Understanding Heat Transmission Through Glass: Beyond Infrared

Introduction

The common belief is that if glass blocks infrared light, you shouldn't feel heat behind it. However, the reality is more complex. This article aims to clarify the process of heat transmission through glass by exploring the role of visible light, UV, and infrared radiation, and the principles of thermodynamics.

Heat Transmission Mechanisms

Heat transmission through glass involves multiple mechanisms, including conduction, convection, and radiation. Understanding these concepts is essential for comprehending why you can still feel heat through glass.

1. Conduction: Heat can be transferred through glass via conduction. This process is particularly significant when there is temperature difference between the inside and outside of a building. Hot air outside can conduct heat to the cooler surfaces of the glass, which can then transfer this heat to the inside.

2. Radiation: Both visible light and infrared radiation play crucial roles in heat transmission. While infrared radiation is blocked by certain types of glass, visible light and ultraviolet (UV) radiation can penetrate the glass and contribute to heat absorption.

3. Convection: Convection occurs when heat is carried by moving fluids or air. In buildings, heated air currents can circulate within the room, causing a feeling of warmth behind even partially insulated windows.

Solar Radiation and Glass

The majority of solar radiation reaching the Earth's surface falls within the range of 0.3 to 2.5 microns, with a portion of this range being infrared radiation. Glass, specifically Heating Glass or Insulated Glass Units (IGUs), is designed to block most of the infrared light while allowing a significant amount of visible and some UV radiation to pass through.

1. Visible Light: The visible light component of solar radiation can pass through glass and be absorbed by the surface of objects or the skin, causing a sensation of heat. This is because any form of electromagnetic radiation can heat up an object if it is intense enough and the object can absorb it.

2. Infrared and UV Light: Even though glass can block most infrared light, it is not entirely effective. Some infrared radiation can still pass through, and UV light, which is also a form of electromagnetic radiation, can contribute to heat absorption.

Theoretical Framework: Thermodynamics

From a thermodynamic perspective, heat is not a specific form of energy. Heat is a measure of the transfer of energy associated with changes in entropy. Entropy, in thermodynamics, is a measure of the energy in a system that is unavailable for doing work. It is often thought of as a fluid that carries energy.

1. Heat Transfer Mechanisms: In thermodynamics, heat transfer is not limited to infrared radiation alone. It can occur through various mechanisms such as conduction (heat flowing from a higher temperature region to a lower temperature region), convection (heat transfer by fluid motion), and radiation (heat transfer in the form of electromagnetic waves).

2. Light's Role in Heat Transfer: Visible light, which includes colors beyond just infrared, can also carry heat. Lasers, for instance, are used for welding because the visible light carries heat due to the kinetic energy of the photons.

3. Work and Heat Energy: In thermodynamics, energy can be carried in two primary ways: by work (e.g., friction) and by entropy (heat). Visible light, therefore, can also contribute to heat transfer in buildings, as it is a form of electromagnetic radiation that can be absorbed and converted into thermal energy.

Conclusion

Understanding how heat is transmitted through glass involves recognizing the complex interplay of different forms of radiation and the principles of thermodynamics. While glass can effectively block some forms of infrared radiation, visible light and UV radiation can still contribute to the sensation of heat. This knowledge is crucial for designing effective insulation and windows that control heat transfer in buildings, enhancing both comfort and energy efficiency.