Waves

Waves are oscillations of particle that transfer energy from one point to another.

Examples of waves:

1. Sound waves - produced by the vibration of matter (solid, liquid and gas). Sources: tuning fork, guitar string.
2. Light waves - produced by the vibration of bundles of photons. Sources: Sun, light bulb.
3. Water waves - produced by the vibration of water molecules. Sources: river, sea.

There are two main types of waves, namely longitudinal wave and transverse wave. A third type of wave constitute the combination between the two.

Longitudinal wave is the oscillation of a particle, in which the direction of wave propagation is parallel to the vector of the particle vibration. Transverse wave is a form of vibration in which the wave propagates at right angle to the vector of particle oscillation.

Longitudinal Waves

Longitudinal waves are oscillations in which the particle vibrates parallel to the direction of wave propagation. Examples of longitudinal waves are sound waves and ultrasound.

In the figure above, disturbance to the spring in the horizontal plane results in its motion parallel to the direction of disturbance. In (c), as the body of the spring transfers the energy applied from the hand, the waves propagation splits the spring into alternative areas of compression (lesser displacement by a fixed number of coils) and rarefaction (higher displacement by the same, fixed number of coils).

Similarly, sound waves occur when the atmospheric particles are alternatively compressed and stretched.

Transverse Waves

Transverse waves are oscillations where the particle oscillates perpendicular to the direction of wave propagation.

As opposed to longitudinal waves, transverse waves have crest (region of maximum displacement) and trough (region of maximum amount of negative displacement).

A ripple on the pond and a strummed guitar chord are examples of transverse waves.

Electromagnetic Spectrum

The electromagnetic spectrum is the range of frequencies of electromagnetic radiation. It is arranged in increasing frequencies (thus, decreasing wavelengths), in the following order:

1 - Radio waves
2 - Microwaves
3 - Infrared (IR) rays
4 - Visible light
5 - Ultraviolet rays
6 - X-rays
7 - Gamma rays

As the frequency of the waves increases, the energy that they carry too increases!
Radio waves, having the lowest frequency, carry the lowest energy relative to that of the members of its spectrum.
Conversely, gamma ray, having the highest frequency, carry the highest amount of energy compared to the rest.

EM radiation in our daily lives
1 - Radio wave
      - produced by electrons oscillating in aerials
      - applications: long-range radio transmission, TV signal transmission
2 - Microwave
      - produced by a microwave transmitter
      - applications: microwave oven used for cooking, microwave lasers
3 - Infrared
      - produced by objects that radiate heat
      - applications: night-vision goggles, human body
4 - Visible light
      - the only component of EM spectrum that are visible to the naked eye
      - produced by the Sun, lamp, lightbulb, etc.
      - applications: sunlight (photography, enables vision), photosynthesis
5 - Ultra-violet rays
      - produced by the Sun
      - applications: fake note detector, sterilisation of surgical tools
6 - X-ray
      - produced by an X-ray tube
      - applications: X-ray scans, luggage scans at the airport
7 - Gamma ray
      - produced by fast-moving electrons
      - applications: radiotherapy for cancer patients, detection of leakage of underground pipes

Electromagnetic Waves

Electromagnetic waves (EM Waves) are waves that are produced when an electric and magnetic field vibrate perpendicular to one another. Thus, it is implied that the direction of wave propagation is at a 90-degree angle to both fields.

From the figure above, the magnetic field (in blue) oscillates in the x-axis while the electric field (in red) oscillates in the y-axis, resulting in the wave propagation in the z-axis (towards the right side of the page).

The properties of EM waves:
1 - They transfer energy from a point to another.
2 - They consist of transverse waves.
3 - They can travel through vacuum.
4 - They travel at the speed of light (c = 3 X 108 m/s2)
5 - They exhibit ALL properties exhibited by waves:
       i. reflection
      ii. refraction
     iii. diffraction
     iv. interference

Application of EM Waves

EM waves play a huge role in our daily lives. We use them for various purposes, such as:

Radio waves (f = 10⁴Hz)
They are used to transmit television and radio programmes - television having a higher frequency than radio.

Microwave (f = 10¹⁰Hz)
We use microwave to cook food and transmit mobile phone signals. Energy from microwaves are absorbed by water molecules, which causes water to heat up and in turn cook the food. As microwaves contain more energy than radio waves, they can be used to transmit information to and from the satellites in orbit.

Infrared (f = 10¹²Hz)
Infrared from the Sun is absorbed by the skin and is felt as heat energy. We use IR in toasters and electric heaters.

Visible light (f = 10¹⁴Hz)
As the name implies, visible light is the only member of the EM spectrum that is visible to the naked eye. It can be split by a prism, which diffracts the different components of the white light based on their speed as they pass through a different medium (solid) from air.

Ultraviolet ray (f = 10¹⁶Hz)
UV ray is naturally found in sunlight. The skin turns darker when exposed due to the increased activity of melanocytes in the skin which produces more of the protective pigment, melanin in turn to the increased exposure towards UV ray. The darker skin (more melanin) absorbs more UV rays, thus causing less of them to reach the deeper aspects of the skin. UV rays have a carcinogenic effect towards the skin, thus having more of melanocytes protects the individual against skin cancer.

X-ray (f = 10¹⁸Hz)
Having a high amount of energy, x-ray can penetrate the living tissues of the human body, except the bone which allows for the use of X-ray in medical imaging to observe bone fractures and presence of fluid in the lungs. Upon penetration, X-rays are absorbed by the bone, thus leaving a clear imprint on an X-ray sheet in contrast to the different body tissues which either partially or do not absorb the rays at all, thus causing them to pass through the body and be detected on the X-ray sheet as black regions. However, low doses of X-rays may induce cancer in human, thus protective vests and lead tags are worn by the medical personnel when dealing with the X-ray machines.

Gamma ray (f = 10²⁰Hz)
Gamme ray, being the most energetic amongst its members of the EM spectrum, has the highest frequency of all. It mostly passes through the skin and body, but some may be absorbed by cells. It can neither be seen nor felt. It is a byproduct of molecules undergoing nuclear reaction. Having a very high amount of energy, it has the potential of disrupting the electrons from their configuration, thus causing disastrous effects on the human DNA when it collides upon them. Therefore, gamma rays are widely used as an anti-cancer therapy to cause the destruction of the cancer cell DNA, rendering them unable to undergo mitosis, thus leading to their death.