The phenomenon you are observing is called "heat shimmer" or "heat waves." It occurs when there are temperature differences between layers of air, and the light passing through those layers is refracted, creating the appearance of shimmering or wavering lines. This effect is more noticeable on hot days or in areas where there is a significant temperature contrast.
The reason you see heat waves when looking at the shadow of sunlight shining through the space above the toaster, but not when looking directly above the toaster, is because of the specific conditions involved:
Heat Source: The toaster is a relatively small appliance with localized heating elements. While it does generate heat, the heat is concentrated around the toaster and may not create significant temperature differences in the air above it. As a result, there may not be enough variation in air temperature to cause visible heat shimmer directly above the toaster.
Sunlight and Shadow: When sunlight passes through the space above the toaster, it is entering an environment that can have temperature variations due to various factors such as the sun's intensity, surface heat absorption, and air circulation. As the sunlight moves through these pockets of air with different temperatures, it gets refracted at varying angles, leading to the appearance of heat waves in the shadow.
Distance: The distance between your eyes and the toaster might be relatively close, and the heat shimmer effect is usually more pronounced when looking at objects at a greater distance. When looking at the shadow of sunlight, the distance between the light source and the shadow area could be greater, enhancing the visibility of heat waves.
Overall, the heat shimmer effect is a visual demonstration of the air's changing refractive index due to temperature variations. It's a fascinating natural phenomenon that is often more noticeable in outdoor environments or areas with significant temperature differences, and it can create interesting visual distortions in the air.