Reading glasses are used instead of distance glasses or no glasses when reading. Objects far away will be somewhat blurred but objects close by, such as a book, will be in focus.
Although bifocal glasses, PALs (progressive addition lenses) and the newer peripheral add lenses (such as MyoVision) have been studied, very little is published specifically on reading glasses, most likely because it is a less used option due to the necessity of having two pairs of glasses if clear distant vision is desired for the person already myopic. This additional complication in lifestyle (wear one pair, carry one pair, switch pairs as needed) and additional expense over one pair of glasses means that it is not often prescribed.
Perhaps the most likely circumstance for such a prescription is when the person is not yet myopic but the exam findings indicate problems focusing accurately and comfortably for reading. In these circumstances reading glasses may be prescribed for reading but the person is instructed to remove them for other activities.
If one were to guess how they work, current research would indicate that they probably don't. Based on what is known about peripheral focus effects on myopic progression, reading glasses will have very little effect. The reason would be that while wearing the glasses, the person would create a clear central focus so that they could read. This creates a peripheral hyperopia in many eyes, which would create more myopia, just as if the glasses were not being worn. The peripheral hyperopia would be there with or without the reading glasses. Again, no studies to date so both sides of the issue are still discussed.
Proponents of reading glasses state they should work because they relax the eye while reading, the assumption being that reading by itself causes myopia. Many studies are showing this not to be true - it is not by any means a given that reading causes myopia and in fact many studies are pointing out that just activating the focusing muscles does not cause myopia. See "Has near work's star fallen?"
The greatest risk is that it won't work for myopia control. Since there is little direct data to suggest a course of action, the rationale for use of reading glasses to reduce myopic progression is mostly "it sounds like a good idea", not the best rationale upon which to base a course of treatment.
Distance vision (across the room, out the window) is blurred with reading glasses.
Reading glasses have what are called single vision lenses, meaning that the entire lens has the same power, as opposed to lenses such as bifocals that have two or more focus distances. The lens power is more "plus" power than the person's distance prescription, so that distance vision is blurred and reading vision is in focus with less effort required of the eye's focusing system.
Reading glasses are normally used for the condition of presbyopia, which is what happens to everyone, usually in their forties, when it becomes difficult to focus on both distant objects such as street signs and then shift focus to near objects such as a book. The reading glasses have a power that shifts the focus so that these near objects come into focus.