Final answer:
Heat shadows happen when heavy furniture blocks infrared radiation from a fire, preventing heat transfer to that area of the wall and creating a 'shadow'.
Step-by-step explanation:
Heat shadows occur when heavy furniture shields the wall from the heat of a fire. This happens because most heat transfer from a fire to the surroundings occurs through infrared radiation. Although visible light from the fire is dramatic, it transfers comparatively little thermal energy. As hot air rises, convection moves energy away from observers, but conduction here is usually very slow. Therefore, the areas not directly exposed to infrared radiation remain cooler compared to the surroundings, creating what can be interpreted visually as a shadow on the wall where the heat has not reached.