11. HUMAN EYE AND COLOURFUL WORLD
Questions:
1. No image is formed on the blind spot of human eye because [SSLC April, 2016] (a) Cones are absent (b) Rods are absent (c) Rods and cones are absent (d) Optic nerve is absent
Ans: Rods and cones are absent
2. A student wears spectacles with concave lenses for proper vision. When he is not using the spectacles the image of object is formed [SSLC June, 2016] (a) In front of the retina (b) On the blind spot (c) Behind the retina (d) On the yellow spot
Ans: (a) In front of the retina
3. Define power of accommodation.
Ans: The ability of the eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of the eye lens is called power of accommodation.
4. What is the principle of the working of the human eye?
Ans: It is like a camera having a lens system forming an inverted, real image on the light sensitive screen, retina inside the eye.
5. How is a normal eye able to see distinctly distant as well as nearer objects? What is the distance of distinct vision?
Ans: Eye lens is made up of fibrous material. Its curvature can be changed by ciliary muscles which changes its focal length. When muscles are relaxed, the lens becomes thin and focal length increases and the eye is able to see distant objects. Similarly when ciliary musdes contract, focal length decreases and eye is able to see nearer objects. 1 The minimum distance at which objects can be seen distinctly is called distance of distinct vision. It is 25 cm.
6. (i) What is meant by ‘least distance of distinct vision’ ? (ii) How does iris controls the size of the pupil in bright light and dim light?
Ans: (i) The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision.
(ii) In bright light, the iris contracts the pupil to allow less light to enter the eye. In dim light, the iris expands the pupil to allow more light to enter the eye.
7. (i) Make a ray diagram to show how the eye defect- myopia is corrected by using a suitable lens. (ii) State two reasons due to which this eye defect may be caused. (iii) A person with myopic eye cannot see objects beyond a distance of 1.5 m. What is the power of the lens required to correct the problem?
Ans: (i)
8. A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him. Name the defect of vision the boy is suffering from. Explain the method of correcting this defect. Draw ray diagram for the: (i) Defect of vision and (ii) For its correction. [SSLC July, 2018] OR A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child suffering from 7 How can it be corrected?
Ans: Myopia on short − sightedness: Inability of an eye in viewing long distance objects. The image in this case falls before the retina. For every myopic eye, there exists a far point beyond which clear image cannot be seen. The short − sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.
9. Draw the vertical section of the human eye and label the following parts: (a) Lens (b) Fovea [SSLC April, 2017]
10. (i) What type of spectacles should be worn by a person having the defect of myopia as well as hypermetropia?
(ii) The far point of a myopic person is 150 cm. What is the nature and the power of lens required to correct the defect?
(iii) With the help of ray diagrams, show the formation of image by
(i) a myopic eye
(ii) correction of myopic eye by using an appropriate lens.
Ans:
11. Red coloured light is used in traffic signals to indicate the vehicles to stop, because compared to other coloured red light [SSLC July, 2018]
(a) Has high frequency (b) Scatters more (c) Has less wavelength (d) Scatters less
Ans: (d) Scatters less
12. What is the cause of dispersion of light on passing through a prism?
Ans: The refractive index of the material of a prism is different for different colours of light as different colours have different speed in the material of a prism. Also, prism has non—parallel surfaces.
13. What is dispersion? What happens when light is passed through a glass prism.
Ans: The process of splitting up of white light into its constituent colour as it passes through a refracting medium is known as dispersion of light. It bends towards the base of the prism after passing through it.
14. Why do different colours get separated when white light passes through a prism? How can we recombine the components of white light after a prism has separated them. Explain.
Ans: A spectrum is the band of distinct colours we obtain when the white light is splitted by a prism. We can recombine the component of white light by passing them through a prism placed upside down near the given prism. When we pass white light through two identical prisms held side by side with their refracting edges in opposite directions; the first prism disperses white light but the second prism recombines them. Thus light emerging from second prism is white.
15. Why did the clear sky appear blue? [NCERT Exemplar] OR Why is the colour of the clear sky blue?
Ans: The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of the visible light.
These are more effective in scattering light of shorter wavelength at the blue end than the light of longer wavelength at the red end. Thus, the blue colour is due to the scattering of sunlight through fine particles in air.
16. What is meant by scattering of light? The sky appears blue and the sun appears reddish at sunrise and sunset. Explain these phenomena with reason. OR What is the difference of colours of the sun observed during sunrise/sunset and noon? Give explanation of each? [NCERT Exemplar]
Ans: (i) Scattering of light: Phenomenon of spreading of light (diffused reflection) by minute particles in a medium.
(ii) The sky appears blue because the blue colour of sunlight scatters much more strongly than the red colour by particles in atmosphere/air due to its shorter wavelength.
(iii) At sun-rise and sun-set most of the blue light and shorter wavelengths are scattered away by the particles in the atmosphere as the light from the sun near the horizon passes through thick layers of air and larger distance. The light that reaches us is of longer wavelength (red colour) giving a reddish appearance.
17. Enlist atleast six phenomena observed in nature subsequent to the scattering of light.
Ans: The various phenomena observed in nature due to scattering of light are the following:
(i) Sun appears red near the horizons (during sunrise and sunset) and white when seen overhead.
(ii) Bluish colour of the sky.
(iii) Visible path of light as it enters a dark room.
(iv) Danger signals or stop signals are usually red.
(v) Blueness of distant mountains.
(vi) Sunlight filtering through clouds.
18. Explain in brief the reason for each of the following: (i) Advanced sun-rise
(ii) Delayed sun-set
(iii) Twinkling of stars
Ans:(i) Advanced sun-rise : When the sun is slightly below the horizon, light rays coming from the sun travel from the rarer to denser medium layers of air because of atmospheric refraction of light, light appears to come from a higher position above the horizon. Thus the sun appear earlier than actual sunrise.
(ii) Delayed sun-set: Same reason as similar refraction occurs at the sunset.
(iii) Twinkling of stars The light coming from the stars gets refracted several times before reaching the observer’s eye. Due to change in physical condition of the atmosphere the light sometimes reaches the observer and sometimes it doesn’t, hence they appear to twinkle.
19. Explain with the help of labelled diagram, the cause of twinkling of stars.
Ans: Light coming from the stars undergoes refraction on entering the Earth’s atmosphere. This refraction continues until it reaches the Earth’s surface. This happens because of uneven heating of atmospheric air. Hence, the atmospheric air has changing refractive index at various altitudes. In this case, starlight continuously travels from a rarer medium to a denser medium. Hence, it continuously bends towards the normal.
The refractive index of air medium gradually increases with a decrease in altitude. The continuous bending of starlight towards the normal results in a slight rise of the apparent position of the star.
Since the physical conditions of the Earth’s atmosphere keeps changing, the apparent position of the star is not stationary. The star changes its position continuously, which makes it twinkle. This happens because starlight travels a very large distance before reaching the observer. However, the path varies continuously because of uneven atmospheric conditions. Hence, the stars seem to be fluctuating, sometimes appearing brighter and sometimes fainter. All this together, gives rise to the twinkling effect of stars.
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.
Ans: (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 thiosuiphate in about 2 L of clean water in the tank and add 1 to 2 mL of conc H2SO 4 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.
Textbook Exercises:
1. The human eye can focus object at different distances by adjusting the focal length of
eye lens. This is due to (a) Presbyopia (b) Accommodation (c) Near sightedness (d) Far sightedness
Ans: (b) Accommodation
2. The human eye forms the image of an object at its (a) Cornea (b) Iris (c) Pupil (d) Retina
Ans: (d) Retina
3. The least distance of distance vision for a young adult with normal vision is about. (a) 25m (b) 2.5 cm (c) 25 cm (d) 2.5 m
Ans: (c) 25 cm
4. 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
Ans: (c) Ciliary muscles
5. 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?
Ans: (i) For distant vision
P = −5.5 D
= 1 = 1 −5.5
= −0.182 m
= −18.2 cm
(ii) For distant vision P = 1.5 D
= 1 = 1 1.5 = −0.667m
= 66.7 cm
6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Ans: = −80 1 P = 1
= −80 P
= −1.25 D
Nature of the lens is concave.
7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is im. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Ans: The diagram is as shown below
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Ans: Because focal length of eye lens cannot be decreased below a certain value.
9. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Ans: For a normal eye, image distance remains fixed, whatever be the object distance.
10. Why do stars twinkle?
Ans: Because of atmosphere refraction.
11. Explain why the planets do not twinkle.
Ans: Refraction is not noticeable due to the small variation in the brightness of light coming from the planet. Thus, planets do not appear to twinkle.
12. Why does the Sun appear reddish early in the morning?
Ans: Since blue colour has’ a shorter wavelength and red colour has a longer wavelength, the red colour is able to reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish early in the morning.
13. Why does the sky appear dark instead of blue of an astronaut?
Ans: The sky appears dark to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light reach the eyes of the astronauts and the sky appears black to them.
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