Is pinhole camera photography a refraction of light?

Shadows, images and light spots are important facts that people explain the linearity of light. The three have the same source and evolved into different optical pictures through different controls.

We know that if the light beam encounters an opaque shelter in the process of going straight, it will form a shadow. Shadows can be observed by contrast between light and shade. The formation of light spots and shadows form displacement symmetry. As shown in figure 1, s is a point light source. If the AB part in front of S is opaque, a cone-shaped dark area will be formed in the back, because light cannot reach it. If only AB is the light-transmitting window in front of S, a cone-shaped bright area will be formed in the back, and a large enough light curtain will pass through the cone-shaped bright area. The former casts a shadow on the screen, and the latter forms a light spot, just like the anti-black and white of photos and negatives. Shadow and light spot are just the opposite. The appearance of both depends on the local contrast between light and shade.

After reflection or refraction, an image can be obtained as long as the single-center beam-divergent beam or convergent beam is maintained. If the light beam emitted from each point on the luminous object becomes a convergent beam under the action of optical instruments, the convergent point is the real image of the object point; If a divergent beam is formed by the action of an optical device, the intersection point of the beam after the reverse extension is the virtual image of the object point. Corresponding to an object point in a certain position, the image point position of an ideal image is unique, and the set of all image points constitutes the image of a luminous object. When the screen is placed in the imaging position of the converging beam group, a clear real image can be drawn, and the shape of the image depends on the object. At other positions of the beam group, the pattern with unclear outline will appear on the screen, which is the light spot, and its shape depends on the shape, boundary and overlap of the beam group. It can be seen that there is a qualitative mutation from the spot to the real image.

Now let's look at the control effect of small holes on light. As shown in Figure 2, a rectangular window is opened on the vertical wall facing the sun in the darkroom. When sunlight enters the window, a rectangular bright area appears on the opposite wall, which looks like a window. This is our common scene. If the window size is gradually reduced, the bright area on the wall will be blurred and lose its edges and corners. When the window is small and becomes a square hole, a round bright block will appear on the wall-similar to the sun, that is, pinhole imaging, just like sunlight scattering bright round spots on the ground through thick shade. Why does the same sun create different shapes of graphics? Imagine a light spot on the surface of the sun. The light from this spot to the window enters the darkroom and becomes a prism beam, so it becomes a rectangular spot on the wall. The light emitted from various points on the surface of the sun will form a prism beam and a rectangular spot on the wall after entering the darkroom through the window. Because the distance between the wall and the window is not large relative to the size of the window, and the sunlight is almost parallel, so these countless rectangular spots overlap together and overlap very closely, and the result is a window-like bright spot. However, when sunlight passes through the small holes, the light from the above points on the sun enters the darkroom through the small holes, and the prism beam is extremely small, causing tiny rectangular spots on the wall. Although these small bright spots overlap each other, they have enough resolution, and each one corresponds to every point on the sun. The overall effect is to outline the shape of the sun. It can be seen that the "image" of the pinhole is essentially a collection of small light spots formed by the light emitted by each point of an object filtered by the pinhole and projected onto the screen, and its shape depends on the luminous object. With the help of dark background, we can see a clear "light map" of similar objects, which can be located at a considerable distance behind the small hole.

According to the symmetrical relationship between light spot and shadow, if the light emitted by an object passes through a small hole after being partially blocked, a local dark area will appear on the screen with the help of contrast between light and shade, forming an "image". In fact, this is the pattern of the luminous part of the object outlined by the collection of small spots.

To sum up, the light emitted by each point of the object only changes due to the action of small holes, but it does not have the condition of forming an image-a single-center beam. The image point is unique, and the optical path between the object and the image point is equal, so the pinhole becomes an "image" only in the sense of its image with the original object, and is essentially a light spot.

Aperture imaging proves that light travels in a straight line.