9.2: Electromagnetic Radiation

Energy from the Sun comes to us in the form of electromagnetic radiation, a term that refers to all the types of radiant energy emanating from stars (e.g., the Sun). Scientists classify different types of electromagnetic radiation according to their wavelengths, the distance between the peaks of their waveform (just like sound waves). The arrangement of different types of electromagnetic radiation according to their wavelengths produces what is known as the electromagnetic spectrum, the range of all types of electromagnetic radiation. From longest to shortest, the electromagnetic spectrum consists of radio waves (the waves you listen to), microwaves (the waves you cook with), infrared light (used in night-vision gear), visible light (so named because you can see it), ultraviolet light (the reason you use sunscreen), X-rays (used by doctors and dentists), and gamma rays (used in medicine; e.g., NASA 2016b). In everyday usage regular folks just refer to the Sun’s electromagnetic spectrum as sunlight, solar radiation, or solar energy. But the spectral divisions are useful, too, as we shall see.

Energy

Most people recognize that light is a form of energy. Few, however, appreciate what the word energy means. Formally, energy is defined as “the ability to do work”—crudely speaking, to move something (e.g., EIA 2023c). I like to think of energy as the stuff that makes things happen. It’s a property of an object or system that can be used to generate a force of some kind and do work.

Broadly speaking, two kinds of energy exist: (1) kinetic energy, the energy of motion; and (2) potential energy, stored energy or energy inherent in an object in a gravitational field. Kinetic energy includes solar radiation (electromagnetic energy), the motion of objects (motion energy), sound waves (sound energy), electricity (electrical energy), and heat exchange (thermal energy). Potential energy may refer to energy stored in chemical bonds (chemical energy), energy stored in objects under tension (mechanical energy), radioactive materials (nuclear energy), or the force experienced by Humpty Dumpty on his epic fall (gravitational energy; EIA 2023a).

More important than the definition of energy is how it operates. Like Mystique of the X-People, energy can shape-shift. It can change from one form to another. Scientists express energy’s ability to change forms in the well-known law of conservation of energy. In nontechnical terms, the law of conservation of energy states that energy can be neither created nor destroyed, but it can change forms. That means that one type of energy can become another type of energy—energy can transform. It’s still the same energy; it’s just in a different package (e.g., EIA 2023b).

Familiar examples will help you grasp this rather abstract concept. A log, the dried-out hunk of a trunk of a tree, was formed when the tree captured electromagnetic energy—sunlight. The tree turned the sun’s energy into chemical bonds, the energy-containing stuff of which the tree is made. When you set the log ablaze, you transform its chemical energy into heat energy that provides warmth. A burning log also releases electromagnetic energy, the red-yellow-orange glow of its fire. And when you heat a marshmallow over a fire and eat it (with chocolate and graham crackers, of course), your body transforms their chemical energy into the kinetic energy of a laugh, a fist bump, or a romantic kiss. The story of energy is the story of its transformations.

Interactions of Light with Matter

Interactions between light (i.e., electromagnetic radiation) and matter (i.e., the atoms of which materials are composed) prove important for understanding transformations of energy. Fortunately, we need only master a few simple definitions to appreciate these interactions.

To understand light–matter interactions, it helps to think of light as rays, arrows that indicate the direction of a light wave. When a light ray encounters an object, many possible things may happen. But let’s consider the four interactions that play a role in our understanding of how solar radiation heats our planet:

• absorption, the energy in the light ray is transferred to the object and becomes part of the object’s energy
• scattering, the light ray interacts with the object and is sent in different directions
• reflection, the light ray bounces off the object at the same angle at which it arrived
• transmission, the light ray travels through an object and some or all of its energy comes out the opposite side from which it entered

Now, these definitions may not be picture perfect in the eyes of physicists, but they hopefully give you an understanding of the basic principles. (See also Ahrens and Henson for a good basic treatment of this topic.)

Temperature and Heat

When an object absorbs electromagnetic radiation, it gains energy. This causes its molecules to move more rapidly. In scientific terms, the kinetic energy of the molecules increases. We detect a gain in energy as a rise in temperature, loosely defined as the average kinetic energy of a system, substance, or object. Temperature provides a convenient way to compare the kinetic energy of systems, substances, and objects.

To be clear, temperature is not a measure of heat energy, total kinetic energy, or even the internal energy in a system or object. It’s simply a convenient way to describe the jumpiness or sluggishness of the molecules in a system or object. By analogy, if kids in a bounce house are jumping all over the place, you might say their temperature is high. If the kids are barely moving, their temperature is low. The higher the temperature, the faster the molecules move, and vice versa. It’s important to note that measurements of temperature are not the same as measurements of energy. An ice cube and an iceberg may have the same temperature, but the iceberg, by virtue of its much larger mass, contains considerably more energy.

Thermometers

A thermometer—a device used to measure temperature—provides a number for the average kinetic energy in an object or system. The level of fluid in a thermometer rises and falls according to the temperature. In a digital thermometer, a digital display corresponds to the expansion or contraction of a strip of metal or changes in its electrical conductivity as temperature changes.

The numbers displayed by a thermometer are completely arbitrary. In the Celsius scale—invented in 1742 by Swedish astronomer Anders Celsius (1701–1744)—the temperature of freezing water is set arbitrarily at 0°. In the Fahrenheit scale—invented in 1724 by physicist David Gabriel Fahrenheit (1686–1736)—the temperature of freezing water is set arbitrarily at 32°. The numbers for the freezing points of water were chosen by Mr. Celsius and Mr. Fahrenheit when they invented their thermometers. The same is true for the boiling points, 100°C and 212°F. These numbers were chosen by their inventors who for various reasons thought that they worked well for the range of temperatures that people who use the thermometers would encounter (e.g., Middleton 1966).

Thermometers work by heat exchange between the external environment and a fluid inside a tube. (Traditionally, the fluid was mercury; now most liquid thermometers contain alcohol.) When the external environment is hotter than the fluid inside the thermometer, heat energy flows from the environment to the fluid, and the fluid expands. The fluid expands because the molecules in the fluid are moving faster, and they take up more space in their motions. We see the fluid rise in the tube. When the external temperature is lower than the fluid inside the thermometer, heat energy flows from the fluid to the external environment; the fluid loses energy and the space between the molecules is reduced—the fluid contracts. We see the level of the fluid fall in the tube. Similar expansions and contractions—or changes in electrical conductivity—accompany heat exchange between the external environment and digital thermometers.

Regardless of the chosen scale, thermometers provide temperature measurements according to agreed-upon methods. They provide us with a means for determining the relative hotness or coolness of objects or systems. They ensure consistency and reproducibility in processes that depend on temperature control, such as cooking, the manufacture of steel, and a whole lot more.

Heat Transfer

Heat only exists when two objects or systems have different temperatures. Think of heat as energy in motion. When two objects or systems reach thermal equilibrium—when they have the same temperature—heat no longer exists (e.g., Ahrens and Henson 2018).

The increase in temperature that occurs when matter absorbs electromagnetic radiation represents one of three ways that heat can be transferred between objects or systems: (1) radiation, (2) conduction, and (3) convection. All three forms of heat transfer become important when we examine what happens when Earth’s surface absorbs sunlight.

Radiation is the transfer of heat via electromagnetic energy. Radiation moves out in all directions from its source until it meets another object. That object may reflect the radiation, in which case no exchange of energy occurs; it may transmit the radiation, in which case the radiation travels right through it and no exchange of energy occurs; or it may absorb the energy, in which case the energy of the object (and its temperature) increases. If you’ve ever felt the hood of a black car on a sunny day, you’ve likely noticed that it’s hotter than the surrounding air. That’s because the metal of a car’s hood absorbs electromagnetic radiation, increasing its temperature. When you touch the hot hood, heat energy is transferred to your finger, and you utter some variation of “ow” (depending on how much it hurts).

A car’s hood also emits infrared radiation, electromagnetic energy at wavelengths beyond visible light. You do not detect this kind of radiation because, unlike visible light (which you can see), your eyes are not sensitive to infrared radiation. All objects, including humans, emit radiation in proportion to their temperature. Hotter objects emit shorter wavelengths, and cooler objects emit longer wavelengths. The relationship between temperature and the wavelength of maximum emission defines a principle known as Wien’s Law. This law provides a quantitative formula for determining the peak emission wavelength from the temperature of an object. As temperature rises, the peak emission wavelength becomes shorter.

The infrared emission of objects, humans, and other forms of life makes them detectable using night-vision technologies. These devices have sensors that detect infrared radiation and electronics that turn the infrared signal into a color the user can see—usually green. They are very useful for seeing in the dark, but generally they are used for clandestine or military purposes.

Conduction occurs when the faster molecules of a higher-temperature object collide with the slower molecules of a lower-temperature object and exchange energy until all the molecules are moving at the same speed (i.e., until they have the same kinetic energy). Conduction is the transfer of heat energy through molecule-to-molecule collisions. Heat transfer by conduction requires contact between two objects. When you put a marshmallow on a metal skewer in a campfire, the end immersed in the fire gains energy and begins to transfer that energy via conduction along the length of the skewer. If you keep it in the fire long enough, the end you’re holding will eventually get too hot and burn you. Skewers made of wood—a poor conductor of heat—protect your hand from burning. Kitchen utensils employ principles of heat conduction to prepare food and to protect chefs. Pans made of copper, aluminum, and carbon steel conduct heat faster than those made of cast iron or stainless steel. Fast-conducting pans are great for searing foods. At the other end of the pan, pot handles made of plastic or other non-conducting materials—or a good pair of cotton pot holders—limit heat conduction and prevent your hands from burning.

Convection involves heat transfer through the motions of fluids, such as air and water. When you heat air or water, you cause it to expand. This expansion lowers the density of the fluid. If the surrounding fluid is more dense, then the heated fluid will rise. This should be familiar to you as the principle behind heating and cooling in your home—heating devices (or their ducts) are generally on or near the floor because warm air rises. Air conditioners (or their ducts) are generally placed at the top of a window or in the ceiling because cold air sinks. The rising or sinking air transfers heat through its movements, which is called convective heating. Convection causes air or water to circulate—to move in a circular motion. On a global scale, convection is responsible for the large-scale motions of the atmosphere and the ocean—the global winds and ocean currents. (See Ahrens and Henson 2018.)