Final answer:
Photons experience different phases depending on the material they pass through, affecting wavelength and interference. This results in changes in contrast under microscopic observation or in the effectiveness of solar panels. Additionally, light direction in photography can add dimensional effects to images.
Step-by-step explanation:
When considering what occurs with photons when the background is lighter than the centre or when the background is darker than the centre, we are addressing the concept of light interaction with matter, mainly through refraction and interference. Light rays passing through a sample under a microscope, for example, emerge with different phases depending on whether they have traveled through the object or the surrounding medium. If the object has a greater index of refraction than the background, the wavelength of light decreases as it passes through the object. The superposition of these varied-phase light rays results in interference patterns, which enhance the contrast between the object and the background. This principle is also applied to understand the cosmic microwave background and to improve the efficiency of devices such as solar panels, where the absorption and penetration depth of photons are key factors.
Additionally, in photography, when light direction is considered, the presence of shadows and the way light interacts with the subject can create visual effects such as a three-dimensional appearance or a silhouette when backlit.