The image formed by a pinhole camera is real. Which of the following…

Rectilinear propagation of light: in a uniform transparent medium, light travels along straight-line paths (rays) unless it is reflected, refracted, diffracted, or scattered. A real image is one formed where light rays actually arrive after physically travelling from the object, so it can be captured and projected on a screen -- unlike a virtual image, where the rays only appear to come from a point (when traced backward) but never actually reach it, so it cannot be thrown on a screen.

In a pinhole camera, light leaving every point of the object travels outward in straight lines in every direction, but the tiny pinhole lets through only the one ray path from each point that happens to be aligned with it. A ray leaving the top of the object crosses through this single opening and then continues in a straight line to physically strike the lower part of the screen, while a ray leaving the bottom of the object crosses through and continues on to physically strike the upper part of the screen -- the crossing at the pinhole is what flips the picture upside down. Because each point of light on the screen is reached by a ray that has genuinely travelled all the way there, rather than merely appearing to originate from that spot, the picture formed is a real (and inverted) image of the object.

• Diffraction (the spreading of light around an edge or small aperture) only becomes noticeable when the aperture size approaches the wavelength of light; for an everyday pinhole this spreading is negligible and, in any case, it is a blurring/washing-out effect, not one that would place light on the screen at all, so it cannot be the reason a sharp image forms there.

• That light is a form of energy is a true general property, but it is a fact about every kind of light in every optical device, so it cannot be what singles out the pinhole camera's image as real rather than virtual or absent.

• Light changing path between media describes refraction, which needs a boundary between two different optical media (e.g. air to glass); the pinhole camera involves no such change of medium at the aperture, so refraction plays no part in forming its image.

So the image is real specifically because light rays travel in straight lines, cross at the pinhole, and physically reach the screen -- producing a real, inverted image.