WEEK 12 (Electromagnetic waves)

Question 1

What are the properties of Electromagnetic waves?

Answer 1

• Electric field lines originate on positive charges and terminate on negative charges
• Magnetic field lines always form CLOSED LOOPS (they don’t begin or end anywhere)
• A varying magnetic field induces an ELECTRIC FIELD
• Magnetic fields are generated by moving charges/currents

Question 2

What is a statement of Faraday’s law?

Answer 2

A varying magnetic field induces an electric field

Question 3

What is a statement of Ampere’s law?

Answer 3

Magnetic fields are generated by moving charges/currents

Question 4

What did James Maxwell do?

Answer 4

• Created a mathematical model and proved that Electricity and Magnetism play a similar role in nature
• Predicted existence of electromagnetic waves
• Predicted that accelerated electric particles could emit electromagnetic waves
• Calculated the speed of electromagnetic waves

Question 5

Are Electromagnetic waves Transverse waves or Longitudinal waves?

Answer 5

Transverse waves

Question 6

What creates an alternating electric field around an antenna?

Answer 6

Oscillating charges

Question 7

What creates an alternating magnetic field around an antenna?

Answer 7

Oscillating current

Question 8

What are the different properties of Electromagnetic waves?

Answer 8

• SPEED
• INTENSITY (power per area)
• TOTAL MOMENTUM

Question 9

What is the visible light wavelength?

Answer 9

0.4-0.7 μm = 400-700 nm = 4000-7000 Å

Question 10

What are the properties of Electromagnetic waves?

Answer 10

• Travel at the speed of light
• Transverse waves since electric and magnetic fields are PERPENDICULAR to the direction of propagation of the wave and to each other
• The RATIO of the electric field to the magnetic field in an electromagnetic wave equals the SPEED OF LIGHT
• Carry both ENERGY and MOMENTUM which can be delivered to a surface

Question 11

Describe Thomson’s model of the atom

Answer 11

Electrons are embedded inside the positive charge like seeds in a watermelon

Question 12

Describe Rutherford’s model of the atom

Answer 12

Rutherford’s model proposed that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it with very high speed in circular paths. He named these circular paths as orbits

Question 13

What were the Geiger-Marsden experiments?

Answer 13

The Geiger–Marsden experiments (also called the Rutherford gold foil experiment) were a landmark series of experiments by which scientists learned that every atom has a nucleus where all of its positive charge and most of its mass is concentrated

Question 14

The circumference of an electron’s orbit must contain an integral number of ________________________

Answer 14

de Broglie wavelengths

Question 15

What is Bohr’s model of the atom?

Answer 15

In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called orbits or shells. Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells

Question 16

Describe Bohr’s model of the atom

Answer 16

• The electron moves in CIRCULAR ORBITS about the proton under the influence of the COULOMB FORCE OF ATTRACTION; the coulomb force produces the electron’s CENTRIPETAL ACCELERATION
• In stable electron orbits, no energy in the form of electromagnetic radiation is emitted so the total energy of the atom remains CONSTANT
• Radiation is emitted by the hydrogen atom when the electron “jumps” from a more energetic initial state to a less energetic state. The FREQUENCY of the radiation emitted in the jump is related to the CHANGE in the atom’s ENERGY

Question 17

When will an Atom emit radiation?

Answer 17

Only at certain frequencies that correspond to the energy separation between the various allowed states

Question 18

What does LASER stand for?

Answer 18

L ight
A mplification
by
S timulated
E mission
of
R adiation

Question 19

What are the properties of Laser?

Answer 19

• Intense, coherent light in a laser is a result of STIMULATED EMISSION
• Voltages can be used to put more electrons in excited states than in the ground state (POPULATION INVERSION)
• Excited state must be a METASTABLE STATE (relatively long lifetime); this allows STIMULATED EMISSION to occur before SPONTANEOUS EMISSION
• Photons produced must be retained for a while to stimulate the production of more photons

Question 20

Describe how the Helium Neon Laser works

Answer 20

1) The current (electrons) excite the helium atoms by COLLISIONS which then collide with neon atoms, which emit light along the length of the tube
[The helium atoms excite the neon atoms to a higher level by collisions & the neon atoms stay long enough to be forced into stimulated emission]
2) The light passes through transparent windows and reflects back and forth through the tube from mirrors to cause more neon atom emissions
3) Some of the light leaks through a mirror to form the laser beam

Question 21

Describe how the Rubi Laser works

Answer 21

1) The tube contains atoms, which represent the active medium
2) An external source of energy is needed to “pump” the atoms to excited energy states
3) The parallel end mirrors provide the feedback of the stimulating wave

Question 22

What is the difference between NMR spectroscopy and EM spectroscopy?

Answer 22

NMR SPECTROSCOPY involves interaction of electromagnetic radiation with the nuclei of atoms in a molecule whereas most form of EM SPECTROSCOPY involve the interaction of light with the electrons in the molecule

Question 23

What is an advantage of NMR (Nuclear Magnetic Resonance) Spectroscopy?

Answer 23

NMR can provide much more structural details (higher resolution) than other forms of spectroscopy

Question 24

Which portion of the EM spectrum does NMR use?

Answer 24

Radio Frequency portion of the spectrum

Question 25

How does NMR work?

Answer 25

NMR involves the application of a strong magnetic field to the sample being studied. This magnetic field ALTERS and LIMITS some of the energy states available to the nuclei which makes it possible to measure the ABSORPTION and EMISSION of EM by the nuclei in the sample

Question 26

What are the different applications of Electromagnetic Waves in Medicine?

Answer 26

• MICROWAVE HYPERTHERMIA THERAPY
  [Repeated heating of a tumour is toxic for cancer cells and makes them more vulnerable to traditional chemo and radiotherapy]
• PULSE OXIMETRY
  [Measure of O2 saturation level in blood]
• INFRARED MEASUREMENT OF TEMP
• THERMOGRAPHY
• ENDOSCOPY
• PHOTOTHERAPY
  [Treatment of newborn jaundice using blue light by lowering BILIRUBIN LEVELS in baby’s blood via PHOTO-OXIDATION & treatment of sleeping disorder]
• STOMAGOLOGY
  [Speed up hardening of tooth filling]
• STERILISATION/GERMICIDE
• DIAGNOSTIC (X-Rays, CT etc)