1. PRINCIPLE OF OPTICAL MEASURING INSTRUMENTS 1.INTERFERENCE: When two wave trains of the same frequency & the same amplitude traveling in the same direction are superimposed on one another they produce interference If at any point the waves are in phase, there is an increase in resultant amplitude, i.e. a constructive interference. If the waves are out of phase & there is decrease in the resultant amplitude, i.e. destructive interference 18/2/20121lec# 13 & 14

2. PRINCIPLE OF OPTICAL MEASURING INSTRUMENTS 2. CONDITIONS FOR INTERFERENCE OF LIGHT WAVES: 1.The two sources should continuously emit waves of the same wave length or frequency 2.For obtaining interference fringes, the amplitude of the two interfering wave trains should equal or very nearly equal 3.The two sets of wave-trains from the two sources should either have the same phase or a constant in difference in phase 4.The two sources should be very narrow 18/2/20122lec# 13 & 14

3. PRINCIPLE OF OPTICAL MEASURING INSTRUMENTS 3. MONOCHROMATIC BEAM OF LIGHT: A Monochromatic beam of light gives light of only one wave length or one color. It is difficult to get a truly monochromatic source of light. However by using light filters it is possible to get a source that gives light within a narrow bend of wave lengths ±5x10 -10 m Light emitted by certain elements eg. Mercury, sodium & cadmium is suitable. In metrology this light is usually obtained from an electrical discharge lamp 18/2/20123lec# 13 & 14

4. PRINCIPLE OF OPTICAL MEASURING INSTRUMENTS Depending upon the optical setup, the interference bands can appear as parallel bands, or as concentric circles The interference phenomenon can be used to make the measurement of high precision by utilizing the fact that the path difference b/w two dark bands or two light bands is one wave length 18/2/20124lec# 13 & 14

5. PRACTICAL # 03 Object: To learn different uses of Combination set Apparatus: Combination set with 300mmblade, square head & centre head, sections for angle measurement Procedure: As Demonstrated Learning Outcome: After performing measurements with combination set we feel confidence to make its application independently 18/2/2012lec# 13 & 145

6. PRACTICAL # 03 Combination Sets can be used as: Outside try square Inside try square Mitre Square Sprit level 45 level Plumb level Depth gauge Height gauge Marking gauge Centre head use Protractor use 18/2/2012lec# 13 & 146