Infrared (IR) light and visible light are both forms of electromagnetic radiation, but they have different wavelengths. Visible light has shorter wavelengths, ranging from about 400 to 700 nanometers, while infrared light has longer wavelengths, typically ranging from around 700 nanometers to 1 millimeter.
When light encounters an object, what happens to it depends on how the object interacts with the specific wavelength of light. In the case of visible light, certain materials may absorb it, reflect it, or scatter it, making the object appear opaque. If an object absorbs most of the visible light that hits it, it will appear dark or solid.
However, with infrared light, things behave differently. Many materials that are opaque to visible light are actually transparent or partially transparent to infrared light. The reason for this lies in the way the atoms and molecules in the material interact with the specific wavelength of infrared light. Unlike visible light, infrared light can be absorbed and re-emitted by the atoms and molecules in a substance without causing a significant interaction with the material's electrons, which is responsible for visible light absorption.
Materials that are transparent in the infrared range allow IR light to pass through them because the energy levels involved in the absorption and emission processes are such that they don't affect the visible light spectrum. So, while some materials might appear completely opaque to visible light, they might allow infrared light to penetrate through them.
Regarding why infrared heat doesn't get blocked as well, it's because heat transfer occurs through a different mechanism. Heat, specifically in the form of infrared radiation, is a type of electromagnetic radiation emitted by objects due to their temperature. This radiation carries thermal energy, and it can travel through space and interact with other objects. When infrared radiation encounters a material, the material's atoms and molecules can absorb some of the radiation's energy, which causes the material to warm up.
Unlike conduction or convection, which require direct contact between the heat source and the object, infrared radiation doesn't need a medium to transfer heat. This property allows infrared heat to pass through certain materials that may be opaque to visible light, as mentioned earlier. So, even though some objects can block visible light due to absorption, they may not block infrared heat as effectively, allowing the heat to penetrate and warm up the material.