What Light Is

1. What is light?


2a. What are photons? Are they particles or waves?

2b. How do the electric and magnetic field waves of a photon move in relation to one another and the motion of the photon?


3a. What is the electromagnetic spectrum?

3b. List the six types of electromagnetic radiation presented in this section in the order of increasing energy level. What characterizes each type?


The Nature of Light Radiated by Our Sun

1a. What is the temperature of the Sun's photosphere?

1b. Describe the Sun's spectral radiance over 99% of its electromagnetic spectrum.


2. What causes the large deviations from an ideal blackbody curve observed in the X-ray and the radio wave portions of the solar spectrum?


3a, Why does the energy output of the Sun vary?

3b. Why does the amount of solar energy reaching Earth remain relatively constant even though there are large variations in certain portions of the solar spectrum?


The Solar Cycle

1a. What is the solar cycle? How far back does the record stretch?

1b. What are sunspots?

1c. What are solar flares? When are they more prevalent?


2. Why are measurements made of radio fluxes from the Sun at 2800 MHz?


3. How much of the temperature and ozone variation in the upper stratosphere is due to the solar cycle in the 200 nm (ultraviolet) wavelength?


How Light Moves Through the Atmosphere

1. TBD

2. TBD

3. TBD

4. TBD


5a. What is Mie scattering?

5b. What is Rayleigh scattering?

5c. What is the chief difference between Mie and Rayleigh scattering?


6. Why is the sky blue?


7a. Why have plants and animals adapted themselves to make use of radiation in visible and near ultraviolet regions?

7b. What is the main biological effect of infrared radiation?


Refer to Figure 4.11 to the lower panel to the curve marked "Surface." This represents the solar flux in watts per square center per nanometer striking the surface of Earth.

8a. Why does no radiation of wavelength shorter than about 315 nm reach the surface?

8b. What happens to the molecular oxygen that absorbs very energetic UV and X-ray radiation from the Sun?

8c. What happens to the ozone molecules formed?


Refer again to Figure 4.11.

9a. What happens to the solar flux between the top of the atmosphere and 30 km?

9b. What happens to the solar flux between 30 km and the surface?


How Objects Emit Radiation: Blackbodies

1a. What is blackbody radiation?

1b. What sort of object emits blackbody radiation?


2a. What sort of radiation do objects at ordinary room temperatures emit?

2b. Why don't objects cool to absolute zero?

2c. What happens when an object is in thermal equilibrium with its surroundings?

2d. What does the nighttime temperature of Earth's surface (also a blackbody) depend on?


3. Why isn't the solar electromagnetic spectrum that of an ideal blackbody?


4a. How is the heat from objects like stove burners and coals transmitted?

4b. What causes a burning candle to emit blackbody radiation?

4c. How do humans and other warm blooded animals behave as blackbody radiators?


5a. Why is a blackbody referred to as "black"?

5b. What characterizes nonideal blackbodies?


6a. What is absorptivity?

6b. What is emissivity?

6c. Why are the absorptivity and emissivity of an ideal blackbody both equal to 1?


7a. What does Kirchoff's Law state?

7b. What does Kirchoff's Law mean?



What Light Is

1. Light is the name we give to electromagnetic radiation that we can see with our eyes. It can also be used more broadly to include ultraviolet and infrared light electromagnetic radiation.

2a. Photons are individual packets of energy that carry both electric and magnetic fields. They display characteristics of both a particle and a wave.

2b. The electric and magnetic field waves of a photon oscillate at right angles to each other and to the direction of the photon (considered as a wave).

3a. The electromagnetic spectrum refers to the complete range of possible electromagnetic radiation energies (or wavelengths or frequencies) that a photon may have.

3b. The six types of electromagnetic radiation in order of increasing energy level are (1) radio waves, (2) infrared radiation, (3) visible light, (4) ultraviolet radiation, (5) X-rays, and (6) gamma rays.
Radio waves are the region of the electromagnetic spectrum with very long wavelengths. Radio, TV, and radar communications occur using these types of waves.
Infrared radiation is the region of the electromagnetic spectrum with wavelengths long enough to cause molecules to vibrate around, increasing the temperature of the molecules. Infrared radiation is heat energy.
Visible light is the region of the electromagnetic spectrum (wavelengths 400-700 nm) where photons have enough energy to interact with certain pigment molecules in the retina of the eye to give us sight. This corresponds to the region of greatest solar output. All the colors of the rainbow fall into this small region, ranging from violet (400 nm) through indigo, blue, green, yellow, orange, to red (700 nm).
Ultraviolet radiation is the region of the electromagnetic spectrumwhere photons are sufficiently energetic to change energy states within atoms and molecules, sometimes even breaking them apart. UV light can damage some biological organisms. Ozone absorbs certain types of ultraviolet radiation from the Sun, protecting life on Earth.
X-rays are very energetic photons produced in nuclear reactions, solar storms, and in bombarding metal surfaces with fast moving electrons. X-rays can change the energy level of electrons within atoms and even the energy of an atomic nucleus. X-rays also have medical applications, since they can be used to investigate the structure of molecules.
Gamma rays are the most energetic of photons in the electromagnetic spectrum that are produced in nuclear fusion reactions that occur inside stellar interiors. Gamma rays can strip electrons away from molecules and atoms, inducing "ionization." The resulting ions are very reactive and the original molecular structures are permanently lost. It is for this reason that gamma rays are so biologically damaging.

The Nature of Light Radiated by Our Sun

1a. 5700 K.

1b. Ninety-nine percent of the Sun's electromagnetic spectrum falls between 10,000 nm (far infrared) and 100 nm (deep ultraviolet). The spectrum of the Sun's spectral irradiance agrees reasonably well with that of a blackbody radiator at about 5700 K. Deviations occur from an ideal blackbody curve because of absorption of light by constituents of the solar atmosphere, and the fact that the photosphere is not uniform in temperature. Absorption lines appear around 700 nm because of elements in the solar atmosphere. At wavelengths shorter than 280 nm, the solar spectrum is well below that of an ideal blackbody because of absorption in the solar atmosphere that results in the atmosphere that results in the atoms being completely ionized with some notable emission bands resulting in upward spikes in the curve.

2. Deviations from an ideal blackbody curve at the X-ray and radio wave ends of the solar spectrum are caused by solar storms. X-ray emissions increase when the Sun is active because of temperature variations in the outer chromosphere, which is far hotter than the Sun's surface. Radio emissions vary because of the interaction between free electrons and the Sun's magnetic field during solar storms. Variations in the 10.7 cm radio wavelength occur because of temperature variations in the solar atmosphere.

3a. The energy output of the Sun varies because of the Sun's rotation, quasi-cyclical changes in solar surface activity and temperature, and episodic events such as solar flares.

3b. The amount of solar energy reaching Earth remains relatively constant over time even though there are large variations in the X-ray and radio portions of the solar spectrum (caused by the reasons listed in answer 3) because over the vast bulk (over 99%) of the solar spectrum (far infrared to deep ultraviolet), the variability in solar output is very small. The amount contributed by X-rays and radio waves to the total solar spectrum is very small, so even though the variations are quite large, the overall contribution from these portions of the spectrum is quite small.

The Solar Cycle

1a. The solar cycle refers to the regular cycle in the number of sunspots on the surface of the Sun. The cycle has a period of about 11 years. The record for the solar cycle stretches back over several centuries.

1b. Sunspots are disturbances on the surface of the Sun where less visible radiation is emitted, hence they appear dark when compared to the overall photosphere (solar disk). However, more UV radiation is emitted from sunspot regions.

1c. Solar flares are giant explosions on the Sun. They are more prevalent during the active portion of the 11-year solar cycle.

2. Measurements of radio fluxes from the Sun at 2800 MHz correspond to 10.7 cm radio wavelength. Measurements are made in this region because the output from this region of the Sun is associated with solar storm activity, which affects Earth's upper atmosphere, including the amount of ozone.

3. Temperature variations in the upper stratosphere of 2-3 K are caused by the solar cycle, hence variations in the 200 nm portion of the solar spectrum. Ozone variations in the upper stratosphere of about 5% of the total are due to the solar cycle at 200 nm.

How Light Moves Through the Atmosphere

1. TBD

2. TBD

3. TBD

4. TBD

5a. Mie scattering is the scattering of light by particles that are much larger than molecular size. Light will be deflected in all directions and will appear white.

5b. Rayleigh scattering is the scattering of light by individual molecules. The molecules are of comparable size to the photon wavelengths.

5c. The chief difference between Mie and Rayleigh scattering involves the exiting radiation. Mie scattering results in exiting radiation whose intensity is about the same in all directions, while Rayleigh scattering is strongly dependent on the viewing angle, with the degree of dependence in turn dependent on the photon wavelength. The shorter the wavelength, the stronger the scattering.

6. The sky is blue because of Rayleigh scattering, which is stronger with shorter wavelengths. Blue-violet light has a shorter wavelength than red light, so blue-violet is scattered more strongly, resulting in a blue sky.

7a. There are no atmospheric constituents that absorb significant amounts of light in the visible and near ultraviolet regions of the spectrum, so solar radiation in these regions pass through the atmosphere and reach the surface. Plans and animals have adapted to make use of this radiation. Animals use the radiation to see, while plans use the radiation to turn carbon dioxide into organic molecules and oxygen via photosynthesis.

7b. The main biological effect of infrared radiation is heating of the biosphere.

8a. Radiation shorter than 315 nanometers, corresponding to the UV-b region, does not reach Earth's surface because of the presence of molecular oxygen and ozone high up in the stratosphere.

8b. The oxygen molecules dissociate to form free oxygen atoms, which then recombine with other oxygen molecules to form the three-oxygen ozone molecule.

8c. The ozone molecules formed absorb strongly in the middle ultraviolet region. The ozone molecules are dissociated into an oxygen atom (O) and oxygen molecule (O2). The free oxygen atom will then recombine with an O2 molecule to reform ozone (O3).

9a. Most of the radiation between 225 nm and 275 nm has been absorbed by oxygen high up in the atmosphere.

9b. Virtually all of the radiation with wavelength shorter than 315 nm has been absorbed, with the difference between 30 km and the surface having been absorbed by ozone.

How Objects Emit Radiation: Blackbodies

1a. Blackbody radiation refers to the phenomenon by which the distribution of electromagnetic radiation (photon energies and fluxes emitted) by an object depends on the object's temperature. Because of this temperature dependence, blackbody radiation is sometimes called thermal radiation or heat radiation.

1b. Any type of object from single molecules (solid, liquid, or gaseous) to entire stars emit blackbody radiation.

2a. Such objects emit infrared radiation, so that even if the object is not glowing, it is still emitting infrared or thermal radiation.

2b. At the same time an object is losing energy to outgoing blackbody radiation, it is bathed in blackbody radiation emitted by all other objects in its surroundings, and it absorbs some of this energy, replacing some that is lost.

2c. Even after thermal equilibrium is reached between different objects, the objects will still exchange radiation with each other, with absorption and emission occurring at the same rate, so that no net heat exchange takes place.

2d. The nighttime temperature depends on the relative rates of absorption and emission by Earth and the atmosphere.

3. The solar spectrum deviates from an ideal blackbody because the photosphere contains materials at different temperatures (since the photosphere is a convective zone in which hotter material is continuously welling up and cooler material is continuously sinking down). The solar atmosphere above the photosphere contains numerous elements, most notably hydrogen and helium, but also small amounts of carbon, nitrogen, oxygen, silicon, potassium, sodium, iron, and nickel. These elements absorb some of the radiation at different wavelengths creating Fraunhofer absorption lines.

4a. Heat from stove burners and coals is transmitted as blackbody radiation without actually heating the air. Molecules in your skin absorb the heat energy emitted from the object. Heating of the air is not the primary way in which radiant energy is transferred.

4b. A burning candle emits blackbody radiation primarily because of the soot particles created in the incomplete combustion of fuel (paraffin) molecules. The soot emits a great deal of blackbody radiation, which appears reddish-orange-yellowish. Other reactions that emit radiation occur in the flame itself, though the soot dominates.

4c. Humans and other warm blooded animals are warmer than their surroundings and emit energy in the infrared region of the spectrum, roughly at a rate of 100 Watts for an average adult at rest. A net loss of energy by way of radiative emission is replaced by metabolic processes.

5a. A blackbody radiator is referred to as "black" because an ideal blackbody is a hypothetical object that absorbs all radiation incident on its surface. It does not reflect any radiation (including visible light), hence appears black.

5b. Nonideal blackbodies emit less radiation at any given wavelength than an ideal blackbody would. Nonideal blackbodies like Earth and the Sun closely approximate ideal blackbodies at the same temperature.

6a. Absorptivity is defined as the fraction of incident radiation that is absorbed by an object at a specific wavelength.

6b. Emissivity is the fraction of radiation emitted at a specific wavelength compared to that emitted by a blackbody at the same temperature.

6c. The absorptivity and emissivity of an ideal blackbody are both 1 because an ideal blackbody absorbs all incident radiation and emits the maximum amount possible at each wavelength.

7a. Kirchoff's Law states that for any object, absorptivity equals emissivity.

7b. Kirchoff's Law means that an object that is a strong absorber at a particular wavelength is also a strong emitter at that wavelength, and an object that is a weak absorber at a particular wavelength is also a weak emitter at that wavelength.