeball.
To correct myopia the concave lens placed in front of the eye forms a virtual image of distant object at far point.
• In a myopic aye, image of distant object is formed in front of the retina (and not on the retina).
The far point of a myopic eye is less than infinity.
5
(a) A student suffering from myopia is not able to see distinctly the objects 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 m 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.
Answer
Detailed Answer: (a) (i) The student is unable to see distinctly the objects placed beyond 5 m from his eyes as he is suffering from myopia i.e., near-sightedness wherein a person can see nearby objects clearly but cannot see distant objects distinctly.
(ii) Concave lens can be used as the corrective lens to restore proper vision. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
u = - ∞, v = - 5m
1/f = 1/v – 1/u
= 1/-5 – 1/∞
= 1/-5 – 0
f = - 5 m
Then, P = 1/f = 1/-5 = - 0.2 D
6
Answer
• Hypermetropia/Long-sightedness
• Defective eye
• Two causes :
(i) The focal length of the eye lens is too long.
(ii) The eyeball has become too small.
• Correction of the defect using appropriate lens
7
Answer
(i) Sometimes, the crystalline lens of people at old age becomes milky and cloudy. This condition is called cataract. This causes partial or complete loss of vision.
It is possible to restore vision through a cataract surgery.
(ii) Focal length for lens for distant vision, = - 1/1 = - 1 m
Focal length for lens for near vision,
= + ½ = + 0.5 m
(iii) The focal length of eve lens cannot be decreased below a certain minimum limit. If we try to read a printed page by holding it very close to our eyes, we may see the image being blurred or feel strain in the eye. To see an object comfortably and distinctly, we must hold it at about 25 cm from the eyes.
8
(i) List three common refractive defects of vision. Suggest the way of correcting these defects.
(ii) About 45 lac people in the developing countries are suffering from corneal blindness. About 30 lac children below the age of 12 years suffering from this defect can be cured by replacing the defective cornea with the cornea of a donated eye. How and why can students of your age involve themselves to create awareness about this fact among people?
Answer
(i) The three common refractive defects of vision are as follows:
Myopia (short-sightedness): It is corrected by using spectacles having concave lenses of appropriate power.
Hypermetropia (far-sightedness): It is corrected by using spectacles having convex lenses of appropriate power.
Presbyopia : This defect is corrected using bifocal lenses of appropriate power in which the upper part consists of a concave lens (to correct myopia) and the lower part consists of a convex lens (to correct hypermetropia).
(ii) Eyes of a dead person can be donated to the person having corneal blindness. It will help him/her see the world. We can also register ourselves to eye donation camps who can preserve our eyes after our death and donate them to the needy.
9
(i) Name the defect of vision in which the eye loses its power of accommodation due to old age.
(ii) The near point of a person suffering from hypermetropia is at 50 cm from his eye. What is the power of the lens needed to correct this defect (near point of normal eye is 25 cm).
(iii) With the help of ray diagrams, show the formation of image by
(a) a hypermetropic eye.
(b) correction of hypermetropia by using an appropriate lens.
Answer
(i) Presbyopia
(ii) v = - 50 cm
u = - 25 cm
Using lens formula,
1/f = 1/v – 1/u
⇒ 1/f = 1/-50 + 1/25
= (-1 + 2)/50
⇒ 1/f = 1/50
∴ f = 50 cm
Power of lens, P = 1/f(m) = 100/50 = + 2D
10
(a) State the reasons which lead to hypermetropia. With the help of suitable diagram explain this defect of vision and its correction.
(b) Draw diagram of an experimental arrangement for observing scattering of light in colloidal solution. Name the two chemicals used in this activity.
Answer
(a) This defect arises either because:
(i) the focal length of the eye lens is too long, or
(ii) the eyeball has become too small.
Such a person has to keep a reading material much beyond 25 cm from the eye for comfortable reading. This is because the light rays from a nearby object are focused at a point behind the retina.
Correction of Hypermetropia:
(i) This defect can be corrected by using a convex lens of appropriate power.
(ii) Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
11
(i) List the parts of the human eye that control the amount of light entering into it. Explain how they perform this function.
(ii) Write the function of retina in human eye.
(iii) Do you know that the corneal-impairment can be cured by replacing the defective cornea with the cornea of the donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?
Answer
(i) The pupil is the perforation in the iris. It is the pupil (or aperture of the eve) through which light enters our eyes. The relaxation and contraction of the muscular fibres of the iris regulate the opening and closing of the pupil. Hence, it is the pupil that controls the amount of light entering our eyes.
(ii) The retina is the "film" of the eye like the film of a camera. The retina is the light-sensing part of the eye. It converts the incident light into electrical signals and sends them to the brain.
(iii) Eyes of a dead person can be donated to a person having corneal blindness. It will help him/her to see the world. We can also register ourselves to donate our eyes. The organisations that put up eye donation camps preserve our eyes after our death and donate them to the needy.
12
(a) A person cannot read newspaper placed nearer than 50 cm from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect List its two possible causes Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
(b) We see advertisements for eye donation on television or in newspaper. Write the importance of such advertisements.
Answer
(a) The person is suffering from hypermetropia.
Two possible causes :
(i) Greater focal length of the lens.
(ii) Eye ball becoming smaller.
Use convex lens of focal length,
u = - 25 cm
v = - 50 cm
1/f = 1/v – 1/u
= 1/-50 – 1/-25
= (-1 + 2)/50
= 1/50 cm
⇒ f = 50 cm
P = 100/50 = 2D is required.
(b) Eye donation advertisements are important as:
(i) They make the people aware about donation of eyes after their death
(ii) Sympathetic nature towards others.
13
(a) What is presbyopia ? State its cause. How is it corrected ?
(b) Why does the Sun appear reddish early in the morning ? Explain with the help of a labelled diagram.
Answer
(a) Presbyopia : Defect of vision when a person is unable to see nearby as well as far off objects clearly.
Causes: Gradual weakening of the ciliary muscles diminishing flexibility of the eye lens.
Correction: By the use of bi-focal lenses.
(b) Due to scattering of blue light/light of shorter wavelengths, light of longer wavelengths/ red component of light reaches our eyes. This gives reddish appearance of the sun.
14
(i) What is dispersion of white light ? State its cause.
(ii) ”Rainbow is an example of dispersion of sunlight.” Justify this statement by explaining, with the help of a labelled diagram, the formation of a rainbow in the sky. List two essential conditions for observing a rainbow.
Answer
(i) Definition of Dispersion: Splitting of white light into seven constituent colours by a prism.
Cause of Dispersion : When white light passes through a glass prism, different angles with respect to the incident ray hence are separated.
(ii)
Conditions for observing a rainbow :
(a) after the rainfall/at a water fountain.
(b) sun is at the back of the observer.
15
(i) Define dispersion of light. How is scattering of light different from dispersion ? Give one example of natural phenomenon based on each of these.
(ii) A beam of light consisting of red, blue and yellow colours is incident on the prisms as shown below. Complete the diagram to show refracted and emergent ray.
Answer
(i) Breaking up of the light into the component colours is called dispersion. Dispersion is caused due to difference in velocity of each colour through a medium. Scattering of light takes place due to interaction of light and particles suspended in a medium. Rainbow - dispersion.
Blue colour of sky, red colour of sky at sunrise or sunset – scattering.
(ii)
16
(a) Draw a ray diagram to explain the term angle of deviation.
(b) Why do the component colour of incident white light split into a spectrum while passing through a glass prism, explain.
(c) Draw a labelled ray diagram to show the formation of a rainbow.
Answer
(a)
(b) Different colour of white light bend through different angles with respect to the incident light, as they pass through the glass prism. Thus, each colour emerges along a different path, forming a spectrum.
(c) 
Detailed Answer :
(i) Emergent ray bends at an angle to the direction of the incident ray and the angle between them is known as angle of deviation.
(ii) When a beam of light enters a prism, it gets refracted and splits into its constituent colours. This splitting of the light ray occurs because of the different angles of bending of each colour. Hence, each colour passing through the prism bends at different angles with the respect to the incident beam. This gives rise to the formation of the colour spectrum.
Diagram to show the formation of rainbow in the sky.
17
What is atmospheric refraction ? Use this phenomenon to explain the following natural events.
(i) Twinkling of stars.
(ii) Advanced sunrise and delayed sun-set.
Draw diagrams to illustrate your answers.
Answer
Atmospheric refraction: Refraction of light caused by the earth's atmosphere due to change in the refractive indices of different layers.
(ii) Twinkling of stars: Stars are distant point sized source of light. The path of the rays of light coming from the star goes on varying due Ray path to atmospheric refraction slightly. Thus apparent position of the stars fluctuates and the amount of star light entering the eye flickers giving the twinkling effect.
(iii) Advanced sun-rise: When the sun is slightly below the horizon, light rays coming from the sun travel from the rarer to denser layers of air.
Because of atmospheric refraction of light, light appears to come from a higher position above the horizon. Thus sun appears earlier than actual sunrise
18
(a) Describe an activity along with a labelled diagram the phenomenon of dispersion through a prism.
(b) Explain in brief the formation of rainbow with the help of figure.
Answer
(a) Place a prism on a white sheet of paper with the triangular face on the sheet and trace its boundary ABC.
(i) Fix two pins T and Son one side.
(ii) Place the prism on the boundary ABC.
(iii) Looking through the other side fix two more pins Q and R in such a way that all the four pins appear to be in the same line.
(iv) Remove the pins and mark their positions.
(v) Join TS and RQ and extend them to meet the faces of the prism at P and O respectively.
(vi) Join PO.
(vii) TP represents the incident ray.
(viii) PO represents the refracted ray.
(ix) And OR represents the emergent ray which is bent towards the base.
(x) Let PN and OM be the normal at the points P and O respectively.
(xi) And let i be the angle of incidence and r the angle of refraction.
(xii) If the incident ray TP is extended forward and the emergent ray OR backwards, they meet at G, forming the angle HGO.
(xiii) Measure the angle HGO.
(xiv) This angle is called the angle of deviation.
(xv) Angle of deviation is the angle through which an incident ray deviates.
(xvi) Repeat this for different values of angle of incidence.
TP - Incident ray, ∠i - Angle of incidence
PO - Refracted ray, ∠r - Angle of refraction
OR - Emergent ray, ∠e - Angle of emergence
A - Angle of the prism, ∠D - Angle of deviation
(b) Rainbow forms when sunlight hits the water droplets suspended in the atmosphere and undergoes total internal reflection. When the sunlight comes out of the drop it disperses, where the drop acts like a small prism. This dispersion or breaking up of light into several colours forms the Rainbow.
19
(i) A person's image when seen through a stream of hot air arising above a fire disperse to waver. Explain.
(ii) Define dispersion. How does a prism disperse white light?
(iii) Which colour of light bends the most and the least?
Answer
(i) In the region above the fire, the hot air is lighter than the cool air above it. Further, the refractive index of hotter region is less than the cooler region and it is not static. These physical conditions are not steady. Subsequently, the image when seen through the hot region is not stationary. The apparent position of the object fluctuates. This gives the wavering effect to the image.
(ii) The phenomenon of obtaining a spectrum of colours by passing white light through the prism is known as dispersion. Dispersion occurs because the refraction of different colours travel with different speeds in a refracting medium. Speed of violet colour is least and that of the red colour is the most. Hence, the refractive index of the medium is largest for violet colour and least for the red colour.
(iii) As a result, the refraction or bending of violet colour is the maximum and that of red colour is minimum This difference in the extent of bending of different colour of light causes dispersion of white light into its constituent colours as they emerge out of prism Red colour bends the least and violet bends the most.
20
(a) Write an activity for observing scattering of light in colloidal solution.
(b) On the basis of this activity explain, why sky appears red at sunrise or sunset.
Answer
(a) (i) Place a strong source of white light at the focus of converging lens.
(ii) Allow the light beam to pass through a transparent glass tank containing clear water. In that clear water dissolve 200g of sodium thiosulphate in about 2 L of clean water in the tank and add 1 to 2 mL of conc. H2SO4 acid.
(iii) Allow the beam of light to pass through a circular hole made in a cardboard. Obtain a sharp image of circular hole on screen by using converging lens.
(b) At sunrise or sunset light travels long distance through thick layers of atmosphere. Blue and other lower wavelength get scattered and only red light reaches earth.