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Learning how energy travels through the vacuum of space via electromagnetic waves.
How does the Sun warm your face across 93 million miles of empty, freezing space without any air to carry the heat?
In conduction and convection, heat needs 'stuff' (atoms or molecules) to move. Radiation is different. It is the transfer of energy through electromagnetic waves. Because these waves don't rely on vibrating particles, radiation is the only form of heat transfer that can travel through a vacuum—a space completely empty of matter. All objects, including your own body, constantly emit and absorb radiation. The hotter an object is, the more radiant energy it gives off. These waves travel at the speed of light, which is approximately .
Quick Check
Why can't convection happen in the vacuum of space?
Answer
Convection requires a fluid (liquid or gas) to move particles; space is a vacuum with no particles to move.
When radiant energy hits an object, three things can happen: it can be absorbed, reflected, or transmitted. The surface's color and texture play a massive role. Dark, matte (dull) surfaces are the 'sponges' of the radiation world; they are excellent at both absorbing and emitting heat. On the other hand, light-colored, shiny surfaces act like shields; they reflect most of the radiant energy away. This is why a black car feels much hotter than a white car after sitting in the sun.
1. Imagine two students standing outside on a day. 2. Student A wears a black cotton t-shirt. 3. Student B wears a white silk t-shirt. 4. The black shirt absorbs nearly all visible and infrared radiation from the sun, converting it to thermal energy. 5. The white shirt reflects most of that energy back into the environment, keeping Student B significantly cooler.
Quick Check
If you wanted to design a container to keep soup hot for as long as possible, should the inner lining be dull black or shiny silver?
Answer
Shiny silver, because it would reflect the heat back into the soup rather than absorbing and emitting it outward.
The Sun is a massive furnace emitting a spectrum of radiation. Most of the heat we feel is Infrared Radiation. As these waves hit Earth's atmosphere and surface, they are absorbed, warming the planet. This energy balance is vital. If the Earth absorbed all radiation without emitting any back into space, the temperature would rise indefinitely. The rate of energy transfer can be modeled by the Stefan-Boltzmann law, which states that the power radiated is proportional to the fourth power of temperature: .
1. Consider the Earth's polar ice caps (white) versus the dark open ocean. 2. The ice has a high albedo, meaning it reflects about of solar radiation. 3. The dark ocean has a low albedo, absorbing about of the radiation. 4. As ice melts due to rising temperatures, more dark ocean is exposed, which absorbs more heat, leading to further melting. This is a 'positive feedback loop'.
1. In space, there is no air to carry heat away by convection, so satellites can overheat just from their own electronics. 2. Engineers wrap satellites in Multi-Layer Insulation (MLI), which looks like gold foil. 3. This foil has a very low emissivity (), meaning it is a poor emitter of radiation. 4. By using multiple shiny layers, the heat is reflected back and forth, preventing it from escaping too quickly or entering from the Sun, maintaining a stable internal temperature of roughly .
Which of the following is required for radiation to occur?
Why are houses in hot, sunny climates often painted white?
A surface that is a good absorber of radiation is also a good emitter of radiation.
Review Tomorrow
In 24 hours, try to explain to a friend why you can feel the heat of a lightbulb without touching it, and which shirt color you should wear for a desert hike.
Practice Activity
Find two identical containers. Wrap one in aluminum foil and paint the other black. Fill both with hot water and measure which one stays warm longer to see radiation in action!