This remarkable feat is accomplished through photochromic technology, and even better is the fact that these special lenses are compatible with a wide range of prescriptions, including bifocals and trifocal recommendations.
If you didn't have bifocals, you'd have to take off one pair of glasses to replace them with a pair of readers every time you needed to shift from viewing something at a distance to looking at something up close.
If you have to wear eyeglasses for reading signs, watching television, driving, or other activities where the ability to see at a distance is required, you'll thank your lucky stars for your bifocals.
Since bifocal lenses often cost a bit more than single vision ones, bifocals that stick onto regular lenses are a good alternative for those who can't afford to spring for the pricier pair of lenses.
Keep in mind that if you're currently wearing no line bifocals, you may not enjoy wearing the stick on variety, as there is a visible line and the shift from distance to near vision is more drastic.