Floaters and Flashes

According to EYEWiki:

Floaters

Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent. At a young age the vitreous is perfectly transparent but during life, imperfections gradually develop. The common type of floater, which is present in most people's eyes, is due to degenerative changes of the vitreous humour. The perception of floaters is known as myodesopsia, or less commonly as myiodeopsia, myiodesopsia, or myodeopsia. They are also called Muscae volitantes (from the Latin, meaning "flying flies"), or mouches volantes (from the French). Floaters are visible because of the shadows they cast on the retina or their refraction of the light that passes through them, and can appear alone or together with several others in one's field of vision. They may appear as spots, threads, or fragments of cobwebs, which float slowly before the observer's eyes. Since these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena.

More information located at the link below:

Floaters

According to EYEWiki:

Flashes

Photopsia (Flashes) is the presence of perceived flashes of light. It is most commonly associated with posterior vitreous detachment, migraine with aura, migraine aura without headache, retinal break or detachment, occipital lobe infarction and sensory deprivation (ophthalmopathic hallucinations). Vitreous shrinkage or liquifaction, which are the most common causes of photopsia, cause a pull in vitreoretinal attachments, irritating the retina and causing it to discharge electrical impulses. These impulses are interpreted by brain as 'flashes'.

This condition has also been identified as a common initial symptom of Punctate Inner Choroiditis (PIC), a rare retinal autoimmune disease believed to be caused by the immune system mistakenly attacking and destroying the retina.

Photopsia can present as retinal detachment when examined by an optometrist or ophthalmologist. However, it can also be a sign of ocular melanoma. This condition is extremely rare (one in 5 million people will be affected, typically fair-skinned, blue-eyed northern Europeans) but if left untreated, will migrate to the liver and shorten life expectancy. Photopsia should be investigated immediately.

Another cause can be nervous spasms.

More information located at the link below:

Flashes