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

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light waves

Light Waves


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

light cannot be described with one model light is either described as a wave or as a particle
Light cannot be described with one model - - - Light is either described as a wave or as a particle.
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… Light described as a wave:
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.

Light described as a particle
which model is correct
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.
the electromagnetic spectrum
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.
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.

Radio waves
  • 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.
infra red
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!!!
the visible spectrum
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.
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.

Ultra Violet
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 celldamage 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.
gamma rays
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.
the energy of a wave is proportional to its frequency
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