Understanding Physics Topics is essential for solving complex problems in many fields, including engineering and medicine.
What is Incident Ray?
If a moving surface wave on water strikes a wall, it is found that another wave starts moving along the surface of water in the opposite direction. Again, standing at a certain distance from a large wall if a loud sound is made, a similar sound comes back to our ears after a while. We know that if light falls on a mirror, it changes its direction. These phenomena are examples of reflection of waves, though the nature of the waves are different in different cases.
We know that the direction along which the energy of a wave is transferred is called a ray. It is represented by an arrow- headed straight line. Suppose, a ray travels through a medium and is incident at a point on an interface with another medium. This point is called the point of Incidence. The ray striking the interface is called the Incident ray. The ray returning to the first medium from the point of incidence after reflection is called the reflected ray.
In Fig. PQ is the interface of two media, or a 2nd medium reflector, O is the point of incidence. AO and OB are incident ray and reflected ray, respectively. The perpendicular ON on the reflector drawn at the point of incidence O is called the normal. The angle between the incident ray and the normal is called the angle of incidence. Similarly, the angle between the reflected ray and the normal is called the angle of reflection.
Laws of reflection: Reflection of waves obeys two laws. These are known as the laws of reflection.
- The incident ray, the reflected ray and the normal to the reflector at the point of incidence lie on the same plane.
- The angle of incidence is equal to the angle of reflection, i.e., i = r.
Reflection of Sound
Sound propagates in the form of waves from one place to another. So, like other waves sound also exhibits the phenomenon of reflection, the phenomenon of reflection.
Plane reflector: A plane reflector R is placed vertically on a horizontal table [Fig.]. A tall partition P perpendicular to the reflector at O divides the table into two halves. Two tubes A and B are placed on the table in such a way that their axes are directed to O and they make equal angles with OP (i.e., ∠AOP = ∠BOP). Now, if a source of sound (e.g., a table clock) is placed in front of the tube A and a receiver (e.g. ear) is placed in front of the tube B, the sound of the clock is heard distinctly.
Due to the partition in the middle, the sound of the clock cannot reach the ear directly; rather after being reflected at O of the reflector it reaches the receiver, i.e., the ear. If ∠BOP is changed a little by rotating the axis of the tube B or by raising the tube from the table slightly, sound is not heard any more. From this experiment it is evident that
- sound has been reflected from the reflector,
- the angle of incidence is equal to the angle of reflection and
- the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane (plane of the table).
Concave reflector: Two concave reflectors A and B are placed in such a way that their axes lie on the same straight line [Fig.]. A source of sound (e.g., a table clock) is placed at the focus of A. The sound waves produced from the clock get reflected from the reflector A and propagate parallel to the axis. These parallel waves are incident on the reflector B and again get reflected from its surface. They meet at the focus of B. If we place our ear there, a distinct sound is heard.
Size of the reflector: To obtain an effective reflection of a wave from a reflector, the size of it should be much greater than that of the wavelength. The wavelength of audible sound in air varies from 1.5 cm to 16 m. On the other hand, the wavelength of visible light varies from 4 × 10-7 m to 8 × 10-7 m, i.e., a sound wave is much longer than a light wave. So, light can be reflected from very small reflectors, but for reflection of sound the size of the reflector should be very large.
Regular reflection of waves takes place from a smooth reflector. If the surface of the reflector is rough, diffused reflection takes place. Whether a surface is smooth or rough is known from the knowledge of the wavelength. For example, a large wall may look smooth to naked eyes, but it has many fine notches. However these notches are much smaller than the wavelength of sound. So in case of reflection of sound, a large wall may be taken as a smooth reflector, and regular reflection of sound takes place from it. On the other hand, the notches of that wall are many times larger than the wavelength of light, and hence, light suffers diffused reflection from the wall.
In short, it may be said that, to get a regular reflection of sound,
- the reflector must be large enough and
- the surface of the reflector need not be very smooth. So buildings, long walls, mountains, rows of trees, etc., act as reflectors of sound.