Sunlight passing through a hole cut in cardboard projects a circular image of the sun onto a wall [1].
This phenomenon demonstrates the fundamental principles of optics and the behavior of light as it interacts with physical apertures. Understanding how light travels in straight lines allows for a simplified explanation of how cameras and the human eye perceive the world.
The process occurs when a piece of cardboard with a hole is positioned so that sunlight passes through it toward a surface. Regardless of whether the hole is a square, a triangle, or an irregular shape, the resulting projection on the wall remains a circle [1]. This effect is not dependent on the size of the aperture, provided the hole allows light to pass through.
The mechanism functions as a pinhole camera. In this setup, the hole acts as the aperture, limiting the light rays that can pass through from any single point on the sun to a single point on the wall [1]. Because the sun itself is a sphere, the projected image reflects that circular geometry.
This projection occurs during daylight hours when the sun is visible and can be aligned with the cardboard cutout [1]. The geometry of the pinhole camera ensures that the shape of the aperture does not dictate the shape of the projected image, as only the shape of the light source does.
The result is a direct visual representation of the sun's disk. By restricting the light, the cardboard transforms a wide beam of sunlight into a focused image, effectively recreating the basic function of a lensless camera [1].
“The hole acts like a pinhole camera, forming an image of the sun.”
This optical phenomenon illustrates the concept of rectilinear propagation of light. By showing that the aperture's shape does not influence the resulting image, it highlights that the projection is a map of the light source itself rather than a shadow of the hole. This serves as a foundational example of how pinhole photography works without the need for glass lenses.
