Features:

1. Pinhole projection gives an point inverted image of a source.

The Image 1 shows exactly how point inversion works better than words using light rays and ray tracing.

Point inversion can also be described as a 180° rotation from source to image.

2. Point inversion of an image also happens in the Keplerian telescope for similar reasons.

3. A simple pinhole projector can be made just with a sheet of cardboard with a hole punched in it and some surface to project the image on.

4. The image with pinhole projection is better focussed (i.e., sharper) the smaller the hole, but less bright---there is a trade-off.

5. In the classroom, a darkened room and single bright light source can be used for pinhole projection experiments. Ask students to put pinholes in sheets of paper and ask them to find the pinhole projection image of the bright light source.

6. If it is a bright clear day, try pinhole projection for yourself. Just use a pinhole in a sheet of paper held close to the ground.

Can you see sunspots? Probably NOT.

Incidentally, can you see narrow dark and bright fringes just near the edges of shadows of an object (e.g., of a pencil)? You need to look really closely. A magnifying glass might help. The fringes are the diffraction patterns set up the object.

7. You can also try using a hand mirror to reflect the Sun's image onto a piece of paper. Cover all but about a dime's worth of the mirror to create a pinhead mirror (FMW-79).

8. Pinhole projection videos (i.e., Pinhole projection videos):
1. Eclipse pin-hole effect | 0:29: An annular solar eclipse observed by pinhole projection with sunlight filtering through a canopy of leaves. Good for classroom.
2. Fine Art Pinhole Photography by Vladimir Zivkovic | 3:13: Pretty, but NOT good for the classroom.

Detailed Explication:

1. Ideal pinhole projection holds in the limit of geometrical optics (i.e., zero zero diffraction) and an infinitesimal aperture. The aperture is the pinhole of pinhole projection.

2. In non-ideal cases (i.e., in reality), the pinhole will have finite size and a general shape.

The situation is that every point in the aperture acts as an infinitesimal aperture in its own right.

The (observed) image is the sum of the infinitesimal aperture images.

However, these do NOT exactly overlap, and so the image is somewhat fuzzy (i.e., lacking perfect sharpness).

As the aperture gets bigger, the image approaches just being the shape of the aperture which is why a rectangular window with sunlight streaming in creates a rectangular spot of illumination.

3. The degree of fuzziness can be estimated using the angular size θ of the source (as seen from the aperture) measured from the optical axis, the distance from aperture to the projection screen "d", and the size scale of the aperture "a".

The imperfectly overlapping aperture images are spread out by of order "a". The ratio of spreading to image size is of order

```          a
f = --------  ,
d*tan(θ)    ```

the fuzziness factor.

If f << 1 sufficiently, the image will look sharp to the human eye.

If f >∼ 1, then the image is totally fuzzed.

Just put the projection screen farther away from the aperture to increase sharpness.

4. Pinhole projection is a safe way to observe the Sun when you do NOT have a guaranteed-safe solar filter.

You may be able to observe sunspots with pinhole projection---but yours truly has always failed. It is probably marginal at best (see, e.g., Cloudy Nights Telescope Review).

Pinhole projection does work well for observing solar eclipses---especially when you are too lazy to do anything more elaborate.

5. Though people do NOT often notice this, highly multiple pinhole projections of the Sun often occur under a canopy of leaves. You see all these bright round light spots---all images of the Sun.

In fact, when the Sun images become crescents ☽, there is a partial solar eclipse. The crescents are sufficiently striking that people do notice them when they never notice the round light spots. In fact, since partial solar eclipses dim the sky similarly to thin cloud cover, maybe the only casual way to notice partial solar eclipses is by being spooked by the crescents.

6. Image 2 Caption: Image 2 shows multiple pinhole projections caused by a canopy of tree leaves during the partial solar eclipse that accompanied the annular solar eclipse of 2005 Oct03. Image 2 was taken at St. Julian's, Malta.