SACE Stage 2 Physics Light and Matter Electromagnetic Waves

Characteristics of Electromagnetic Waves Light is considered to consist of oscillating electric and magnetic fields at 90 o to each other and at right angles to the direction of travel. Y Z X E E B B Direction of propagation

Characteristics of Electromagnetic Waves

The frequency of the wave refers to the frequency of the periodic variations in the electro-magnetic fields. The wavelength of light refers to the distance between two consecutive points in space where the electric field (or magnetic field) is in phase. Electromagnetic induction suggests that a changing magnetic field will induce a changing electric field which in turn will induce a changing magnetic field and so on. Waves that consist of oscillating electric and magnetic fields are known as electromagnetic waves.

Characteristics of Electromagnetic Waves Electromagnetic waves are generated by accelerating (vibrate) electrons. Electrons which vibrate in a single plane will generate electromagnetic waves with the electric field restricted to a single plane. Similarly the magnetic field will also be restricted to a plane at right angles to the electric field. A polarised wave is one in which the electric field is confined to a single plane and the magnetic field vectors are confined to a single plane at right angles to the electric field. We define the plane of polarisation to be the plane of the electric field in the polarised electromagnetic wave.

Production of Electro-Magnetic (E-M) Waves by an Antenna When any charged particle (eg. an electron) accelerates, an E-M wave is produced. If a circuit, therefore, has a vibrating or alternating current in it the charges are continually accelerating thus will radiate E-M waves. An alternating voltage applied to a length of metal (an antenna) will thus radiate an E-M wave. ~~ E E E E ~ E ~ ~ Two metal rods are connected to an AC generator. The charges on each rod then alternate, creating an alternating Electric field, thus radiating an E-M wave - which travels at the speed of light.

Reception of an E-M Wave by an Antenna: When an EM wave hits an antenna, the free electrons in the antenna will be forced to vibrate (by induction). If a simple circuit is connected to the antenna and the circuit is tuned, so that a narrow band of frequency will cause the electrons to resonate. This signal is the amplified and sent to the appropriate audio visual device. The orientation of the antenna should match the plane of polarization of the EM wave.

Speed, Frequency and Wavelength In a vacuum any electromagnetic wave will travel at the speed of light c = 3 x 10 8 m s -1. (This is a constant and is the fastest speed possible.) Since for a wave, the time taken for one complete wavelength (  s = ) to pass a point is the period (T) of the wave. The frequency of the wave is the number of waves past a point in one second, hence, f = 1 / T

Thus, v = c,  s =, and  t = T = 1/f Then, becomes, Ie, Speed, Frequency and Wavelength

Example A ray of green light has a wavelength of 540nm in a vacuum. Find its frequency.

Speed, Frequency and Wavelength Example A ray of green light has a wavelength of 540nm in a vacuum. Find its frequency.

Speed, Frequency and Wavelength Example A radio station broadcasts at a frequency of 720kHz. Find its wavelength.

Speed, Frequency and Wavelength Example A radio station broadcasts at a frequency of 720kHz. Find its wavelength.

Application: LADS LADS (Laser Airborne Depth Sounder) is a system that uses an aircraft to fly over a body of water to automatically measure the depth of the water. The LADS system works by emitting a laser at the water. When the laser ‘hits’ the water, it is partially reflected and transmitted. The reflected laser the returns to the plane. The transmitted laser continues through and is reflected of the bottom of the water and back up to the plane. Knowing the speed of light in water, the time delay between the reflected and transmitted ray can be used to determine the depth.

Application: LADS The extra distance travelled by the second pulse (transmitted ray) is to the bottom of the water and back, i.e., twice the depth of the water. Therefore the depth of the water is half of the extra distance travelled.

Application: LADS Example An aircraft carrying a LADS system is flying horizontally above a lake. The laser pulses are reflected of the surface of the water and of the bottom of the lake return to the aircraft 3.33  s and 3.55  s after being emitted. If the speed of light in fresh water is 2.25 x 10 8 ms -1, calculate the depth of the water where the sounding was taken.

Application: LADS Example A aircraft carrying a LADS system is flying horizontally above a lake. The laser pulses are reflected of the surface of the water and of the bottom of the lake return to the aircraft 3.33  s and 3.55  s after being emitted. If the speed of light in fresh water is 2.25 x 10 8 ms -1, calculate the depth of the water where the sounding was taken. The extra distance travelled by the second pulse is 49.5m, therefore the depth of the lake is 24.75m deep.

The Necessity To Use A Powerful Laser Lasers used in the LADS system are 1Megawatt. This is a very powerful laser! Why? Suspended sediment in the water can scatter the laser reducing the amount of energy reaching the bottom. Water tends to absorb the light that passes through it. The seabed also absorbs light. Only a small fraction of the energy actually return to the aircraft as it scans from side to side and the surface of the water and seabed is quite rough. The laser used is very power full and could blind someone below, therefore the laser is spread out over a distance to reduce the risk of injury below.