Orbs...

Source: http://www.ourcuriousworld.com/TypesofGhosts.htm

An orb is basically a ball of light that appears unexpectedly in a photo. Normally white or bluish in color, one or more may appear on an otherwise normal photograph, and are occasionally captured on videotape as well (in which case they are usually moving, often at a high rate of speed).

They are hypothesized by some researchers to be basic manifestations of spiritual energy in its simplest and most primitive form. In other words, it is thought by some that an orb is a ghost that is in the most early stage of manifesting, its essence being contained within a tiny globe of pure energy like air inside a bubble.

Many researchers, however, reject this notion.

Even some 'ghost hunters' do not accept the 'orb as energy' theory, dismissing the little balls of light as lens flare or merely reflections of light off airborne particles such as dust, snow, drizzle, or even insects that have wandered in front of the camera just as the flash goes off.

They correctly point out that many times pictures are taken in dusty, dark rooms where the smallest particles of airborne material can reflect the flash of a camera to stand out in stark contrast to the darkness of the background.

In the hands of inexperienced or overly enthusiastic ghost hunters, then, every blob of light on a photo thus becomes a 'ghost'-much to the chagrin of their more careful and somber colleagues.

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Image source: http://www.bostontours.us
Source: http://www.ourcuriousworld.com/TypesofGhosts.htm