PRINCIPLE OF OPTICAL MEASURING
INSTRUMENTS
1. INTERFERENCE:
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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
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lec# 13 & 14
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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
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lec# 13 & 14
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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 ±5x10-10m
• Light emitted by certain elements eg. Mercury,
sodium & cadmium is suitable.
• In metrology this light is usually obtained from an
electrical discharge lamp
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lec# 13 & 14
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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
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lec# 13 & 14
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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
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lec# 13 & 14
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PRACTICAL # 03
Combination Sets can be used as:
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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
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lec# 13 & 14
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