The camera obscura is a black box with a curved lens or aperture that projects the image of an external object to a screen/wall inside. The name 'camera obscura' comes from the Latin words meaning 'darkened room'. The first record of the camera obscura technique goes back to Ancient Greece, when Aristotle noticed how light passing through a small hole into a darkened room produces an image on the wall.
The Camera Obscura is an ancient optical device. In its most basic form it is, quite simply, a dark room with a small hole in one wall. On the wall opposite the hole, an image is formed of whatever is outside. This image is upside-down (inverted) and back to front (laterally transposed). The size of the hole has a great effect on the picture that is being projected. A small hole produces a sharp image, which is dim, while a larger hole produces a brighter picture which is less well focused. This happens because light travels in straight lines, a property known as the rectilinear propagation of light.
This image is showing how the image outside is being projected on the inside. This is done through being in a darkened room and the scene being projected through a tiny hole. As you can see, the image on the inside is upside down. It is explained why below.