Possible cause one: modern photographic equipment and retroreflection.
Orbs are common with compact cameras, where the short distance between the lens and the built-in flash decreases the angle of light reflection to the lens so directly illuminating the aspect of any particles facing the lens and increasing the camera's ability to capture the light reflected off normally sub-visible particles.
In this manner retroreflection can create orbs when light from the flash returns to the lens with a limited scattering of light off a solid particle (e.g., dust, pollen), liquid particles (water droplets, especially rain) or other foreign material within the camera lens.
Such images usually appear as either white or semi-transparent circles, though they may also occur with whole or partial colour spectrums, purple fringing or other chromatic aberrations. With rain droplets, an image may capture light passing through the droplet creating a small rainbow effect.
Possible cause two: ghosts, spirits and life forms
Some ghost hunters believe orbs are ghosts in the form of balls of light or that they are the life forms now taken on by the human soul or life force of those that once inhabited a physical body here on earth. Such orbs are encountered with relative frequency and some psychics claim to be able to talk to them.
Some people believe these orbs to be spirits who have willingly decided not to move to the next level of existence because they feel bound to their previous life or location for some reason and can, consequently become similar to a psychotic human being. Some people also believe that the longer these spirits stay behind, the harder it is for them to find their way to the next level.
Possible cause three: visible energy transference
Another theory considers orbs to be energy transferring from a source (i.e. power lines, heat energy, batteries, people, etc) to the spirit so they can manifest. It is possible that this is simply a natural way for them to get their energy.
Why this shape?
Orbs are the most photographed anomalies caught by ghost hunters and can be quite photogenic; ranging from completely transparent to a bright solid form.
The ‘energy transference’ theory would, according to the laws of Physics, explain why these ‘spirits’ appear as round balls; while in the process of transferring, energy would assume its natural shape of a sphere. This theory can also be tied into the EMF readings obtained during spirit activity.
Another theory is that ghosts prefer the form of an orb because this shape uses the least energy with small orbs requiring far less energy than apparitions and other fuller shapes. Some believe that in the colder parts of the year ghosts can find it easier to take on shapes other than orbs as this is when static electricity most commonly occurs in the atmosphere particularly in the months from October to February. It is also believed that they are able to draw on our own energy when needed.
But how do we explain…?
Orbs appear in photographs taken in grave yards, alleged haunted locations and before important life changing events. While some people believe these orbs to be spirits and paranormal energies, some skeptics believe them to be caused by insects, moisture, dust particles or malfunctioning camera diodes and while some ‘orbs’ can be accredited to natural phenomena, there are some that simply just cannot.
For example; photographs taken of the hands of healers showing orbs as the healer is projecting their spiritual powers, or orbs that can appear in photos around the heads of mediums when they are communicating with the dead. So what are the orbs that appear on photographs taken at spiritually significantly times? As of yet no one can say for certain so it is down to individual belief.