Key relationship
I=I₁+I₂+2γ√(I₁I₂)cosφ; visibility=2γ√(I₁I₂)/(I₁+I₂).
Physics · Class 12
Coherence and visibility
A stable phase relation preserves interference over time. Random relative phase washes out the cross term in the averaged intensity. Even perfectly coherent unequal sources do not reach complete darkness.
Open the interactive experiment →Coherence and visibility
நிலையான சார்பு கட்டம் குறுக்கீட்டைத் தொடரச் செய்கிறது. சீரற்ற கட்டத்தின் காலச் சராசரி குறுக்கீட்டு கூறை நீக்கும்.
Key notes, relationships and model limits
Remember
A stable phase relation preserves interference over time. Random relative phase washes out the cross term in the averaged intensity. Even perfectly coherent unequal sources do not reach complete darkness.
Common trap
Coherent sources need not have equal intensities, but unequal intensities reduce contrast.
Model limits
γ is a prescribed time-averaged degree of coherence. γ=0 represents decorrelated phase over measurement time; no stochastic temporal process is claimed.
What you will learn
Relate coherence and amplitude balance to visibility.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
I₁=I₂=1 and γ=1 ranges from0 to4; γ=0 stays at2.
Why this matters
The model connects coherence and visibility with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
Equal sources with γ=0 give time-averaged intensity?
- Zero
- I₁+I₂
- Four times I₁+I₂
Read the explanation
The coherence term vanishes.
Course connection: Class XII Chapter 10. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.