wave motion

• Wave motion: a propagation of disturbance that travels from one location to another.
• Displacement: distance of a point from its undisturbed (equilibrium) position
• Amplitude: maximum displacement of particle from undisturbed position
• Period: time taken for one complete oscillation
• Frequency: number of oscillations per unit time

  f=1/T
• Electromagnetic Waves

  • As electromagnetic wave progresses, wavelength decreases and frequency increases
• electromagnetic waves

  
• • Visible light: 400 nm - 700 nm
• All electromagnetic waves:

  • All travel at the speed of light: 3*10^8ms^-1
  • Travel in free space (don’t need medium)
  • Can transfer energy
  • Are transverse waves
• Radio waves: 1 mm – 1 km
• Microwaves: 1mm – 1 m
• Infrared radiation: 1μm – 1 mm
• Visible Light: 400nm-700nm
• Ultraviolet radiation: 25nm – 400nm
• X-rays: 0.01nm – 10nm
• If two or more waves overlap, the resultant displacement is the sum of the individual displacements. Displacement is a vector quantity. The overlapping waves are said to interfere. This occurrence is called as interference.
• If two waves arrive in phase (their crests arrive at exactly the same time), they will interfere constructively. A resultant wave will be produced, which has crests much higher than the two individual waves, and troughs much deeper.
• If the two waves arrive in antiphase (with a phase difference of π radians or 180°), the peaks of one wave arrive at the same time as the troughs from the other, and they will interfere destructively. The resultant wave will have smaller amplitude.
• Interference of sound waves is possible when two or more sound waves having the same frequency collide. The waves with the same frequency (constant phase difference between them) are called coherent waves.
• Young’s double slit experiment is used to study the interference of light waves. In Young’s double slit experiment, the distance on the screen between successive bright fringes is called the fringe width.
• Fringe width can be calculated by the equation (λD)/d. where λ – Wavelength of the light source (m).

  D – Distance between screen and double slit (m).

  d – Distance between centres of two slits S1 & S2 (m).
• Defining the Huygens Principle

  Huygens principle is a method of analysis applied to wave propagation problems both in the far-field limit and near-field diffraction and reflection. It states that:

  “Every point on a wavefront is in itself the source of spherical wavelets which spread out in the forward direction at the speed of light. The sum of these spherical wavelets forms the wavefront”.
• Advantages and Disadvantages of Huygens Principle

  Advantages:

  • Huygens concept proved the reflection and refraction of light.
  • The concepts like diffraction of light, as well as interference of light, were proved by Huygens.

  Disadvantage:

  • Concepts like emission of light, absorption of light and polarisation of light were not explained by Huygens principle.
  • Huygens principle failed to explain the photoelectric effect.
  • A serious drawback is that the theory proposes an all-pervading medium required to propagate light called luminiferous ether. This was proved to be false in the 20th century.
• Characteristics of Sound Waves

  Sound cannot travel through a vacuum. This is very much in contrast with the property of light.
• Amplitude

  Amplitude in light refers to the amount of energy in an electromagnetic wave and its meaning is the same here. Amplitude refers to the distance of the maximum vertical displacement of the wave from its mean position. Larger the amplitude, the higher the energy. In sound, amplitude refers to the magnitude of compression and expansion experienced by the medium the sound wave is travelling through. This amplitude is perceived by our ears as loudness. High amplitude is equivalent to loud sounds.

  
• Wavelength

  The waveform representation converts the pressure variations of sound waves into a pictorial graph which is easier to understand. A sound wave is made of areas of high pressure alternated by an area of low pressure. The high-pressure areas are represented as the peaks of the graph. The low-pressure areas are represented as troughs on the graph. The physical distance between two consecutive peaks in a sound wave is referred to as the wavelength of the sound wave. It is labelled in the image above.
• Frequency in a sound wave refers to the rate of the vibration of the sound travelling through the air.
• Frequency decides whether a sound is perceived as high pitched or low pitched.
• In sound, the frequency is also known as Pitch.
• The frequency of the vibrating source of sound is calculated in cycles per second.
• The SI unit for frequency is hertz and its definition is ‘1/T’ where T refers to the time period of the wave.
• The time period is the time required for the wave to complete one cycle.
• The wavelength and frequency of a sound wave are related mathematically as:
• The velocity of Sound = Frequency * Wavelength.
• The first graph represents a sound wave from a drum while the second graph represents the sound wave from a whistle.
• You probably already know the difference in the sounds but have a look at the difference in their frequencies.
• frequency or pitch