Eighth Grade Activity:Waves and Waves Activity

WAVES

A Wave is a transfer of energy, in the form of a disturbance through some medium, without translocation of the medium. A wave having a short duration is called a pulse. Waves that vibrate in repeating cycles illustrate periodic motion or harmonic motion. One complete oscillation is called a cycle.

The first three examples are in simple harmonic motion:

TYPES OF VIBRATION

Transverse vibration occurs when vibration of the medium is perpendicular to the direction of energy transfer. The maximum and minimum positions on a transverse wave are called the crest and trough.

  

The direction of the wave is from left to right (green line) but the oscillation occurs up and down.

Longitudinal vibration occurs when the medium vibrates parallel to the direction of energy transfer. Longitudinal vibrations consist of regions of compression and refraction.

The direction of the wave is from left to right (green line) and the oscillations occur from left to right also.

Frequency (f) describes the number of cycles occurring in a given unit of time. The SI unit for frequency is hertz (Hz). The units cycles / second are also used.

Frequency = Number of cycles / Time Interval

Period (T) is the time required to complete one full cycle.

Period and frequency exhibit a reciprocal relationship.

The Amplitude (A) of a wave is the maximum displacement from the equilibrium or the rest position.

Wavelength (l) is the distance between successive crests or successive troughs. The wavelength is the shortest distance between two points in phase (or two points that have identical characteristics).

The universal wave equation (v = fl) is said to be universal because it applies to all types of waves. The universal wave equation can be used to solve various problems relating to wave motion.

PROPERTIES OF WAVES

Reflection of Waves. When a series of waves strikes a surface, the waves are reflected. Waves falling upon the barrier are called incident waves and waves reflecting from the barrier are called reflected waves. Reflection of waves can be summarized in the following law of reflection: the angle of incidence (q_i) is equal to angle of reflection (q_r). Here is a diagram illustrating the reflection of waves:

Refraction of Waves. Refraction occurs at the boundary between two different media and causes the waves to change direction as shown in the following diagram:

Diffraction of Waves. Diffraction is the bending that occurs when a wave passes around the edge of an obstacle. Waves having longer wavelengths are diffracted more than those with shorter wavelengths. When waves pass through a slit, diffraction is maximized when the wavelength and the slit width are within the same order of magnitude.

In the figure above the waves are coming from the left, they hit a barrier (in red).  Those waves passing the slit will create patterns depending on the wavelength.

LAWS OF REFLECTION

A light ray is a stream of light with the smallest possible cross-sectional area. (Rays are theoretical constructs.) The incident ray is defined as a ray approaching a surface. The point of incidence is where the incident ray strikes a surface. The normal is a construction line drawn perpendicular to the surface at the point of incidence. The reflected ray is the portion of the incident ray that leaves the surface at the point of incidence. The angle of incidence is the angle between the incident ray and the normal. The angle of reflection is the angle between the normal and the reflected ray.

The Laws of reflection:

   - The angle of incidence is equal to the angle of reflection
   - The incident ray, the normal, and the reflected ray are coplanar

Specular reflection (regular reflection) occurs when incident parallel rays are also reflected parallel from a smooth surface.

If the surface is rough (on a microscopic level), parallel incident rays are no longer parallel when reflected. This results in diffuse reflection (irregular reflection). The laws of reflection apply to diffuse reflection. The irregular surface can be considered to be made up of a large number of small planar reflecting surfaces positioned at slightly different angles. Indirect (or diffuse) lighting produces soft shadows. It produces less eye strain than harsher, direct lighting.

WAVES CLASS ACTIVITY

Answer the following questions using the reading about waves.

1.       What is a wave?

2.        

a.       What are the two main types of vibration?

b.      Write the definition for both types of vibration.

3.       Define the following terms:

a.       Frequency

b.      Period

c.       Amplitude

d.      Wavelength

4.       Make a drawing where you show the following terms: incident wave, reflected wave, angle of incidence, angle of reflection, reflecting surface, normal or perpendicular line.

5.       What does the law of reflection say?

6.       Make a drawing showing refraction of light.

7.       What is diffraction of light?

8.       What is specular or regular reflection?

9.       What is diffuse or irregular reflection?