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K-12 Science Backgrounder

On this page we cover some basic science questions about the nature of light and radiation. It assumes very little math background and ties to build on concepts gradually. We hope you find this material useful, and if you have suggestions please do not hesitate to email us.

What is light?

Light is electromagnetic radiation, and behaves a lot like radio waves and X-rays, except that it typically cannot penetrate objects the way radio waves can.

Why do we call light radiation?

From the root "radiate" light from a source radiates outward in all directions. To be precise, we should call light electromagnetic radiation to differentiate it from its close cousin, nuclear radiation. The term "light" should really be used only in the range that humans can see, although the literature frequently incorrectly cites "infrared light" or "ultraviolet light" which are incorrect.

Isn’t all radiation bad? What about the kind of radiation that is in a nuclear power plant?

Nuclear radiation is a more commonly used term, and yes, nuclear radiation can harm your cells in your body. Nuclear radiation is alpha particles, beta particles, or gamma photons that are emitted when an unstable heavy atom breaks apart (this is called nuclear fission). Because nuclear radiation is created by the high energy breakup of larger atoms and is made up of actual physical particles such as electrons protons or neutrons, it typically has much higher energy – that is, it has the ability to do much more work like destroy a healthy cell in your body.

So why do we hear so much about nuclear radiation and nuclear waste?

Nuclear radiation is potentially dangerous because it has the potential to break up the atoms in your body if you are near a source of nuclear material. Cell damage can occur when these particles rip though your body. Depending on many factors, your cells may die or heal, or they may deform and lead to cancer or even birth defects in your offspring. Old nuclear material that is decomposing, depending on its exact weight and purity, can literally take thousands of years to fully deteriorate. This means that it will remain toxic for a very very long time, depending on its half-life (the rate of spontaneous breakup of the material into lighter atoms.) Because it is extremely difficult to get small amounts of nuclear fissionable material out of the body once ingested, some of the most toxic materials known to man are nuclear materials.

What is ionizing radiation?

When a heavy atom is split in nuclear fission, an alpha, gamma or beta particle is emitted. This high energy particle can travel through your body and if it comes in contact with an atom in a cell, can strip off an electron without splitting the atom’s central core nucleus. This lack of electron will create a charged ion. We call this form of radiation ionizing radiation. Most forms of nuclear radiation is considered to be ionizing radiation, as are X-rays which are very short wavelength electromagnetic radiation.

So light is much safer than nuclear radiation?

Well, no – you have to also consider the intensity of the source. Light is typically non-ionizing radiation, however, keep in mind that very high energy light power levels (for example, light from a welder’s bright torch) can harm you by blinding or burning you. On the other side, low level nuclear radiation is given off continuously by ordinary red brick - it won’t harm you because it is at a ver low intensity level.

What does frequency mean?

Electromagnetic radiation alternates back and forth, changing its polarity each cycle. Frequency refers to the rate at which electromagnetic radiation changes its direction (or polarity) each second. Think of a jump rope – the faster the rope is spun around each second, the higher the frequency of oscillation.

How fast does light travel?

Light travels extremely fast through free space– 300,000,000 meters per second (or about 186,000 miles per second). That’s about 7 times around the globe every second! Its speed does slow a bit when traveling though medium other than free space.

Can we see all light in the spectrum?

Light that we humans can see (that is, that the cells at the back of our eyes can detect), is limited to the photopic range which covers about 450-700(nm)of the electromagnetic spectrum .

What is the electromagnetic spectrum?

Electromagnetic radiation has both an electric field and a corresponding magnetic field. It changes its direction a certain speed per second based on its frequency, which is measured in cycels per second or Hertz. The electromagnetic spectrum is simply what we call the entire range of wavelength from direct current (0 cycles per second) to infinity.

What is the photopic range of light?

The photopic range covers about 450-700 nanometers (nm) of the spectrum.

What is a nanometer (nm)?

A nanometer is one billionth of a meter and signifies the length of each oscillation of electromagnetic radiation. Although you can think of light photons as particles, it also behaves like electromagnetic radiation (this characteristic is called the wave-particle duality nature of light).

What about wavelengths beyond the photopic?

The sun emits radiation over a broad range, form about 190 nm to 3000 nm. Light shorter than what we can see is called ultraviolet radiation (UV). UV radiation is further sub-divided into UV-A (400-320 nm), UV-B (320-280 nm) and UV-C (280-190nm) or extraterrestrial radiation, since no UV-C thankfully reaches the earth’s surface. Radiation with wavelengths longer than the photopic falls into the near infrared (700-1100 nm) and then the thermal infrared (longer than 1100 nm out to about 100,000 nm). At wavelengths longer than this we start to call the radiation millimeter waves, then microwaves, then radio waves.

How is light converted into heat (thermal) energy?

All objects that are warmer than absolute zero (–273° Celsius or 0° Kelvin) emit electromagnetic radiation according to a well-understood curve of wavelengths, called Planck’s function. The warmer the object, the further the curve moves towards the visible/NIR. Our sun’s extremely hot surface, the photosphere, puts the center of this curve at about 500 nm, in the visible/NIR photopic range. When these photons strike an absorbing surface, it is converted directly to thermal energy and heats the receiving surface. This is how most scientific-grade pyranometers work.

Why is atmospheric (or solar) radiation so important to mankind?

The sun is an intense source of light that covers a wide spectral bandwidth, to much shorter wavelengths than which we can measure on the ground. The shortest wavelength solar radiation is deadly to life forms and is filtered by a fragile layer of ozone (O3) up high in the atmosphere, in a layer we call the "stratosphere." Our atmosphere thus sustains most of the life forms on earth. Without it, the harsh solar UV-C radiation would not be filtered out and life would cease to exist. The delicate balance of light transmission, absorption and re-radiation in the thermal infrared make up a critical shield that protects us from the harsh radiation of the sun.

What is the ozone layer and why is it so important?

The earth’s protective ozone layer, located high in the earth’s stratosphere filters out all of the deadly high energy UV-C photons in the 190-280 nm range. Incoming UV-C photons are absorbed by very stable diatomic oxygen molecules (O2) located high in the earth’s stratosphere and then breaks into two single oxygen atoms (O). The unstable oxygen atoms quickly combines to form a tri-atomic oxygen molecule (O3), called ozone. After a period of time, the ozone spontaneously disintegrates back into diatomic oxygen (O2), until another incoming high energy UV-C photon starts the cycle over.

What is causing ozone depletion?

Man made chemicals such as those found in common air conditioners (called chlorofluorocarbons) are believed to be altering this precise balance though a multi-stage fairly complex chemical reaction. The temperature of the earth’s upper atmosphere is critical to regulating the speed of this continuous and fragile chemical reaction. During the attack due to chlorofluorocarbons, the ozone layer thins significantly, letting many more UV-B photons through than we should normally see. The southern hemisphere is slightly colder than the northern hemisphere (because the Antarctic continent reflects amore solar radiation back to space), ozone depletion problem tends to be more acute "down under." Our understanding of this reaction was considered so crucial that the three scientists who are generally attributed to discovering the model to describe it won the Nobel prize in chemistry for their work!

I heard on the radio the ozone level was high today. Isn’t this a good thing?

Not exactly. Yes it is the same molecule, but a very different physical place, as well as it is not in a high enough concentration to help block UV radiation. Ground level, or tropospheric ozone is created by man-made pollution as a result of fossil fuel consumption (internal combustion engines.) It is not healthy for humans or animals to directly inhale highly reactive ozone molecules that are found in high concentrations during brief periods of summer pollution, when the environmental protection agencies issue health hazards. First of all, tropospheric ozone is mainly limited to urban areas and is not blanketed evenly over our population. However, even in these polluted urban areas, about 90% of the total column ozone above you is still located up in the stratosphere, so the 10% portion of the tropospheric ozone does not help to block the UV radiation. And while we could throw away our cars tomorrow if we really had to, we cannot "fix" the problem in the stratosphere because the ozone reaction half-life is on the order of decades. So stratospheric ozone depletion is therefore a more serious and difficult problem for the world to solve; even if we stopped all fluorocarbons from being released from air conditioners tomorrow it will take decades to slow down the reaction.

Why is measuring light so difficult and expensive?

Consider the following analogy:

  • It costs about $10 to measure time to one part in a million (using a common quartz wrist watch)
  • It costs about $1000 to measure current to one part in ten thousand (using a modern DVM)
  • It costs about $10,000 to measure distance to one part in a hundred thousand (using an optical interferometer)
  • But it costs about $100,000 to measure light, or spectral irradiance, to just one part in a hundred (via a NIST/NPL/PTB-traceable spectroradiometer)

If that seems like a ridiculous sum of money to achieve just a 1% measurement, consider further that measuring radiation (that is, spectral irradiance, or optical power-per-unit-area-per-unit-wavelength) significantly than one part in a thousand is not achievable by practical means at any cost!

The reason for this paradox lies in the reality that measuring radiation requires several different separate physical measurements and the errors from each one adds up:

  1. One must measure area, (two separate distance measurements) plus the distance from the irradiance source to the detector, a third distance measurement.
  2. One must measure the current though the standard lamp source, and this is typically done practically by measuring the voltage precisely across a standard resistor that is placed in series with the lamp. (One must also somehow check that the standard resistor’s value has not changed since its last calibration, as it will change over time and temperature).
  3. Finally, one must be able to measure wavelength accurately. This is an often complex problem to do in the lab and typically relies upon the availability of standard Hg or Cd emission lines near the spectral region of interest. Typically a separate transfer calibration is done to calibrate the wavelength of the detector via a spectroradiometer

Why is making accurate atmospheric radiation measurements in the field a challenge?

Once we leave the comfort of the laboratory, there are several important factors that affect the measurement of solar atmospheric radiation outdoors:

  • On the ground, our sun appears as a moving source and thus either the angular response of the flat-plate (pyranometer) detector much be precisely known, or else it must be made to track the sun. Errors in solar tracking historically have amounted to the greatest source of error. Over time, the angular response of a flat-plate detector can change due to exposure.
  • Intense solar radiation (especially in the UV-B range) tends to degrade or solarize the detector and shift its sensitivity over exposure in both absolute (integrated) and wavelength accuracy terms.
  • Dust or aerosol deposits on the detector can affect its sensitivity. These can be agriculturally generated (biomass burning or pollen) and therefore be seasonally biased
  • Daily and seasonal variations in local air temperature can greatly affect the detector’s sensitivity depending on technology used.
  • Vibration and shock during the transport of the detector from the calibration laboratory to the field can affect its calibration.
  • A flat-plate detector must be kept level at all times.
  • Ignoring the problems associated with detector, the twin requirements of making precise geometrical measurements (area) and electrical measurements of the detector output over time and temperature must be maintained.

What is causing global warming? Can’t we just measure air temperature to track it?

All of the energy that drives our climate comes from the sun. The balance of absorption, reflection, transmission and re-emission of incoming solar radiation by clouds and the earth’s surface is quite sensitive to the balance of the ratio of a variety of molecules, in particular carbon dioxide, a by product of fossil fuel consumption. CO2 is a major driver of the greenhouse effect, which tilts the balance slightly in favor of trapping more radiation than is radiated back into outer space. Measuring the temperature at the ground is one approach to track global warming.

However, the mixing of air in the troposphere due to convention and gaseous diffusion are only loosely coupled with daily and yearly changes in solar zenith angle, making simple trend extraction quite difficult. By measuring solar radiation in conjunction with air temperature we can extract these trends more readily. Further trace gas measurements are also used to expose a correlation between climate drivers. The computer models used to describe the earth’s climate in order to predict the future are called "global circulation models." For more information about global climate change, you may want to check out these web sites:



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This page was last updated on Monday, September 11, 2006 .