E lectric stove-top heating coil: “Off” setting (room temperature)

1. The surface of the sun:~ 10,000 °F “white” hot Radiative Emission Spectra:Intensity with which objects at four different temperaturesemit radiative energy at different wavelengths Electric stove-top heating coil: “High” setting “red” hot Electric stove-top heating coil: “Medium” setting 50% 40% 10% Electric stove-top heating coil: “Off” setting (room temperature) V I B G Y O R “Near” IR (longer) ( shorter) Longwave infrared (LWIR) radiation (wavelengths of radiationthat the earth emits, mostly) Solarradiation (wavelengths of radiationthat the sun emits, mostly) Note: “V I B G Y O R“ = Violet, Indigo, Blue, Green, Yellow, Orange, and Red (selected visible colors)

2. Notes on Emission Spectra The plots shown illustrate the first three basic laws of radiation: • Most things emit radiative energy, of many wavelengths (The plot for each object shows it emitting radiation at a range of wavelengths) • The intensity with which an object emits radiative energy depends on the object’s temperature: the warmer the object is, the more radiative energy it emits (The warmer the object is, the higher the plotted curve is, at every wavelength) • Objects emit most of their radiative energy at shorter wavelengths when they are warmer, at longer wavelengths when they are cooler (The warmer the object is, the more the peak of its emission intensity curve shifts to shorter wavelengths)

3. Notes on Emission Spectra: A kitchen example of the basic laws of radiation • When the heating coil on an electric stove is “Off” (at room temperature), it emits radiative energy but no differently than other things in the room, so we don’t feel that radiation any more than we feel radiation emitted by other objects in the room. The radiation is mostly infrared radiation, which we can’t see, and the coil looks black. • When set on “Medium”, the heating coil is warmer and it emits more radiative energy. A short distance away our skin absorbs it, converting it to heat, which we can feel. However, the radiation is still mostly infrared so we still can’t see it, and the coil still looks black.

4. Notes on Emission Spectra: A kitchen example of the basic laws of radiation • When set on “High”, the heating coil is warmer still and emits even more radiative energy, which we can feel when our skin absorbs it and converts it into heat. Not only does it emit more radiation at every wavelength, but the peak of its emission has shifted to shorter wavelengths. As a result, it emits enough visible light to see. However, it emits more longer wavelengths of visible light (red) than shorter wavelengths (blue), and so to us it looks red. We call it “red hot”! • The sun is much hotter than the heating coil set on “Hight”, so the sun emits radiation much more intensely and its emission peak is at still shorter wavelengths—in fact, the emission peak is in the middle of the visible part of the spectrum, and the sun emits similar amounts of all wavelengths of visible light. To us, this looks white. Hence, we call something as hot as the sun “white hot”.

5. Solar Radiation vs. Longwave Infrared (LWIR) Radiation • Most of the radiation that the sun emits consists of: • Relatively short wavelengths of infrared (IR) radiation, which we call “near IR” radiation (about 50% of the total energy emitted by the sun) • Visible light (about 40% of the total energy) • Ultraviolet (UV radiation (about 10% of the total energy) • Collectively, we call these wavelengths “solar radiation”. Solar radiation consists of wavelengths that the sun mostly emits. • In contrast, the earth is at a range of temperatures such that it emits mostly longer wavelengths of infrared radiation, which overlap with solar radiation very little. • We call these wavelengths “longwave infrared (LWIR) radiation”. LWIR consists of wavelengths that the earth emits, mostly.

6. Notes on Emission Spectra:The color of the sun and sky • When white light from the sun enters the atmosphere, air molecules scatter (redirect) some of it, but not all wavelengths equally. They scatter shorter wavelengths (blue ) much more effectively than longer wavelengths (red). • As a result, light reaching us directly from the sun has had blue wavelengths removed from it, which shifts its color to yellow. • Also as a result, light now comes at us from all directions, not just directly from the sun. Light coming from all other directions is the scattered (redirected) blue light. Hence, the sky looks blue. • When the sun angle is low, solar radiation must travel a greater distance through the atmosphere, and more blue light is scattered out of the direct rays of the sun. As a result, the sun can look orange or even red. (This is what happens near sunrise and sunset, for example.) • On the moon, which has no atmosphere, there is not scattering. As a result, the sun looks white and the sky looks black.