CBSE Class 10 Science Human Eye and Colorful World Assignment Set E

Very Short Answer Type Questions

Question. Define the power of accommodation of human eye.
Answer: The ability of eye to see nearby as well as far off objects at the same time is called power of accommodation.

Question. In which type of eye defect far point of the eye gets reduced?
Answer: Myopia.

Question. What is dispersion of light?
Answer: The splitting of light into its various components (i.e., 7 colours) is called dispersion of light.

Question. Which part of the human eye helps in changing the thickness of lens?
Answer: Ciliary muscles.

Question. Why do birds fly back to their nest in the evening?
Answer: Birds lack light sensitive cells called rods, due to lack of these cells they cannot see the objects clearly in less/dim light.

Question. When white light enters the prism, which colour of light deviates/bends the least and which colour bends the most?
Answer: The light that bends the least is red colour and the light that bends the maximum is violet colour light.

Question. Which is the range of vision of normal eye?
Answer: 25 cm to infinity.

Question. Explain why the planets do not twinkle.
Answer: Planets do not emit light. However, they become visible due to reflection of light falling on them. The planets are much closer to the earth and thus can be considered as the extended source of light. The fluctuations in the light coming from various points of the planet due to atmospheric refraction get averaged out. As a result, no twinkling of planets is seen.

Question. What is Tyndall effect?
Answer: Scattering of light in the nature due to small particles present in the atmosphere is called Tyndall effect.

Question. Name the disease in which crystalline lens of human eye becomes opaque.
Answer: Cataract.

Question. In which type of eye defect near point of the eye becomes more than 25 cm?
Answer: Hypermetropia.

Short Answer Type Questions

Question. A person is unable to read a book clearly when kept at a distance of 25 cm from his eye. Name the defect.
How can it be corrected? Draw ray diagrams for (i) defective eye (ii) corrected eye and explain them.
Answer: The person cannot read a book at a distance of 25 cm from his eyes. So he is suffering from long sightedness/ hypermetropia. It can be corrected by using convex lens of suitable focal length.  

Question. With the help of a diagram, explain the formation of a rainbow in the sky.
Answer: The water droplets in the atmosphere act like small prisms. These droplets refract and disperse the incident sunlight, then reflect it internally and finally refract it again when it comes out of the rain drop.
Due to distortion of sunlight and internally reflection, different colours reach to the observer. 

Question. Explain the formation of rainbow in the sky with the help of a diagram. 
Answer: After a rain-shower, the sunlight gets dispersed by tiny droplets, present in the atmosphere. The water droplets acts like a small prisms. They refract and disperse the incident sunlight, then reflectit internally, and finally refract it again when it comes out of the raindrop. Due to dispersion of light and internal reflection different colours reach the observer’s eye.     

Question. Give reasons:
(i) The extent of deviation of a ray of light on passing through a prism depends on the colour.
(ii) Lights of red colour are used for danger signals.
Answer: (i) The extent of deviation of a ray of light on passing through a prism depends on the colour because the refractive index of glass for different colour is different. It depends on wavelength of a particular light.
(ii) Since the wavelength of light is maximum in the spectrum, its penetration power in the air is maximum and so we can see red colour from further distances. Thus, danger signal uses red colour.

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 
Answer: Hypermetropia : It is a defect in an eye in which a person is not able to see nearby object distinctly but can see far objects clearly.
Hypermetropia is caused due to following reasons:
(a) Shortening of the eyeball
(b) Focal length of crystalline lens is too long. 

Question. What eye defect is myopia? Describe with a neat diagram how this defect of vision can be corrected by using a suitable lens. 
Answer: Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
Myopia can be corrected by using concave lens of appropriate focal length.   

Question. With the help of scattering of light, explain the reason for the difference in colours of the Sun as it appears during sunrise/sunset and noon.
Answer: At the time of sunrise or sunset it appears red where as at noon it appear white when it is overhead because at sunrise or sunset blue colour and other shorter wavelength light get scattered away while passing through the atmosphere. Scattering of longer wavelength is least which reach to the earth. Due to this sun appears red during morning and evening time.
When sun is overhead at noon, sun rays travels smaller distance, due to little scattering of blue and violet colour wavelength almost all wavelengths reach to earth and due to this sun appears white.

Question. Account for the following:
a. Part of the human eye that helps in changing the focal length of the eye lens.
b. The condition resulting due to the eye lens becoming cloudy.
c. The factors on which colour of the scattered white light depends.
d. The range of vision of a normal eye.
e. The sky appears dark to the astronauts in the space.
Answer: a. Ciliary muscles.
b. Cataract.
c. Size of the suspended particles in the medium through which white, light passes.
d. 25 cm to infinity is the range of vision of normal eye.
e. Due to absence of atmosphere in the space, there is no scattering and hence space appears black/ dark.

Question. Draw a labelled diagram to explain the formation of a rainbow in the sky. 
Answer: A rainbow is a natural spectrum caused by dispersion of sunlight by tiny water droplets, present in the atmosphere.   

Question. In the figure given a narrow beam of white light is shown to pass through a triangular glass prism. After passing through the prism it produces a spectrum XY on a screen.
(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) At X, violet colour is seen and at Y, red colour is seen.
(b) Different colour of white light bend through different angles with respect to the incident of light because the refractive index of glass is different for different colours.

Question. What is meant by spectrum of white light?
How can we recombine the components of white light after a prism has separated them?
Draw a diagram to illustrate it.
Answer: Spectrum of white light is the ordered distribution of frequency or wavelength of the colour of white light.
We can recombine the components of white light after splitting, by placing inverted prism in the path of spectrum.   

Question. A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m.
(a) 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: (a) The two possible reasons due to which the defect of vision arises are :
excessive curvature of the eye lens elongation of the eye ball.
(i) A student with myopia has the far point nearer than infinity, thus, the image of a distant object is formed in front of the retina. In the given case the student is not able to see distinctly the objects placed beyond 5 m from his eyes are formed in front of the retina hence, it appears blurred.

(ii) This defect can be corrected by using a concave lens of suitable power as it brings the image back on to the retina, thus the defect is corrected.     

Question. Explain giving reason why the sky appears blue to an observer from the surface of the Earth. What should the appearance of the sky be during the day for an astronaut staying in the international space station orbiting the Earth? State reason to justify your answer.
Answer: When sunlight passes the atmosphere, the fine particles in air scatter blue colour more strongly than red. This scattered blue light enters our eye and the colour of clear sky appears blue. For an astronaut staying in the international space station orbiting the Earth, the appearance of the sky will be black due to absence of air molecules to scatter the light coming from the Sun.

Question. State the difference in colours of the sun observed during sunrise/sunset and noon. Give explanation for each.
Answer: During sunrise or sunset, the suns rays pass through a maximum length of the atmosphere.
Most of the blue and shorter wavelength get scattered. Only the red colour of light reaches the observer. That is why the Sun observed during sunset and sunrise appear red. At noon, the distance to be travelled is least. All wavelengths are scattered equally and hence sun appears white.

Question. (a) If a person wears lens of power – 6D for distant vision and for correcting his near vision he needs a lens of +2D. Determine the focal length of the lenses in both the case.
(b) Give reason for the following natural phenomenon:
(i) Stars twinkle
(ii) Planets do not twinkle
(iii) Stars appear raised in the sky.
Answer: (a) (i)  

(b) (i) Due to continuous changes in the densities of the atmospheric layers the apparent position of the star also changes; which make the light coming from the distant point sized star brighter and dimmer. The light coming from the stars therefore gives a shaking appearances, which gives the impression of twinkling of a star.
(ii) Since the planets are closer to us. Due to this they appear a combination of large point-sizes source of light, and change in the path of light coming from the planets is not significant. So planets do not appear twinkling.
(iii) Due to atmospheric refraction, a star appears to be slightly higher than its actual position in sky.

Question. (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. In hypermetropia a person cannot see near by objects clearly. Its is due to shortening of the eye ball or due to increase in focal length of eye lens.
To correct hypermetropia, a convex lens of suitable focal length is needed.

Question. About 3.5 crore people in the developing world are corneal blind. About 30 lac blind children below the age of 12 can be cured by replacing the defective cornea with the cornea of a donated eye. What can a student of your age do in order to create awareness about this fact among people of your locality and why?
Answer: To create awareness among the people, we as a student can do the following.
(i) Organise a team of 3-4 members for eyedonation champion.
(ii) We can organise a play to aware people about the importance of donating an eye.
(iii) We can demonstrate poster and banner and take a walk around the area.
We must spread such awareness to encourage people to donate their eyes after their death so that the children below 12 years have to see this world as they are the future of our country.

Question. (a) Draw a diagram to show the formation of image of a distant object by a myopic eye.
How can such an eye defect be remedied?
(b) State two reasons due to which this eye defect may be caused.
(c) A person with a myopic eye cannot see objects beyond a distance of 1.5 m. What would be the power of the corrective lens used to restore proper vision?
Answer: 

This defect can be correct by using concave lens.
(b) Myopia can be caused due to the following reasons.
(i) Excessive curvature of the eye lens
(ii) Elongation of the eyeball
(c) The far point of this myopic person is 1.5 m (This means a person can see a distant object kept at infinity clearly if the image of the object is formed at his infinity i.e., at 1.5 m)
u = ∞, v = –1.5 m, f = ?   

Question. State the cause of dispersion, when white light enters a glass prism. Explain with a diagram.
Answer: When white light passes through a prism it splits into its constituent colours, because each coloured light has different speed in prism. Prism has different refractive index for these light rays and so deviates through different angles when emerge out from the prism.
Deviation for violet colour (shortest wavelength) is most and for red colour (largest wavelength) deviation is least.  

Question. What is a spectrum? How can we recombine the components of white light after a glass prism has separated them? Illustrate it by drawing a diagram. 
Answer: Spectrum : The band of the colored components of a light beam is called its spectrum.
We recombine the components of white light by placing a second identical prism in an inverted position with respect to first prism.   

On passing white light to the first prism it splits into colours of the spectrum. Then this spectrum is allowed to pass through a second identical prism in an inverted position with respect to the first prism. Then, we found a beam of white light.

Question. Draw a ray diagram to show the formation of a rainbow and mark the point where (i) dispersion, (ii) internal reflection occurs.
Answer: (i) Point A denotes dispersion and (ii) point B denotes internal reflection.