Long years ago, you have to wear two sets of eye glasses, then doctors discovered the lens at the bifocal eye-glasses ,then recently they do the same thing in a bifocal contact lens. The lens is divided into two halves ,one upper and the other is lower and sometimes it can be in concentric circles ,one lens on the interior part and the other is on the outside part.

Who need bifocal lenses?
People who suffer from Presbyopia that causes trouble focusing on objects which are near and this doesn't develop after the age 40.They are nearsightedness and farsightedness and suffer from the eye-glasses that are usually with thick lenses and a line across them (they act as two lenses in one).These eye-glasses have several problems: First :They are heavy, Second: They are almost unattractive, Third: It is hard to adjust to them as you have to look up or look down .All these problems cause dizziness during adapting to these bifocal lenses .The only choice for these people for long years ago was the eye-glasses even after contact lenses came on the market.
Now ,contact lenses offer bifocal contact lenses which are soft ,rigid and with gas permeable materials .It is dual lens ,as if you are wearing two sets of glasses at the same time.
By using a set of bifocal contact lenses ,your eye begins gradually to adjust to the difference of portions as the muscles of the eye coordinate and be trained automatically from and to the correct lens that is needed at this time .Example: If someone is reading ,so the vision has to be corrected to view the close things at this time ,so the pupil of the eye will be directed to the lens that correct the vision and light will be refracted into the angle of the retina that will help in close reading .But if you begin to look off at things which are far away-as looking to the sky-the focus of the eye will be directed at this time to the lens that assist you to see the distant thing .All of this is due to the different prescription lens of the contacts that have the ability to refract the light at different angles and correct the vision due to different viewing.

Which is better? A bifocal lens in the traditional eye-glasses OR a bifocal contact lens?
You have to choose which type you prefer .As some people have great problems and troubles in wearing the contact lenses as they considered this difficult routine to put in and take out them ,even by using special kind that stay for all the day ,whole the week or even more than a week- they have to change them eventually ,so they consider wearing contact lens a big problem .These people can get some hope by the innovation of the manufacture and the designs of contact lenses as these contacts became much easier to maintain and very simple to manage.
Contact lenses can be very useful and fit more than a pair of traditional eye-glasses for those who work in a hazardous environment or who wear any kind of equipment like goggles .Conversely ,for people that work in certain toxic gaseous chemicals environment ,contact lenses won't suits them .So you have to take the advise from your eye care specialist to help you to take the correct decision and to choose the best option either to wear eye-glasses or contact lenses.

There are a lot of types of lenses ,like:

1-Alternative-Design:
They are the same to bifocal eye-glasses ,half the lens for near objects and half for distant objects.

2-Translating-Design:
They are also the same as bifocal eye-glasses ,the near correction is under the distance correction and there is a line separating them .It is flat on the bottom for the reason of keeping without movement in your eye while blinking.

3-Concentric-Design:
Inner part can see near objects while the outer part can see the distant objects or vice versa.

4-Mono-vision-Design:
You wear one lens in one eye and the other in the other eye .You wear to view the distant objects in the dominant eye.

5-Asferic-Design:
The near and the distant vision are located at the center of the lens .Usually the correction for near vision is at the center while the correction of the distant vision surrounds it and this can be reversed in some situations according to the advise of the eye care specialist.

6-Simultaneous-Design:
This kind has the ability to blend both the near sight and the distant sight ,they fill the area of the pupil (part of the eye) and the eyes can adjust to that and can interpret the circle choice according to how far or close you are viewing .This is because the light all the time from near and far objects can be focused on at the same time .These kinds of lenses may be problematic as by looking through the lens ,the eye takes both : in focus and out of focus at the same time .This is due to some light from the far objects go through the near vision and vice versa ,so the brain has to correct the desired image.

Finally ,when you decide to wear bifocal contact lenses ,you have to:
1-Ask your doctor to know if contact lenses can suits you or not and also which kind of them you can buy.
2-Ask your friends or relatives if any of them have the experience of wearing contact lenses.
3-Put in your consideration that it may take time to find contact lenses that can fit you and be comfortable.
4-Some companies can allow you to try their lenses firstly ,so by this offering you can have a free trial wear period to see if these lenses can adjust with your eyes or not.
5-You can check the discounted stores for contact lenses ,they are online stores.
6-You have to know that the bifocal lenses will depend on your budget.