Light WavesLight Waves

Objectives:Objectives:1. Recognize that light has wave and 1. Recognize that light has wave and

particle characteristicsparticle characteristics2. Relate energy of light to its 2. Relate energy of light to its

frequencyfrequency3. Know the parts to the EM 3. Know the parts to the EM

spectrumspectrum

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

Light described as a wave:Light described as a wave:

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

Light described as a particleLight described as a particle

– We think of light as little We think of light as little packets of energy. Blue packets of energy. Blue light has one size packet light has one size packet that is greater than the Red that is greater than the Red light, which has another size light, which has another size packet.packet.

– The name we give these The name we give these packets are PHOTONSpackets are PHOTONS

– These photons are NOT These photons are NOT particles – they have no particles – they have no mass – think of them as mass – think of them as bundles of energy.bundles of energy.

– We have this model because We have this model because of the photoelectric effect, of the photoelectric effect, which shows that a dim blue which shows that a dim blue light can knock an electron light can knock an electron out of a metal atom where a out of a metal atom where a very bright red light cannot.very bright red light cannot.

Which model is correct?Which model is correct?

– Different situations call for a different Different situations call for a different modelmodel

• When examining interference, the wave model When examining interference, the wave model is better suitedis better suited

• When examining how light travels in space, When examining how light travels in space, the particle model helps to explain this the particle model helps to explain this occurring without a mediumoccurring without a medium

– Most scientists think of light having a Most scientists think of light having a dual-nature, so light has different dual-nature, so light has different characteristics depending on the characteristics depending on the situation.situation.

The Electromagnetic The Electromagnetic SpectrumSpectrum• Consists of light at all possible frequencies and Consists of light at all possible frequencies and

wavelengths.wavelengths.• All the waves are similar in certain ways; they All the waves are similar in certain ways; they

also have unique properties.also have unique properties.

Radio waves Radio waves

• Radio Waves are used for radio Radio Waves are used for radio broadcasts, amateur radio, broadcasts, amateur radio, television, and mobile phones.television, and mobile phones.

• Different parts of the radio Different parts of the radio spectrum have been allocated to spectrum have been allocated to the various services. Radio waves the various services. Radio waves have a much longer wavelength have a much longer wavelength that light waves.that light waves.

• The longest waves are several The longest waves are several kilometers in length. The shortest kilometers in length. The shortest ones are only millimeters long. ones are only millimeters long.

• The really nice thing about radio The really nice thing about radio waves is that they will make the waves is that they will make the electrons in a piece of copper electrons in a piece of copper wire move; this means that they wire move; this means that they generate electric currents in the generate electric currents in the wire. wire.

• In fact it works both ways: In fact it works both ways: alternating currents in a copper alternating currents in a copper wire generate electromagnetic wire generate electromagnetic waves, and electromagnetic waves, and electromagnetic waves generate alternating waves generate alternating currents.currents.

Microwaves Microwaves

• Microwaves have such a short wavelength that Microwaves have such a short wavelength that they are very easily absorbed by water. they are very easily absorbed by water.

• This is why they are used in microwave ovens. This is why they are used in microwave ovens. What happens is that when the water in your TV What happens is that when the water in your TV dinner absorbs the microwaves, the energy of the dinner absorbs the microwaves, the energy of the microwaves is converted into heat: it makes the microwaves is converted into heat: it makes the water molecules vibrate faster. water molecules vibrate faster.

• Some people are frightened that the radio waves Some people are frightened that the radio waves coming out of their mobile phones are short coming out of their mobile phones are short enough to cook their brains. enough to cook their brains.

• Microwaves are good for transmitting information Microwaves are good for transmitting information (like telephone calls or computer data) from one (like telephone calls or computer data) from one place to another because microwave energy can place to another because microwave energy can penetrate haze, light rain and snow, clouds, and penetrate haze, light rain and snow, clouds, and smoke. smoke.

Infra Red Infra Red

• You probably think of Infra-red waves as heat, because You probably think of Infra-red waves as heat, because they're given off by they're given off by hot objectshot objects, and you can feel them , and you can feel them as warmth on your skin. as warmth on your skin.

• Because every object gives off IR waves, we can use Because every object gives off IR waves, we can use them to "see in the dark". Night sights for weapons them to "see in the dark". Night sights for weapons sometimes use a sensitive IR detector. Remember the sometimes use a sensitive IR detector. Remember the film, "Predator"?film, "Predator"?

• Radio/light waves have a very short wavelength yet is Radio/light waves have a very short wavelength yet is longer than visible light. Infra-red can be detected by longer than visible light. Infra-red can be detected by special infra-red film. If the police shine an infra-red special infra-red film. If the police shine an infra-red light on you they will be able to take a picture of you light on you they will be able to take a picture of you using infra-red film: you will not know that they have using infra-red film: you will not know that they have taken your photo. You have been warned!!! taken your photo. You have been warned!!!

• Animals like the pit-viper have infra-red detectors so Animals like the pit-viper have infra-red detectors so that they can find their prey in the dark. You have been that they can find their prey in the dark. You have been warned again!!! warned again!!!

The Visible Spectrum The Visible Spectrum

• Our eyes can detect only a Our eyes can detect only a tinytiny part of the electromagnetic part of the electromagnetic spectrum, called spectrum, called visible lightvisible light. This means that there's a . This means that there's a great deal happening around us that we're simply not great deal happening around us that we're simply not aware of, unless we have instruments to detect it.aware of, unless we have instruments to detect it.

• Red, Orange, Yellow, Green, Blue, Indigo, and Violet are the Red, Orange, Yellow, Green, Blue, Indigo, and Violet are the colors of the visible spectrum. colors of the visible spectrum.

• We cannot see Infra-red, but we can feel it warm our skin 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 when we sit in the sun. Infra-red has a longer wavelength (less energy) than Red light. (less energy) than Red light.

• We cannot see Ultra-violet light, but we feel our skin has 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 been burnt by the sun if we were in the sun too long yesterday. It is the Ultra-violet which is thought to cause yesterday. It is the Ultra-violet which is thought to cause skin cancer. UV light has a shorter wavelength (more skin cancer. UV light has a shorter wavelength (more energy) than visible light. energy) than visible light.

Ultra Violet Ultra Violet

• Ultraviolet rays can be used to kill Ultraviolet rays can be used to kill microbes. Hospitals use UV lamps microbes. Hospitals use UV lamps to sterilize surgical equipment to sterilize surgical equipment and the air in operating theatres. and the air in operating theatres.

• Large doses of UV can damage Large doses of UV can damage the retina in your eyes, so it's the retina in your eyes, so it's important to check that your important to check that your sunglasses will block UV light. sunglasses will block UV light.

• These waves have very high These waves have very high energy and very short wave energy and very short wave lengths. lengths.

• Some animals like honey bees Some animals like honey bees can see ultra-violet light. can see ultra-violet light.

• Some plants have white flowers, Some plants have white flowers, at least you think that they are all at least you think that they are all white, but they may appear to be white, but they may appear to be different colors to a honey bee different colors to a honey bee because of the amounts of ultra-because of the amounts of ultra-violet light which they reflect. violet light which they reflect.

X-raysX-rays

• X-Rays have so much energy and such a short wavelength that they can X-Rays have so much energy and such a short wavelength that they can go right through you. go right through you.

• However, they cannot get through bone as easily as they can get through However, they cannot get through bone as easily as they can get through muscle. This is because your bones contain so much Calcium. 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 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 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 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 your guts and find your ulcers. You get ulcers by worrying about your exams.exams.

• X-Rays can cause cellX-Rays can cause cell damage and cancers. This is why Radiographers in damage and cancers. This is why Radiographers in hospitals stand behind a shield when they X-ray their patients. Although 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 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.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 These are nasty ones. They have very high energy and will even go through thin metals. So they can be used for even go through thin metals. So they can be used for finding tiny cracks in metals. finding tiny cracks in metals.

• They are extremely high frequency waves, and carry a They are extremely high frequency waves, and carry a large amount of energy. They pass through most materials, large amount of energy. They pass through most materials, and are quite difficult to stop - you need lead or thick and are quite difficult to stop - you need lead or thick concrete in order to block them out. concrete in order to block them out.

• Gamma rays kill microbes, and are used to sterilize food so Gamma rays kill microbes, and are used to sterilize food so that it will keep fresh for longer. This is known as that it will keep fresh for longer. This is known as "irradiated" food. "irradiated" food.

• Gamma rays are also used to sterilize medical equipment. Gamma rays are also used to sterilize medical equipment. • Some radioactive materials produce gamma rays. Gamma Some radioactive materials produce gamma rays. Gamma

rays and X-Rays can cause cancer, but gamma rays can rays and X-Rays can cause cancer, but gamma rays can also be used to destroy cancer cells: this is radio-therapy. also be used to destroy cancer cells: this is radio-therapy.

The Energy of a wave is The Energy of a wave is proportional to its Frequencyproportional to its Frequency

• A photon with twice as much energy as another source A photon with twice as much energy as another source would have a frequency also twice the second source.would have a frequency also twice the second source.

• On the other hand, as frequency (or energy) increases, On the other hand, as frequency (or energy) increases, wavelength decreases.wavelength decreases.

• Light waves with the greatest energy have the greatest Light waves with the greatest energy have the greatest frequencies and the smallest wavelengths.frequencies and the smallest wavelengths.

•Increasing Energy