There are varying theories attempting to explain optics in the Flat Earth Model. The most common are the electromagnetic accelerator, limitations of perspective and a combination of refraction and optical density of the atmolayer. In some models, more than one of these explanations are found. At the moment, they are somewhat incomplete, and several flat-earthers are working to improve or replace them.
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The Electromagnetic Accelerator Theory calls for light to be "bent" upwards as it travels towards the earth. The path of light is a parabolic arc. It is commonly abbreviated to EA.
Horizon Limits with Refraction and Opacity
Horizon limits are easily explained by the fact that air is not transparent and refraction. As light travels through a denser medium, the object will appear to be smaller because light is refracted towards the normal. Furthermore, air is not transparent so it is not possible to see past a certain distance.
Horizon Limits with Electromagnetic Accelerator
Light from objects too far away either hits the ground or is bent upwards before it reaches us. This also explains the "sinking ship" effect: the bottom portion of the ship appears to sink into the ocean because all of the light either hits the ocean or is bent upwards, but light from the top portion will be able to go further down before being bent upwards and becoming visible to us, since the ocean is lower relative to it. Also, this allows for an extremely high Greater Ice Wall to keep the atmosphere in, while remaining invisible.
-Robosteve, father of EA
Horizon Limits with Perspective
Proponents of objects disappearing on the horizon due to perspective allege that light travels in straight lines and that perspective naturally creates the effect that portions of objects become indistinguishable to the eye due to great distance.
Day/Night with Electromagnetic Accelerator
When the sun is too far away rays are bent in a parabolic arc before they reach earth, resulting in night time.