Physics for future-biologists or future-physicians - Part 2

Physics for future-biologists or future-physicians who what to learn the least possible of physics, but still menage to finish university.

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See http://lasers-in-the-jungle.blogspot.com/2008/06/attribution-sharealike.html

PART2 – Radiation (includes: sound, optical spectrum, radioisotopes)

Q) What is radiation?

A) Radiation is something that radiates.

Q) Can you give more details?

A) I can list the type of radiations we are going to describe here. Electromagnetic radiation, alpha radiation, beta radiation, sound radiation. We are not going to speak of another type of radiation (just an example: neutrino radiation).

Q) What about gamma radiation, x-ray radiation, ultraviolet radiation, light radiation, infrared radiation?

A) They all are forms of “electromagnetic radiation”. More about that soon.

Q) Alpha, beta, gamma. Are those Greek letters?

A) Yes, and you can use the Greek letters instead. But I do not have Greek letters in my keyboard, so I must write them as I am doing NOW.

Q) What is the other possible name for “electromagnetic radiation”?

A) Light! I agree that this is difficult to understand because we are used to think in “light” as something we associate with vision. But that is not always the case. We can see violet, but we cannot see ultraviolet. We can see red, but not infrared. Believe me there are a lot of other “types” of light we cannot see - but can be detected by our instruments or apparatus. We will call “visible light” to the type of electromagnetic radiation we can see with ours yes, and simply “light” to all kind of electromagnetic radiation.

Q) If there is a lot of “types” of light, was is the difference between then?

A) Frequency. To be more precisely range (or interval) of frequencies.

Q) I began to see, but can you systematize?

A) Here we go. A list of different type of light (or electromagnetic radiation), and their interval of frequencies:

Radio (TV, etc) from 3 Hz to 300 GHz

Infrared – from 300 GHz to 400 THz

Visible light - from 400 THz to 790 THz

Ultraviolet – from 790 THz to 30 PHz

X – rays – from 30 PHz to 30 EHz

Gamma rays – from 30 EHz to 300 EHz (or more)

(But nobody know those numbers by heart)

As you could imagine those divisions are a little arbitrary and, a lot of sub-divisions are possible and are use in specific fields of work or research. But here we would try to keep things as simple as possible. Just an example, microwave ovens work with a radiation near 2.5 GHz, in a band called “microwaves”, but for us, at the moment, is just radio.

Q) What is a “band” (in this context)?

A) That one is easy to understand. Is just a range or interval of frequencies.

Q) What happen below 3Hz?

A) Nothing really interesting or useful (as far as we know at the moment). So no specific name.

Q) What above 300 EHz?

A) We continue to call them gamma rays.

Q) I suppose visible light is an important subject. Can you give us more details?

A) Sure. Visible light is a band between 400 THz to 790 THz, but can be considered divided in smaller bands. Those bands are simply the “rainbow colors”, i.e., red, orange, yellow, green, blue, indigo, and violet (mnemonic: ROY G. BIV - or - "Richard Of York Gave Battle In Vain”).

Q) What does the word “optic” mean?

A) Although we rarely use that word alone, that mean something related to the visible light (the word has the same root of optician)

Q) What is an “optical spectrum”?

A) Just the “rainbow colors” set aside exactly the way they appear in rainbow. The rainbow itself is a good example of the optical spectrum.

Q) What is the plural o spectrum?

A) Spectra.

Q) The rainbow is an example of a spectrum. Are there other spectra?

A) The rainbow is the spectrum of the sun (white) light. Other sources can have different spectra. In physics sometimes we can know the composition of a source of light just studding carefully the spectrum of the light coming from it.

Q) Everybody can see those colors well?

A) Most normal people can see the colors, but usually indigo is quite difficult to differentiate from blue or violet (some people are better at that that others, but most people do not know if they themselves are sensitive to indigo).

Q) What is “color blindness”?

A) Is the inability to perceive differences between some (or all) of the colors that other people can see.

There are some different types (forms) of “color blindness”. In the most severe form people cannot differentiate any color. Some times people can differentiate some (but not all) colors

The word “Daltonim” is also used, but it's mean is no exactly the same, so it's meaning should only be used in a more detailed context. “Color blindness” is, or can be considered, a disability. Problems with “indigo”, as described above, are not considered “color blindness”.

Q) You mean a color blind cannot see one (or more colors)?

A) No. He or she can see, but cannot DIFFERENTIATE the colors although they see them. It's like looking at a black-and-white picture. You can see everybody in the picture but it is impossible to know if a person is using it's red or blue dress.

Q) We are speaking about human, what about other animals.

A) We, as human have a quite good color vision. Some other animals could be considered “color blind” if comparer with humans.

Some animals can see frequencies we cannot see. Some insects can see the radiation just above violet (in a range called ultraviolet). Some reptiles can sense infrared (frequencies just below red) – but not using their eyes.

All in all most animals can see only the colors we see, with small differences. To be all to differentiate them all is a different thing. Some other animals have, however, better color vision then we do (some birds, fishes, etc).

Q) But there are some colors mission from the “rainbow colors”. Like pink, purple, magenta, brown. Why?

A) They are combination of different “rainbow” colors.

Q) What is “white”?

A) The mixture of all rainbow colors in the same proportions seen in the rainbow.

Q) What is “black”?

A) The absence (nonexistence) of ANY rainbow colors (or others colors).

Q) I would like to see the rainbow colors, but I do not have the time to wait for a rainbow.

A) In most laboratories you can find a not expensive apparatus called a “prism”. If you see some light to one of its faces you can probably see the rainbow colors from other face (some try and error could be necessary to find the right positions). Just remember to use a white source of light (sunlight is OK).

Q) What is “intensity”?

A) Is the scientifically correct word for what we can call, in colloquial speech, "amplitude", "strength" or "level".

Using an example with visual light is easier. Now I have a 50 W light bulb over my desk. It's OK. But if I change to a 100 W light bulb I double the intensity of the light in my desk.

The correct definition is “energy divided by time (duration), divided by surface”.

Q) What does mean “wave”, “ray(s)”, in this context.

A) Without going into more details, means the same thing as radiation.

Q) What is the effect of electromagnetic radiation in the human body?

A) We will see that is more details, but it depends of the “type” of radiation, the intensity and the duration of the exposition as is easy to understand. As a rule all radiation is harmful. Also, as a rule, the greater the frequency the more the damaging, but this is just a rule, that does not work all time.

There is a lot of research (an controversy) about radio waves. The radio frequencies used by cellular phones, and wireless computer networks, are not very far from frequencies use by microwaves ovens. So, in a sense, each time we use our cellular we “microwave” (like in “microwave oven”) our brain. HOWEVER most specialists will tell you that the intensity of the radiation is too small to cause you any harm. I keep the use of my phone as short as possible. It is less expensive this way.

Q) So radiation is always dangerous?

A) Yes, but sometimes the destruction can be useful if you destroy unwanted tissue (s). Also sometimes radiation is used in a resourceful way.

For example (we would see more soon) x rays are dangerous, but very useful to take “pictures” of the inside of your body. Since one received only a few seconds of exposition every year (or something around that) we are not at danger, and x-rays are very useful for diagnostic (however someone operating the machine day after day requires some appropriate protection, usually in the form of distance from the machine, special vestiary, protective walls, etc).

Sun light (visible light, plus some infrared and ultraviolet), if used in a controlled way is healthy.

Q) What are “photons”?

A) We sometimes describe the electromagnetic radiation (also known as light) as a stream of particles called photons (same root as photography). We will not explore a lot that concept here.

Q) What time of radiations has not been explained yet?

A) Sound, alpha radiation and beta radiation - are NON-electromagnetic radiation.

Q) What is sound?

A) Well, almost everybody knows the meaning of sound and the common perception is right. In the physical context sound can be considered to be changes in pressure (more on pressure, late).

Q) We have “frequencies” in sound?

A) Yes.

Q) Can we ear all frequencies?

A) No. The human audio range is between 20 Hz and 20KHz, but that’s a very good audition. Some people can listen only between 70 Hz and 13 KHz. Also with age same lost of audition is usual.

Q) Can you explain the change of sensibility with age?

A) Some young people use high pitched (high frequency) tone ring in their phones, so adults cannot ear and are not aware of what is going on. Some old people use high-pitched sound, with high intensity, to “repel” young ones.

Q) How do we sense “frequency” related to sound.

A) As pitch (in colloquial terms: highness or lowness of sound). For example, in a piano, when we strike different keys, we get different frequencies. On the left we got low frequencies (low sound, low pitch). On the right we get high frequencies (high sound, high pitch).

Q) And what about other animals?

A) That changes a lot. Just two examples. Dogs can ear above 25 MHz, sometime to 45 MHz. A dog whistle works around 25 MHz, so dogs can hear, but people cannot. Bats can ear frequencies as high as 80 KHz, or even more, but they use the sound for the purpose of locating things around, like sonar, used by boats to locate fish.

Q) What is the beta radiation?

A) Just a flux of electrons at high velocity (we exclude were a more exotic form of beta radiation, which include an exotic particle called positron, just forget).

Q) What is alpha radiation?

A) Just a flux of alpha particles

Q) OK, but what is an alpha particle?

A) It's a particle that consists as two neutrons plus two protons. It sure looks like the nucleus of element n. 2 (which, by the way, is called helium).

Q) What is a radioisotope (also known as radionuclide)?

A) Any isotope that radiate some kind of radiation

Q) What kind of radiation radiates a radioisotope?

A) (Sounds funny) A radioisotope can radiate radiation alpha beta and/or gamma (other “things” can be radiate, but we are not going to talk about that).

Q) What is radioactive decay?

A) The process a radioisotope goes through when it radiates.

Q) What is the importance of radioisotopes?

A) There are a lot of radioisotopes (some natural, most artificial). Choosing the right isotopes we can usually get the kind of radiation we need for a specific task. The radioisotopes are just the way the nuclear industry arrives to the laboratory, to medicine and to the industry – and to the home.

Q) Do people have radioisotopes at home?

A) Probably – If they have a fire/smoke detector. Most of the times radioisotopes should be handle with extreme care, always using a protocol for the given source.

Q) Are gamma rays dangerous?

A) Gamma rays are usually the most destructive radiation. It can “burn” the tissues, but also alter the DNA of the cells, which eventually leads to cancer and other anomalies.

Q) Are gamma rays useful?

A) Yes. They have a lot of useful applications. It can be used to sterilize medical equipment. It can be used to kill bacteria in the food, preventing that stuff to go bad in a short period of time (however consumers have some kind of reserve about eating irradiated food – if they have the choice)

Also we can use gamma radiation to destroy cancer tissues, or other kind of tissue harmful to the patient. The main problem is to keep other tissues from the gamma rays. Usually the source is cobalt-60, an artificial radioisotope of cobalt (element n.27).

Gamma rays are hard to shield. We need 1 cm of lead or 5 cm of concrete, just to reduce to 50% the intensity of a beam.

Also people can receive the radioisotope technetium-99, which irradiates gamma rays. While the technetium circulates through the body if, it radiates gamma rays, that are recorded, from outside, with a camera that records gamma rays. This allows a diagnostic of many problems in the patient.

Q) Are x-rays dangerous?

A) Yes. The some kind of danger of the gamma rays, but they are not so damaging.

Q) Are x-rays produced by radioisotopes?

A) No. Radioisotopes do not generated X-rays. X-rays are created within a sort of light bulb, but very different inside (and much more complex) that a normal light bulb.

Q) Is beta radiation armful?

A) Yes, it can do the same of gamma rays.

Q) Is beta radiation useful?

A) Yes. It is not very different from gamma. The source of beta radiation is usually the Strontium-90.

Q) Is alpha radiation armful?

A) Most of the time is not, because it is not very penetrating, and does not penetrate the skin. However if someone managed to swallow a source of alpha radiation, that can be very damaging because the alpha particle is shouted to cells at very short distance.

Q) But is easy to swallow an alpha source?

A) The smoke detectors in the houses have a radioisotope, americium-241, that irradiates alpha. Manipulation the americium is not dangerous because the skin protects us from the rays. However it would be very damaging if we ingest the alpha source.

Q) Is alpha radiation useful in medicine?

A) Not really.

Q) Any other way to produce radiation?

A) Particle accelerators. They have been used more and more on medicine. X-rays produced by accelerators have been replacing gamma-ray treatments (that, see above, use usually a cobalt-60 radioisotope as source). The radiation is more precisely delivered and the source is more reliable.

However accelerators are big.

Q) Does the human body radiate visible light.

A) It does not. That is why we cannot see ourselves in the dark.

Q) Does the human body radiate any radiation?

A) Yes. It, the human body, as any hot-blooded animal radiates in the infrared. Although we cannot see it, some cameras record it and translate the different frequencies of infrared to the visible range (which is called false color). Recently they have used this kind of cameras to fast detection of people with flu (with temperature above normal) in some airports.