Light Waves

Question 1

How are Electromagnetic Waves different from Sound Waves?

Answer 1

• They are generated in an entirely different way and they can travel much faster.
• They do not need a medium to propagate and so can travel through the vacuum of space faster than sound waves can in air.
  eg. Light waves, radio waves, x-rays and microwaves.

Question 2

What is Polarization?

Answer 2

It is a phenomenon that can only occur with electromagnetic waves because of the transverse nature.

Question 3

What is a Polaroid made of ?

Answer 3

It is made of long chain molecules all orientated in the same direction and so all electric field directions apart from one particular angle are absorbed.

Question 4

What happens if the chains of molecules in the Polaroid are stretched horizontally?

Answer 4

• Then only light waves that have their electric field in the vertical direction will be allowed to pass through it.
• This is how Polaroid sunglasses reduce reflected glare.

Question 5

When does Band Spectrum tend to occur?

Answer 5

• Band emission spectra are emitted from excited molecules in the gaseous state.
• Molecules have many more energy levels than atoms and the energy levels are in close groups.
• The extra energy levels result from overlap of the energy levels of the individual atoms in the molecule.

Question 6

When does a Continuous Spectrum occur?

Answer 6

• Continuos spectra are emitted from hot solids such as the filament of an incandescent light globe or hot liquids such as molten iron from a smelter.
• In these substances, the atoms are so close together that their outer electrons overlap.
• The energy levels in the atoms are spread out into bands.
• The bands are very close together because of the very large number of atoms.
• Consequently os many different electron transitions are possible that the full range of wavelengths is produced.
• This type of radiation is called black-body radiation and the specific characteristics of the continuous spectrum depend on the temperature.

Question 7

How is light absorbed?

Answer 7

• If white light passes through a gas then the wavelengths corresponding to the permissible electron transitions are absorbed.
• Because these particular energies are absorbed from the spectrum black lines are left where these wavelengths are missing.
• The position of the black absorption lines will correspond to the mission lines of the elements in the gas absorbing the light.

Question 8

What is the function of the absorbed energy?

Answer 8

It is used to promote electrons into higher energy states.

Question 9

What is Stellar Spectra.

Answer 9

• In astronomy, stellar classification is the classification of stars based on their spectral characteristics.
• Light from the star is analysed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colours interspersed with absorption lines.

Question 10

What is Doppler Shift?

Answer 10

• Doppler shift is an effect whereby the wavelength of light is changed due to the motion of the star.
• If the star is moving away from us the wavelength of a particular line in its hydrogen spectrum, say, will be lengthened.

Question 11

What is Red Shift?

Answer 11

• By the time a wave is emitted, the star has moved away and so the wavelength appears longer to an observer.
• The apparent lengthening of wavelengths, so all waves move closer to the red end of the spectrum as a star recedes, moving away.

Question 12

Why do some compounds that are illuminated with ultraviolet light fluoresce, or emit visible light of their own?

Answer 12

• In other words, they convert UV to visible light.
• This occurs because the electrons that are promoted to higher energy levels by the UV fall down in a series of steps instead of falling straight down to the ground state.

Question 13

Explain how fluorescent washing powder work?

Answer 13

• These contain a chemical that absorbs UV and emits white light.
• This makes the clothing washed in a powder containing fluorescent material appear much brighter than normal (albeit no cleaner!).

Question 14

How do Fluorescent lamps work?

Answer 14

• UV light is generated from within the tube, using mercury vapour emission.
• The UV emitted falls on the tube coating and causes it to fluoresce with one of a variety of colours - bluey-white, warm white, pink or other colours.

Question 15

Why Phosphorescent and fluorescent materials different?

Answer 15

This is because fluorescent ones will glow only whilst is being illuminated with UV light and phosphorescent ones will remain glowing after the incident radiation has stopped.

Question 16

How were X-rays discovered?

Answer 16

Willhelm Rontgen (1895) found that when electrons were accelerated through a high potential difference and then collided with a heavy metal target rays were given off which caused objects in his room to fluoresce.

Question 17

What is an X-ray?

Answer 17

It is identified as very short wavelength electromagnetic radiation that could penetrate solid objects.

Question 18

How does modern X-ray work?

Answer 18

• A modern X-ray tube (Coolidge Tube) comprises an evacuated glass tube containing a metal target anode at one end and heated filament cathode at the other.
• When a voltage of about 30kV is applied electrons fly from the filament and strike the target (tungsten, copper or any heavy metal) and a large amount of heat is generated from the loss of Ek by the electrons.
• X-rays are emitted form the side of the tube.

Question 19

What 3 distinct features does a spectrum of x-rays from the tube show?

Answer 19

1. A continuous (whale-shaped) spectrum of wavelengths.
2. A short wavelength cut-off point,
3. Spiked peaks superimposed over the continuous spectrum.

Question 20

How does Bremsstrahlung (Braking Radiation) occur?

Answer 20

1. The electrons accelerated as they leave the negative filament to cross the tube until they strike the target at a very high velocity (around half the speed of light) where they lose their Ek in slowing down as they hit the metal.
2. As these electrons approach the metal atoms they are repelled by the electron shells of the metal in a random way but as they decelerate they emit electromagnetic radiation of varying wavelengths.

Question 21

Explain the shot wavelength cut-off point.

Answer 21

• It is possible that instead of the electron weaving its way though the metal electron shells and gradually slowing, it could lose all its energy in one lump by colliding head-on with a metal atom.
• This would result in the maximum amount of energy absorption from the electron and therefore the shortest emitted wavelength.

Question 22

Explain Spiked peaks superimposed over the continuos spectrum.

Answer 22

• These lines are produced by the innermost orbiting electrons of the target metal being promoted to higher energy levels and then falling to emit X-rays of specific wavelengths.
• It requires a great deal of energy to promote the inner electrons in a heavy metal - in the region of tens of thousands of electron volts - because the repulsive force from the shells above the inner electrons.