Understanding the Reflection of Sound: Principles and Laws | 10th Science Acoustics

Reflection of Sound

When you speak in an empty room, you hear a soft repetition of your voice. This is nothing but the reflection of the sound waves that you produce. Let us discuss about the reflection of sound in detail through the following activity.

When sound waves travel in a given medium and strike the surface of another medium, they can be bounced back into the first medium. This phenomenon is known as reflection. In simple the reflection and refraction of sound is actually similar to the reflection of light. Thus, the bouncing of sound waves from the interface between two media is termed as the reflection of sound. The waves that strike the interface are termed as the incident wave and the waves that bounce back are termed as the reflected waves, as shown in Figure 5.3

Figure 5.3: Reflection of sound

1. Laws of reflection

Like light waves, sound waves also obey some fundamental laws of reflection. The following two laws of reflection are applicable to sound waves as well.

• The incident wave, the normal to the reflecting surface and the reflected wave at the point of incidence lie in the same plane.
• The angle of incidence ∠i is equal to the angle of reflection ∠r.

These laws can be observed from Figure 5.4.

Figure 5.4: Laws of reflection

In the above Figure 5.4, the sound waves that travel towards the reflecting surface are called the incident waves. The sound waves bouncing back from the reflecting surface are called reflected waves. For all practical purposes, the point of incidence and the point of reflection is the same point on the reflecting surface.

A perpendicular line drawn at the point of incidence is called the normal. The angle which the incident sound wave makes with the normal is called the angle of incidence, ‘i’. The angle which the reflected wave makes with the normal is called the angle of reflection, ‘r’.

2. Reflection at the boundary of a denser medium

A longitudinal wave travels in a medium in the form of compressions and rarefactions. Suppose a compression travelling in air from left to right reaches a rigid wall. The compression exerts a force F on the rigid wall. In turn, the wall exerts an equal and opposite reaction R = – F on the air molecules. This results in a compression near the rigid wall. Thus, a compression travelling towards the rigid wall is reflected back as a compression. That is the direction of compression is reversed (Figure 5.5).

Figure 5.5: Reflection of sound at a denser medium

3. Reflection at the boundary of a rarer medium

Consider a wave travelling in a solid medium striking on the interface between the solid and the air. The compression exerts a force F on the surface of the rarer medium. As a rarer medium has smaller resistance for any deformation, the surface of separation is pushed backwards (Figure 5.6). As the particles of the rarer medium are free to move, a rarefaction is produced at the interface. Thus, a compression is reflected as a rarefaction and a rarefaction travels from right to left.

Figure 5.6: Reflection of sound at a rarer medium

4. Reflection of sound in plane and curved surfaces

When sound waves are reflected from a plane surface, the reflected waves travel in a direction, according to the law of reflection. The intensity of the reflected wave is neither decreased nor increased. But, when the sound waves are reflected from the curved surfaces, the intensity of the reflected waves is changed. When reflected from a convex surface, the reflected waves are diverged out and the intensity is decreased. When sound is reflected from a concave surface, the reflected waves are converged and focused at a point. So the intensity of reflected waves is concentrated at a point. Parabolic surfaces are used when it is required to focus the sound at a particular point. Hence, many halls are designed with parabolic reflecting surfaces. In elliptical surfaces, sound from one focus will always be reflected to the other focus, no matter where it strikes the wall.

This principle is used in designing whispering halls. In a whispering hall, the speech of a person standing in one focus can be heard clearly by a listener standing at the other focus.

The principle of whispering halls using elliptical surfaces.

Lens Maker’s Formula Explained | 10th Science Optics Study Material

Study Material | 10th Science | Chapter 2: Optics

Lens Maker’s Formula

LENS MAKER’S FORMULA

All lenses are made up of transparent materials. Any optically transparent material will have a refractive index. The lens formula relates the focal length of a lens with the distance of object and image. For a maker of any lens, knowledge of radii of curvature of the lens is required. This clearly indicates the need for an equation relating the radii of curvature of the lens, the refractive index of the given material of the lens and the required focal length of the lens. The lens maker’s formula is one such equation. It is given as

where µ is the refractive index of the material of the lens; R₁ and R ₂ are the radii of curvature of the two faces of the lens; f is the focal length of the lens.

Apparent Weight, Weightlessness, and Newton's Law of Gravitation Explained

Apparent Weight

Understanding Apparent Weight

APPARENT WEIGHT

The weight that you feel to possess during up and down motion, is not same as your actual weight. Apparent weight is the weight of the body acquired due to the action of gravity and other external forces acting on the body.

Let us see this from the following illustration:

Let us consider a person of mass m, who is travelling in lift. The actual weight of the person is W = mg, which is acting vertically downwards. The reaction force exerted by the lift’s surface ‘R’, taken as apparent weight is acting vertically upwards.

Let us see different possibilities of the apparent weight 'R' of the person that arise, depending on the motion of the lift; upwards or downwards which are given in Table 1.2

Table 1.2: Apparent weight in different scenarios.

1. Weightlessness

Have you gone to an amusement park and taken a ride in a roller coaster? or in a giant wheel? During the fast downward and upward movement, how did you feel?

Its amazing!!. You actually feel as if you are falling freely without having any weight. This is due to the phenomenon of ‘weightlessness’. You seem to have lost your weight when you move down with a certain acceleration. Sometimes, you experience the same feeling while travelling in a lift.

When the person in a lift moves down with an acceleration (a) equal to the acceleration due to gravity (g), i.e., when a = g, this motion is called as ‘free fall’. Here, the apparent weight (R = m (g – g) = 0) of the person is zero. This condition or state refers to the state of weightlessness. (Refer case 4 from Table 1.2).

The same effect takes place while falling freely in a roller coaster or on a swing or in a vertical giant wheel. You feel an apparent weight loss and weight gain when you are moving up and down in such rides.

2. Weightlessness of the astronauts

Some of us believe that the astronauts in the orbiting spacestation do not experience any gravitational force of the Earth. So they float. But this is absolutely wrong.

Astronauts are not floating but falling freely around the earth due to their huge oribital velocity. Since spacestation and astronauts have equal acceleration, they are under free fall condition. (R = 0 refer case 4 in Table 1.2). Hence, both the astronauts and the spacestation are in the state of weightlessness.

3. Application of Newton’s law of gravitation

• Dimensions of the heavenly bodies can be measured using the gravitation law. Mass of the Earth, radius of the Earth, acceleration due to gravity, etc. can be calculated with a higher accuracy.
• Helps in discovering new stars and planets.
• One of the irregularities in the motion of stars is called ‘Wobble’ lead to the disturbance in the motion of a planet nearby. In this condition the mass of the star can be calculated using the law of gravitation.
• Helps to explain germination of roots is due to the property of geotropism which is the property of a root responding to the gravity.
• Helps to predict the path of the astronomical bodies.

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail for 10th Science : Chapter 1 : Laws of Motion : Apparent Weight.

Mastering Notice and Article Writing | The Attic by Satyajit Ray | Prose Guide

Writing (Notice, Article writing) - The Attic | by Satyajit Ray | Prose

Writing

NOTICE:

A notice is a formal means of communication. The purpose of a notice is to announce or display information to a specific group of people. Notices are generally meant to be pinned up on specific display boards in schools or in public places.

How to write a notice:

1. The name of the school or the institution must be prominent It should be clear, legible and in CAPITAL letters.
2. The name of the program for which the notice is drafted should be highlighted.
3. The date of drafting the notice, must be written on the top left/right corner of the box.
4. You can start the notice by using expressions like……
   • This is to inform all the students…..
   • All the students are informed.....
5. You must include details such as …….
   • What/when/why/where/for whom is the programme…
   • Date of registration, last date....
6. The final sentence can be…
   • For further information, details contact…..
   • For further details contact………
   • Contact the undersigned person….

Sample 1. You are Nikil/Nikitha, school pupil leader of GHSS, Trichy. Prepare a notice on behalf of your school inviting the grandparents of the students to celebrate World Elders’ Day in your school auditorium on the 20th of next month.

Prepare notice for the following

i. You are the school monitor, of Modern Matriculation School, Villupuram. Your school Principal has requested you to inform the students about a trip to Yercaud for 3 days. Prepare a notice giving the details such as date of journey, mode of transportation, amount , dress code etc.

NOTICE

MODERN MATRICULATION SCHOOL, VILLUPURAM

TRIP TO YERCAUD

16 November 2019

All the students are informed that our school management has arranged for a trip to Yercaud for three days. Interested students are requested to pay Rs.500 on or before 20 November 2019. Our school bus will start at 8.00 a.m on 25 November 2019.

Please bring casuals and sweaters.

Kamal/Kamala
School Monitor

ii. You are the Secretary of Park Circus Residents Welfare Association. Write a notice to inform the residents of your colony of a Meditation program under the guidance of Dr. P. Ranjit with a view to understanding the self better. The program is exclusively for the residents It will be conducted on the second Saturday of the following month from 7.00 a.m. to 9.00 a.m. at the children’s park nearby.

NOTICE

Park Circus Residents Welfare Association

Meditation Program

17 November 2019

This is to inform all residents of our colony that there will be a meditation program under the guidance of Dr. P. Ranjit for understanding the self better. The program will be conducted on the second Saturday of the following month from 7.00 a.m to 9.00 a.m at the nearby children’s park.

Kumar
Secretary

iii. You are Ganesh/Gayathri Head boy/ Head girl, of your school. Write a notice for your school notice board informing the students about the ‘Fancy Fete’ that is going to be organised in your school campus on the 10th of next month.

NOTICE

ST. JOSEPH’S COLLEGE HR. SEC. SCHOOL, TRICHY

FANCY FETE

21 November 2019.

Our school is going to organise ‘Fancy Fete’ for all our students on 10th December 2019. All students are invited to attend the function in our school auditorium at 3.00p.m. Interested students can enroll their names to participate in the competitions on or before 25th November 2019.

Ganesh/Gayathri
Head boy/Head girl

Article writing

Article writing is the process of creating non-fictitious text about current or recent news. It can be items of general interests or specific topics. They are published in print forms, such as newspapers and magazines. Article writing is a skill that needs to be practised.

Steps involved in writing an article:

1. Decide the theme: Choose an interesting, relevant or a current issue.
2. Decide the title: The title suggests the core idea of the article. It has to be brief and captivating, kindling the interest of the readers.
3. Form an outline: Forming an outline of the article is very essential. It can be done in three steps:
   • *Introduction
   • *Body
   • *Conclusion
4. Draft the content: When your outline is complete start expanding on the title.
5. Edit it: Never submit an article in its first draft. Revise the article until it expresses your thoughts completely. Give it a final reading. Edit it and correct the errors.
6. Final Reading: Once the article is edited, give it a final read. Check if it adheres to the requirements.

Sample: The following is an article by Arjun / Anjana on the causes and effects of pollution.

Pollution a major concern

-By Arjun / Anjana

Pollution is a major issue in India. Anything added into the environment that results in producing harmful or poisonous effect on living things is called pollution. It is one of the considerable issues for the whole world. It is a kind of impurity in natural environment that is harmful for all the living beings on earth. Pollution whether it is air, land, noise or water always has adverse effects.

India is the world’s largest consumer of fuel wood, agricultural waste and biomass for energy purposes, which releases millions of tonnes of pollutants into the air every year. Vehicle emissions, another source of air pollution, get worsened by fuel adulteration and poor fuel combustion efficiencies from traffic congestion. Factories pollute air through fossil fuel emissions. These emissions include carbon dioxide, methane and nitrous oxide.

Air pollution is the main cause for the monsoon to be delayed. Air pollution is the major cause for several health hazards. It damages vegetation and animal life too.

Steps must be taken to clean smokestacks and exhaust pipes in factories. Vehicles must be checked periodically and maintained meticulously. We can opt for renewable or alternative energy sources. Using such renewable and sustainable energy sources reduces pollution. Creating awareness is the remedial measure to check pollution.

We as responsible citizens, must willingly contribute to the reduction of air pollution. Ecological issues are an integral part of environmental issues that challenges India. So it is high time we take stringent steps to stop pollution that affect us. It’s time for action.

R. Write an article for the follow

i. You are Jansi/Avinash of Class X studying in GHSS, Chengalpet. You believe that physical activities improve our health and reduce the risk of sickness. It has got immediate and long term benefits. Write an article in not more than 150-200 words for your school magazine stressing the importance of physical activities in a students day to day life.

A Sound Mind in a Sound Body

- By Jansi/Avinash
Class X

The proverb A Sound Mind in a Sound Body insists on both healthy body and stable mind. A person’s health is a combination of physical, mental and social well being. A healthy body is essential for a healthy mind. First and foremost, everyone should be health-conscious. Good health is ensured by a balanced, diet and good exercise. Balanced diet supports our body avoiding excessive fats, minerals, salts, etc. Physical activity or exercise enhances our health and provides us happiness. The second asset of a well composed person is mental health. Cognitive functions are ensured by the stability of one’s mind. Yoga is invariably advisable to be a mentally balanced person. It enhances and reinforces concentration and attention at a task in hand.

Both physically and mentally stable persons possess a positive attitude towards life. They become socially responsible, individually productive and globally connected. They are constructive, creative, optimistic, and innovative throughout their life. The world is in need of such people. To sum up, physically strong people possess mental health and are sociable. Hence, all people must be health conscious and avoid all sorts of diseases that are the fruits of idleness and sluggishness.

ii. The service provided by the conservancy workers in your city is very poor. You find all the street corners dumped with garbage thrown by the residents of the locality. It causes a menace for the public at large. You are Ramya/Rajan of Class X, studying in TM Model School, Dharmapuri. Write an article in about 150-200 words to the editor of The Indian Express, about this and suggest ways by which the situation could be improved.

Cleanliness is Next to Godliness

- By Ramya/Rajan
Class X
TM Model School, Dharmapuri

The proverb ‘Cleanliness is Next to Godliness’ indicates that cleanliness leads to the health of humanity. Maintaining cleanliness provides hygiene and good health to the people. Every one is responsible for such a noble task. Residential areas are to be strictly kept clean whereas in our city heaps of garbage are found in the densely populated areas. Unclean and untidy streets cause health hazards to the inhabitants. Undeniably, vendors dispose off the waste wherever they set up a business. Residents of the city dump the waste on bins. Dogs and cattle squatter upon the waste and consume it. Garbage is hazardous to one’s health and the surroundings.

Hospitals, schools and industries become the target of such nuisance in our city. Pedestrian's throw litter along the streets and roads. People, young and old defecate on roads and streets polluting the environment. The mounds of garbage near schools and hospitals pose a great threat to the health of the residents. Nauseating stench all over the city is a menace to the well being of the city dwellers. Heaps of garbage, lying around remain uncleared for weeks. They are the breeding place for mosquitoes that cause diseases such as Dengue fever, Malaria, Hepatitis, Dysentery and Typhoid

Grievances have been submitted to the officers. Government’s apathy in maintaining congested traffic, controlling dust pollution and fixing filthy potholes is intolerable. Increasing garbage leads to environmental degradation and it also affects our health. To fight against the worsening conditions, regular clearance of garbage is necessary. The city administration must pump men and machines into action. Cleanliness Awareness programmers should be organized in the city. More men and women should be employed to carry out the task. I sincerely, request your intervention to keep the city clean and green.

iii. Recently while returning home from school you were knocked down by a speeding motorcycle. You escaped with minor injuries. You are Kishore/ Kavitha of class XI, studying in GHSS, Coimbatore. Write an article to The Hindu, in about 150-200 words expressing your concern about the increasing number of road accidents due to reckless driving. Also stress the importance of following traffic rules.

Importance of Road Safety Rules

~ By Kishore/ Kavitha
Class XI
GHSS, Coimbatore.

Accidents have become a day-to-day affair. Whether on national highways or city roads, most of the accidents are either due to reckless driving or negligence of road safety rules. Besides, the increase in the number of vehicles causes traffic jam. It is to be regulated. Road safety and accidents are crucial issues of today. Almost every day people face accidents, sometimes causing death. In spite of observing road safety, every year, people are ignorant of the rules and impatient while driving. Most of the accidents are caused due to reckless driving. Impatient, negligent drivers overlook the safety rules and speed up their vehicles causing confusion in the traffic.

Highway drivers drive as fast as they can, assuming no hurdles on their way. Rail roads, bypass roads, bordering villages are dangerous due to the cattle crossing. To avoid accidents, immense attention is required while driving. Drivers and pedestrians should strictly follow the safety rules in order to avoid deadly accidents. Safety rules, awareness programmes should be organised at school and college levels. Each panchayat, municipality and city corporation should spread road safety awareness among people. Importance of wearing helmets and seatbelts must be emphasized. Road safety rules should be displayed along the roads as reminders. Let us all join together and spread awareness about road accidents and the importance of following road safety rules to save humanity.