Class 9 Science Chapter 11 Sound

1. How does the sound produced by a vibrating object in a medium reach your ear?

Answer: When an object vibrates, it forces the neighbouring particles of the medium to vibrate. These vibrating particles then force the particles adjacent to them to vibrate. In this way vibrations produced by an object are transferred till it reaches the ear.

2. Explain how sound is produced by your school bell.

Answer : When the school bell is struck with a hammer, it starts vibrating and as a result of these vibrations, sound waves are produced in air.

3. Why are sound waves called mechanical waves?

Answer : Sound waves are mechanical waves because they need a material medium for propagation, like air or liquids like water, or metals like silver.

4. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Answer : No, because sound waves need a medium through which they can propagate. Since there is no material medium on the moon due to the absence of atmosphere, you cannot hear any sound on the moon.

5. Which wave property determines (a) loudness, (b) pitch?

Answer : (a) The loudness of sound is determined by the amplitude of sound waves.

(b) The pitch of sound is determined by the frequency of vibration of the sound producing source.

6. Guess which sound has a higher pitch: guitar or car horn?

Answer : Guitar has a higher pitch than car horn because sound produced by the strings of guitar has high frequency than that of car horn. High the frequency higher is the pitch.

7. What are wavelength, frequency, time period and amplitude of a sound wave?

Answer : Wavelength: The distance between two consecutive crests or troughs is known as the wavelength. Its SI unit is metre (m).

Frequency: The number of complete oscillations per second is known as the frequency of a sound wave. It is measured in hertz (Hz).

Amplitude: The maximum height reached by the crest or trough of a sound wave is called its amplitude.

8. How are the wavelength and frequency of a sound wave related to its speed?

Answer : we know that ,speed=wavelength×frequency, so this means that speed is directly proportional to the wavelength and frequency ,so if any of them increase then the speed also increases.

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CLASS 10 SCIENCE CHAPTER - 11 THE HUMAN EYE AND THE COLOURFUL WORLD NCERT SOLUTION

 THE HUMAN EYE AND THE COLOURFUL WORLD 

1. What is meant by power of accommodation of the eye?

Answer: 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.

2. 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?

Answer: The person is able to see nearby objects clearly, but he is unable to see objects beyond 1.2 m. This happens because the image of an object beyond 1.2 m is formed in front of the retina and not at the retina. To correct this defect of vision, he must use a concave lens. The concave lens will bring the image back to the retina.

3. What is the far point and near point of the human eye with normal vision?

Answer: The near point of the eye is the minimum distance of the object from the eye, which can be seen distinctly without strain. For a normal human eye, this distance is 25 cm.

The far point of the eye is the maximum distance to which the eye can see the objects clearly. The far point of the normal human eye is infinity.

4. 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?

Answer: The student is suffering from myopia or short-sightedness. The defect can be corrected by the concave lens of an appropriate power.

5. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Answer: A normal eye is not able to see objects closer than 25 cm because the eye is not able to decrease the focal length beyond a limit since ciliary muscle  of eyes are unable to contract beyond a certain limit, then the object appears blurred and produces strain in the eyes.

6. What happens to the image distance in the eye when we increase the distance of an object from the eye?

Answer: When we increase the distance of an object from the eye, the image distance in the eye does not change. The increase in the object distance is compensated by the change in the focal length of the eye lens. The focal length of the eyes changes in such a way that the image is always formed at the retina of the eye.

7. Why do stars twinkle?

Answer: The twinkling of stars is due to atmospheric refraction of star-light. The atmosphere is made of several layers and the refractive indices which keep on changing continuously due to this path of light rays from the star keep on changing their path continuously. As a consequence the number of rays entering, the pupil of the eye goes on changing with time and the stars appear twinkling.

8. Explain why the planets do not twinkle?

Answer: Planets do not twinkle because they appear larger in size than the stars as they are relatively closer to earth. Planets can be considered as a collection of a large number of point-size sources of light. The different parts of these planets produce either brighter or dimmer effect in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling effects of the planets are nullified, and they do not twinkle.

9. Why does the sky appear dark instead of blue to an astronaut?

Answer: The sky appears dark instead of blue 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 reaches the eyes of the astronauts and the sky appears black to them.

Class 10 Science Chapter 10 Light- Reflection and Refraction NCERT Solution

 Light- Reflection and Refraction NCERT Solution

1. Define the principal focus of a concave mirror.

Answer: The principal focus of a concave mirror is a point on its principal axis to which all the light rays are parallel and close to the axis converge after reflection from the concave mirror.

2. Name a mirror that can give an erect and enlarged image of an object.

Answer: Concave 

3. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Answer: Convex mirrors are commonly used as rear-view  mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

4. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

Answer: It bends towards the normal. This is because it travels from an optically rarer to , optically denser medium.

5. You are given kerosene, turpentine and water. In which of these does the light travel fastest?

Answer: Speed of light in a medium is inversely proportional to refractive index of the medium,as refractive index of water is minimum, so light travels fastest in water.

6. The refractive index of diamond is 2.42. What is the meaning of this statement?

Answer: The refractive index of diamond is 2.42 this means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in air.

7. Define 1 dioptre of power of a lens.

Answer: 1 Dioptre is defined as the power of a lens of focal length 1 meter.

8. Name the type of mirror used in the following situations. (a) Headlights of a car. (b) Side/rear-view mirror of a vehicle. (c) Solar furnace. Support your answer with reason.

Answer: (a)    Concave mirror is used in the headlights of a car. This is because concave mirrors can produce powerful parallel beam of light when the light source is placed at their principal focus.

(b) Convex mirrors are commonly used as rear-view  mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.

(c)  Concave mirrors are convergent mirrors. That is why they are used to construct solar furnaces. Concave mirrors converge the light incident on them at a single point known as principal focus. Hence, they can be used to produce a large amount of heat at that point.

9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations

Answer: The convex lens will form complete image of an object, even if its one half is covered with black paper. It can be understood by the following two cases.

Case I: When the upper half of the lens is covered

In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.

Case II: When the lower half of the lens is covered

In this case, a ray of light coming from the object is refracted by the upper half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.

10.The magnification produced by a plane mirror is +1. What does this mean?

Answer: The magnification produced by a plane mirror is +1 means that the image formed is virtual, erect, and of the same size as that of an object.