The over head projector is the most used in all a.v. aids. It projects transparencies with brilliant screen images suitable for use in a lighted room. The teacher can write or draw diagrams on the transparency while he teaches; these are projected simultaneously on the screen by the OHP.
During presentation:
§   Keep the screen above the heads of the participants.
§   Keep the screen in full view of participants
§   Make sure you are not blocking any ones view when presenting.
§   Darken the room appropriately by blocking out sunshine and dimming near by.
§   Turn the screen off between slides if you are going to talk for more than two.
§   Talk to the audience, not to the screen
§  To develop concepts and sequences in a subject matter area.
§  To make marginal notes on the transparencies for the use of the teacher that can carry with out exposing them to the class.
§  To test students performances, while other classmates observe.
§  To show relationships by means of transparent overlays in contrasting color.
§  To give the illusion of motion in the transparency.

§  It permits the teacher to stand in front of the  class while using the projector, thus enabling her to point out features appearing on the  screen by pointing to the materials at the  projector it self  and at the same  time, to observe the students reactions to her discussion.
§  Gains attention of the student
                           Transparencies are popular instructional medium. They are simple to prepare and easy to prepare and easy to operate with the over head projector which is light weight.
A 10*10 inches sheet with printed, written or drawn material is placed on the platform of the projector and a large image is projected on a screen behind you.
The projector is used from near to the front of the room with the teacher standing or sitting beside, facing the student.
Guidelines for making effective transparencies:
·         Have one main idea an each transparency.
·         Include only related figures and diagrams.
·         Use simple lettering style in writing.
·         Use diagrams in proposition to its lettering.
·         Keep the message clear and simple.
·         Emphasize the key messages.
·         Use color and lettering with discretion.
·         Permits face to face interaction with the students.
·         Can be used in daylight conditions.
·         Can present information in systemic developmental sequences.
·         Requires limited planning and can be prepared in variety of inexpensive methods.
·         Easily available.
                              Opaque projector is the only projector on which you can project a variety of materials ex: - book pages, objects, coins, postcards, or any other similar flat material that is non-transparent.
                               The opaque projector will project and simultaneously enlarge, directly from the originals, printed matter, all kinds of written or pictorial matter in any sequence derived by the teacher. It requires a dark room, as projector is large and not reality movables.
·         Stimulates attention and arouses interest.
·         Can project a wide range of materials like stamps, coins, specimen, when one copy is available.
·         Can be used for enlarging drawings, pictures and maps.
·         Does not require any written or typed materials, hand-written material can be used.
·         Helps students to retain knowledge for longer period.
·         Review instructional problems.
·         Test knowledge and ability.
·         Simple operation.
·         Costly equipment.
·         Needs to use it with care.
·         Needs a dark room for projectio
                                           A slide is a small piece of transparent material on which a single pictorial image or scene or graphic image has been photographed or reproduced otherwise.
                                        Slides are a form of projected media that are easy to prepare. They are still pictures on positive film which you can process and mount individually yourself or send to a film laboratory. The standard size of the slides is 2 “X 2 “any 35mm camera will make satisfactory slides.
Types of slides
1.      Photographic slides: 2” X 2”
3” X 4”
a)      Black and white
b)      Colored  
     2. Hand made slides: can be made with
a)      Acetate sheet
b)      Cellophane
c)      Etched glass
d)      Plain glass
e)      Lumarith

                                    Slides can be made from photographs and pictures by teachers and pupils taking photographs and snapshots when they go on fieldtrips for historical, geographical, literacy or scientific excursions.
The arrangement of slides in proper sequence, according to the topic discussed, is an important aspect of teaching with them.
1.      Requires only filming, processing and mounting by self or laboratory.
2.      Results in colorful, realistic, reproduction original subject.
3.      Preparation with any 35mm camera for most uses.
4.      Easy to revise and up-date.
5.      Easily handled, stored and re-arranged for various uses.
6.      Can be combined with tape narration or can control time for discussion.
7.      May be adapted to group or individual use

                                             Film strips are sequence of transparent still pictures with individual frames on 35mm film. A tap recorded narration can be synchronized with film strip.
Each strip contains from 12 to 18 or more pictures. It is a fixed sequence of related stills on a roll of 35mm film or 8mm film.
1.      Preview filmstrips before using them and selected carefully to meet the needs of the topic to be taught.
2.      Show again any part of the filmstrip needing more specific study.
3.      Use filmstrip to stimulate emotions, build attitudes and to point up problems.
4.      It should be introduced appropriately and its relationship to the topic of the study brought out.
5.      Use a pointer to direct attention, to specific details on the screen.
Types of filmstrip:
1)      Discussion filmstrip: it is continuous strip of film consisting of individual frames arranged in sequence usually with explanatory titles.
2)      Sound slide film: it is similar to filmstrip but instead of explanatory titles or spoken discussion recorded explanation is audible, which is synchronized with the pictures.
1)      Are compact, easily handled and always in proper sequence.
2)      Can be supplemented with recordings.
3)      Are inexpensive when quantity reproduction is required.
4)      Are useful for group or individual study at projection rate are controlled by instructor or user.
5)      Are projected with simple light weight equipment.