Light and heat are connected through electromagnetic waves. When sunlight hits an object, it activates particle motion, converting into thermal energy, and this process raises the temperature.
The Sun, Light, and Heat
Why is the sunny side warmer than the shady side? No matter how hot the Sun is, how can it heat objects on distant Earth across the vast, empty space of the universe without any suitable medium? To understand this, we must first clearly grasp the nature of heat and light.
The Historical Understanding of Heat
Until the mid-18th century, scholars believed heat was the action of an invisible substance—the ‘caloric’—and explained that solids melting or liquids evaporating were a kind of chemical interaction between caloric and the particles composing solids or liquids. However, thanks to the research achievements of Rumford, Mayer, Joule, and others, the existence of ‘caloric’ was disproved, and the concept of ‘thermal energy’ was established instead. It was Clausius who specifically clarified the nature of heat, arguing that the thermal energy of a gas is the kinetic energy of its molecules, and therefore temperature represents the degree to which gas molecules are moving rapidly. Furthermore, Maxwell demonstrated that at a constant temperature, the speeds of gas molecules are distributed around an average value. This ultimately revealed that heat is the ‘average kinetic energy of the particles constituting an object’. The particles of all objects are constantly vibrating or rotating around their average positions, and temperature represents the magnitude of this kinetic energy.
Electromagnetic Theory and Light
Understanding the nature of light also requires grasping electromagnetic theory, as light is a type of electromagnetic wave. The existence of electromagnetic waves was inferred through Ampère’s experiments demonstrating that electric currents (electric fields) generate magnetic fields, Faraday’s experiments confirming that magnetic fields induce electric currents, and Maxwell’s theory synthesizing these findings. Ampère confirmed that passing current through parallel wires creates a magnetic field and that a coil—a wire wound into a cylindrical shape, called a ‘solenoid’—becomes a powerful magnet when current flows through it. Faraday confirmed that passing a magnet through a coil with no current flowing generates a current in response to the change in the magnet’s magnetic field. An electric field generates a magnetic field, and a magnetic field in turn generates an electric field. Maxwell systematized the results of these experiments to establish the theory known as ‘Maxwell’s Equations,’ and through this theory, the existence of electromagnetic waves could be inferred.
Principle of Electromagnetic Wave Propagation
When a current is suddenly passed through a conductor or its intensity is changed, a magnetic field is generated around it. This magnetic field creates a secondary electric field, which in turn creates a secondary magnetic field. This process, where an electric field creates a magnetic field, which in turn creates another electric field, repeats and propagates outward as a wave. This is precisely an electromagnetic wave. After calculating that the speed of this wave was identical to the speed of light, Maxwell reached the brilliant conclusion: “Light itself is a type of electromagnetic wave.” Unlike mechanical waves like sound, where matter physically vibrates, light propagates as an electromagnetic wave through the continuous alternation of electric and magnetic fields. Subsequent scientists confirmed that electromagnetic waves propagate even without a medium, explaining how sunlight can traverse the empty space of the universe.
The Propagation of Solar Energy and Its Delivery to Earth
What comes from the Sun is not particles of heat, but electromagnetic waves. When these waves strike an object, they cause it to vibrate through interference. This vibration interacts with the object’s particles, causing them to move and, consequently, raising the object’s temperature. Through this process, sunlight can traverse the universe without any medium and warm objects on Earth.
Temperature Difference Between Sunlit and Shaded Areas
The temperature difference between sunlit and shaded areas can be explained by this principle. In sunlit areas, the Sun’s electromagnetic waves directly strike objects, causing their particles to vibrate vigorously and raising the temperature. Conversely, in shaded areas, sunlight does not strike directly, so this process does not occur, resulting in relatively lower temperatures. Furthermore, the degree to which an object absorbs solar energy varies depending on its color and properties. Consequently, even in the same sunny location, the surface temperatures of different objects can vary significantly. For example, dark-colored objects absorb more solar energy and heat up faster, while light-colored objects have higher reflectivity and thus experience a relatively smaller temperature increase.
Conclusion
These principles help us understand why sunny areas are warmer than shaded ones. Solar energy is transmitted as electromagnetic waves, crossing space without a medium to warm objects on Earth. Areas directly exposed to light become warmer. Understanding natural phenomena through such scientific principles is fascinating and provides deep insight into easily observable everyday occurrences.
