#### To verify laws of reflection of sound - Science Practicals

Aim
To verify laws of reflection of sound.

Theory
Sound is reflected following the same laws as followed by light rays. That is, the reflected ray lies in the same plane of incidence (in which the incident ray and normal to the reflecting surface at the point of incidence lies), and the angle of reflection (∠r) is equal to the angle of incidence (∠i ).
First Law:  The incident wave, the reflected wave, and the normal at the point of incidence lie on the same plane.
Second Law: The angle of incidence is equal to the angle of reflection.

Materials Required
Two identical plastic pipes of length approximately 1 m and of diameter approximately 10 cm or less, a protractor (preferably of big size), a meter scale, and a source of low-amplitude sound such as a table-clock.

Procedure
Step 1: In this experiment, very low-amplitude sound waves are heard so peaceful atmosphere is important. Fan is also off to hear the sound clearly.
Step 2: Using a chalk piece or a pencil, draw a line ON on the table (as shown in Fig. 44.2) normal to the wall surface.
Step 3: Now draw a line OC making an ∠i 1 (say 30°) with the line ON.
Step 4: Put one of the two plastic pipes (say PQ) along this line OC such that the end P of the plastic pipe is very close to point O on the wall. Now the axis of pipe PQ lies over the line OC (Fig. 44.2).
Step 5: Now put the second plastic pipe RS on the table, keeping its end R towards the wall on the other side of the normal ON. Mark the position of end R on the table.
Step 6: Keep the table-clock close to the open end Q of pipe PQ.
Step 7: Bring your ear close to the end S of pipe RS. Try to hear the sound of the table-clock through this pipe. Do you hear any sound? Keeping the position of the end R, adjust the position of pipe RS on the table to hear the sound of the table-clock. Mark the position of end S of the pipe RS where you hear the maximum sound.
Step 8: Draw a line OD joining the point O on the wall, the point representing the position of end R, and the point representing the position of end S.
Step 9: Measure ∠NOD. This is the angle of reflection (say ∠r 1 ) for angle of incidence ∠i 1 . Record observations.
Step 10: Keeping the position of the end R fixed, lift the end S of pipe RS vertically to a small height. Are you able to hear the sound of the table-clock through the pipe RS? If yes, lift end S pipe vertically to some more height. Do you still hear any sound? You will observe that on raising the height of end S, the sound of the table-clock either weakens or completely diminishes.
Step 11: Repeat step 2 onwards for three different values of angle of incidence ∠i and find the corresponding values of angle of reflection ∠r.
Step 12: You might have drawn several lines on the table. As courtesy to the students coming to perform this experiment next, it is advised to remove all the lines drawn on the table.

Observations
 Sl. No. Angle of incidence, ∠i (°) Angle of reflection, ∠r (°) ∠ i ∼ ∠r(°) 1. 30° 30° 2. 30° 30° 3. 45° 45° 4. 60° 60°

Results and Discussions
1. The angle of reflection is equal to angle of incidence in all cases.
2. When the pipe through which the sound is heard, is lifted vertically the sound of the table-clock is either weakened or diminishes completely. It shows that the reflected ray lies in the same plane of incidence.
These observations verify that the sound reflects at the surface of a solid and follows the same laws of reflection as in case of light. In case if your observations are different from what are expected, discuss the reasons.

Precautions
1. It is obvious that to hear a clear reflected sound, the incident sound must be clear and smooth.
2. When sound falls on any surface, it is not only reflected but a part of it is also absorbed by the surface of the wall. Hence, the sound that you hear through the pipe depends on the nature of the wall. For a smooth reflecting wall, the reflection will be more. It is thus important to have
a smooth reflecting surface.
3. If we take a larger-amplitude sound source, then you may hear the sound of the source directly (that is the waves coming reaching directly to your ear and not after traveling through pipe QP, reflection from the wall, and through the pipe RS). It is therefore important to have low-amplitude sound source. And for the reasons mentioned here, it is advised to close your other ear while taking observations.
4. In this experiment it is assumed that the table-clock produces a ray
of sound that is incident on the wall along the path QPO and reflected along the path ORSD (Fig. P44.2). That is, the sound source is directional. In reality it is not so. Because of unwanted sound, it is advised to take the pipes of larger length and smaller diameter.
5. Measurements of angles should be done taking the axis of the pipes as incident and reflected rays. Take utmost care and precaution in placing
the two pipes and in drawing the lines OC and OD.
6. Since you are dealing with relatively larger dimensions, it is suggested to use a bigger protractor to measure angles

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