Homework: Electromagnetic Energy and the Earth

Mr. Brown's Science Labs

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Electromagnetic Energy and the Earth

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PART 1

The energy that leaves the Sun

Almost all of the energy that runs Earth's surface arrives as electromagnetic energy — radiation that travels as waves and needs no material to travel through. That last point matters. Between the Sun and Earth there is essentially nothing, so conduction and convection cannot carry energy across that gap. Radiation can.

Every electromagnetic wave travels through the vacuum of space at the same speed, about 3.0 × 10⁸ meters per second. What separates one kind of radiation from another is wavelength: the distance between one crest and the next.

The trade-off that runs this whole topic: as wavelength gets shorter, frequency gets higher and each wave carries more energy. Gamma rays sit at the short, high-energy end. Radio waves sit at the long, low-energy end. Visible light is a narrow strip in the middle.

The hotter the object, the shorter the wavelength it radiates most strongly. That single rule is why sunlight is visible and Earth's glow is not.

Reference Tables check: open to page 14. Find where visible light begins and ends. On the Regents you will be given a wavelength in meters and asked to name the band. Practice reading that scale now.

Part 1 questions

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PART 2

Short waves in, long waves out

Energy arriving from the Sun is called insolation — a compression of incoming solar radiation. Because the Sun is roughly 6000 K, its insolation is dominated by short wavelengths: visible light, with ultraviolet on one side and near-infrared on the other.

When that insolation is absorbed by rock, soil, water, or pavement, the energy does not disappear. The surface warms, and a warm surface radiates too. But Earth's surface averages about 288 K, nowhere near 6000 K, so what it radiates is far longer in wavelength. Earth's outgoing radiation is almost entirely infrared. You cannot see it, which is exactly why a parking lot at night looks dark while still pouring energy into the sky.

The wording the Regents uses: incoming solar radiation is described as short-wave. Radiation given off by Earth's surface is described as long-wave or terrestrial radiation. If a question contrasts the two, the answer almost always hinges on that difference in wavelength.

The atmosphere is largely transparent to visible light on the way in and largely opaque to infrared on the way out. That asymmetry is the greenhouse effect.

Notice what this means for how the atmosphere is heated. Visible light mostly passes straight through the air without warming it. The ground absorbs that light, warms up, and radiates infrared. Greenhouse gases absorb that infrared. So the troposphere is heated from the bottom up, which is why air temperature generally drops as you climb.

Common Regents distractor: answers that say greenhouse gases "trap visible light" or "absorb ultraviolet." They do not. Ozone handles the ultraviolet. Carbon dioxide, methane, and water vapor handle the infrared.

Part 2 questions

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PART 3

Absorbed, reflected, or somewhere in between

Insolation that reaches a surface has a few possible fates. It can be absorbed, converting to heat and raising the surface temperature. It can be reflected, bouncing off with no temperature change. It can be scattered in many directions by particles in the air, or refracted as it bends passing into a new material, or transmitted straight through a transparent material like clear water or glass.

Only absorption heats anything. A white roof and a black roof receive identical insolation; the black roof is hotter because it absorbs a larger share of it.

Albedo is the percentage of incoming radiation a surface reflects. High albedo means highly reflective and slow to warm. Low albedo means highly absorptive and quick to warm. Fresh snow and thick clouds sit near the top. Asphalt and open ocean sit near the bottom.

What controls how much gets absorbed

Property	Absorbs more when…	Reflects more when…
Color	dark	light or white
Texture	rough	smooth or polished
Angle of insolation	high (near vertical)	low (near horizon)
Surface type	asphalt, forest, plowed soil, open water at noon	snow, ice, clouds, sand, water at sunrise

Angle is the one students skip. When the Sun is high, a beam of light is concentrated onto a small patch of ground and strikes nearly straight on, so most of it is absorbed. When the Sun is low, the same beam is spread across a much larger patch and a greater share glances off. This is the whole reason summer is warmer than winter, and the reason the equator is warmer than the poles.

Also worth memorizing: a good absorber is also a good radiator. The dark, rough surface that heats up fastest during the day is also the surface that cools down fastest at night.

Albedo explorer

Click each surface. Watch how the split between reflected and absorbed energy shifts, then use what you see on the questions below.

Select a surface

Reflected (albedo)—

Absorbed—

Each surface receives exactly the same insolation. What changes is how much of it becomes heat.

Part 3 questions

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PART 4 · REGENTS CLUSTER

Reading an energy-balance graph

This graph shows a single clear day at a location in New York State. One curve is the intensity of insolation reaching the surface. The other is the intensity of terrestrial radiation leaving the surface. Base your answers on the graph, the diagrams you have already studied, and your knowledge of Earth science.

The curves cross twice. Everything this cluster asks you comes back to what is happening at, between, and outside those two points.

Part 4 questions

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PART 5 · WRITING

Explain it in a sentence

Regents constructed-response items reward precision, not length. Finish each sentence below so that it is scientifically accurate and complete. One good sentence beats a paragraph of hedging.

Because gives a cause. But gives a contrast or limitation. So gives a result or consequence. The conjunction tells you what kind of thinking the sentence needs.

A. Finish the sentence

B. Sentence scramble

Click the words in order to build one correct sentence about the greenhouse effect. A word turns green when it is sitting in the right position. Click a word in the answer line to send it back.

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