Key relationship
r₁+r₂=A; δ=i+e−A; δmin=2sin⁻¹(n sin(A/2))−A.
Physics · Class 12
Prism deviation and dispersion
Ray Optics & Optical Instruments
Two refractions turn the ray through a prism. Symmetric passage gives minimum deviation. Different refractive indices for different colours create dispersion, with a higher index producing a larger minimum deviation.
Open the interactive experiment →Prism deviation and dispersion
பட்டகத்தில் இருமுறை விலகுவதால் கதிர் திரும்பும். சமச்சீர் பாதையில் விலகல் சிறியது; நிறங்களின் ஒளிவிலகல் எண்கள் வேறுபடுவதால் பிரிகின்றன.
Key notes, relationships and model limits
Remember
Two refractions turn the ray through a prism. Symmetric passage gives minimum deviation. Different refractive indices for different colours create dispersion, with a higher index producing a larger minimum deviation.
Common trap
Minimum deviation belongs to a particular incidence, not every ray through a prism.
Model limits
Air-prism-air; homogeneous reference n with an imposed violet/red difference. Dispersion readout compares each colour at its own minimum-deviation angle. TIR exits are omitted from the graph.
What you will learn
Trace prism deviation and distinguish its wavelength dependence.
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
For A=60°,n=1.5, δmin≈37.18°.
Why this matters
The model connects prism deviation and dispersion with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
At minimum deviation, the internal refraction angles are?
- Zero
- Both A/2
- Both A
Read the explanation
Symmetry gives r₁=r₂ and their sum equals A.
Course connection: Class XII Chapter 9. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.