Light Waves

1
Light Waves

• Objectives
• 1. Recognize that light has wave and particle
  characteristics
• 2. Relate energy of light to its frequency
• 3. Know the parts to the EM spectrum

2
Light cannot be described with one model - - -
Light is either described as a wave or as a
particle.
3
Light described as a wave

• Light reflects, refracts, diffracts, undergoes
  interference, and exhibits the Doppler effect in
  the same manner that any wave would reflect,
  refract, diffract

4
Light described as a particle

• We think of light as little packets of energy.
  Blue light has one size packet that is greater
  than the Red light, which has another size
  packet.
• The name we give these packets are PHOTONS
• These photons are NOT particles they have no
  mass think of them as bundles of energy.
• We have this model because of the photoelectric
  effect, which shows that a dim blue light can
  knock an electron out of a metal atom where a
  very bright red light cannot.

5
Which model is correct?

• Different situations call for a different model
• When examining interference, the wave model is
  better suited
• When examining how light travels in space, the
  particle model helps to explain this occurring
  without a medium
• Most scientists think of light having a
  dual-nature, so light has different
  characteristics depending on the situation.

6
The Electromagnetic Spectrum

• Consists of light at all possible frequencies and
  wavelengths.
• All the waves are similar in certain ways they
  also have unique properties.

7
Radio waves

• Radio Waves are used for radio broadcasts,
  amateur radio, television, and mobile phones.
• Different parts of the radio spectrum have been
  allocated to the various services. Radio waves
  have a much longer wavelength that light waves.
• The longest waves are several kilometers in
  length. The shortest ones are only millimeters
  long.
• The really nice thing about radio waves is that
  they will make the electrons in a piece of copper
  wire move this means that they generate electric
  currents in the wire.
• In fact it works both ways alternating currents
  in a copper wire generate electromagnetic waves,
  and electromagnetic waves generate alternating
  currents.

8
Microwaves

• Microwaves have such a short wavelength that they
  are very easily absorbed by water.
• This is why they are used in microwave ovens.
  What happens is that when the water in your TV
  dinner absorbs the microwaves, the energy of the
  microwaves is converted into heat it makes the
  water molecules vibrate faster.
• Some people are frightened that the radio waves
  coming out of their mobile phones are short
  enough to cook their brains.
• Microwaves are good for transmitting information
  (like telephone calls or computer data) from one
  place to another because microwave energy can
  penetrate haze, light rain and snow, clouds, and
  smoke.

9
Infra Red

• You probably think of Infra-red waves as heat,
  because they're given off by hot objects, and you
  can feel them as warmth on your skin.
• Because every object gives off IR waves, we can
  use them to "see in the dark". Night sights for
  weapons sometimes use a sensitive IR detector.
  Remember the film, "Predator"?
• Radio/light waves have a very short wavelength
  yet is longer than visible light. Infra-red can
  be detected by special infra-red film. If the
  police shine an infra-red light on you they will
  be able to take a picture of you using infra-red
  film you will not know that they have taken your
  photo. You have been warned!!!
• Animals like the pit-viper have infra-red
  detectors so that they can find their prey in the
  dark. You have been warned again!!!

10
The Visible Spectrum

• Our eyes can detect only a tiny part of the
  electromagnetic spectrum, called visible light.
  This means that there's a great deal happening
  around us that we're simply not aware of, unless
  we have instruments to detect it.
• Red, Orange, Yellow, Green, Blue, Indigo, and
  Violet are the colors of the visible spectrum.
• We cannot see Infra-red, but we can feel it warm
  our skin when we sit in the sun. Infra-red has a
  longer wavelength (less energy) than Red light.
• We cannot see Ultra-violet light, but we feel our
  skin has been burnt by the sun if we were in the
  sun too long yesterday. It is the Ultra-violet
  which is thought to cause skin cancer. UV light
  has a shorter wavelength (more energy) than
  visible light.

11
Ultra Violet

• Ultraviolet rays can be used to kill microbes.
  Hospitals use UV lamps to sterilize surgical
  equipment and the air in operating theatres.
• Large doses of UV can damage the retina in your
  eyes, so it's important to check that your
  sunglasses will block UV light.
• These waves have very high energy and very short
  wave lengths.
• Some animals like honey bees can see ultra-violet
  light.
• Some plants have white flowers, at least you
  think that they are all white, but they may
  appear to be different colors to a honey bee
  because of the amounts of ultra-violet light
  which they reflect.

12
X-rays

• X-Rays have so much energy and such a short
  wavelength that they can go right through you.
• However, they cannot get through bone as easily
  as they can get through muscle. This is because
  your bones contain so much Calcium.
• X-Ray can also be used to find other problems in
  your body. If the doctors want to look for ulcers
  in your guts, they can give you a Barium meal.
  Like Calcium, the Barium absorbs X-Rays so the
  doctors can look at parts of your guts and find
  your ulcers. You get ulcers by worrying about
  your exams.
• X-Rays can cause cell damage and cancers. This is
  why Radiographers in hospitals stand behind a
  shield when they X-ray their patients. Although
  the dose is not enough to put the patient at
  risk, they take many images each day and could
  quickly build up a dangerous dose themselves.

13
Gamma Rays

• These are nasty ones. They have very high energy
  and will even go through thin metals. So they can
  be used for finding tiny cracks in metals.
• They are extremely high frequency waves, and
  carry a large amount of energy. They pass through
  most materials, and are quite difficult to stop -
  you need lead or thick concrete in order to block
  them out.
• Gamma rays kill microbes, and are used to
  sterilize food so that it will keep fresh for
  longer. This is known as "irradiated" food.
• Gamma rays are also used to sterilize medical
  equipment.
• Some radioactive materials produce gamma rays.
  Gamma rays and X-Rays can cause cancer, but gamma
  rays can also be used to destroy cancer cells
  this is radio-therapy.

14
The Energy of a wave is proportional to its
Frequency

• A photon with twice as much energy as another
  source would have a frequency also twice the
  second source.
• On the other hand, as frequency (or energy)
  increases, wavelength decreases.
• Light waves with the greatest energy have the
  greatest frequencies and the smallest wavelengths.

• Increasing Energy