Refractive index measurement by interference microscopy: corrections for dispersion

Area of Science:

• Optical physics
• Materials science
• Crystallography

Background:

• Refractive index measurement is crucial for characterizing particles.
• Microscopy techniques can determine refractive index via interference.
• White light illumination can introduce errors in refractive index measurements for dispersive materials.

Purpose of the Study:

• To investigate the phenomenon of apparent refractive index in particles with strong dispersion.
• To differentiate between apparent and true refractive indices using white light microscopy.
• To validate theoretical predictions relating apparent index to group velocity.

Main Methods:

• Utilized microscope equipment to measure interference patterns in transmitted light.
• Employed white light illumination for particles with path differences of several wavelengths.
• Calculated apparent refractive index based on observed interference and compared with theoretical models.

Main Results:

• Observed that strong dispersion under white light illumination leads to a significantly higher apparent refractive index than the true value.
• Demonstrated that the apparent refractive index correlates with the group velocity of light waves.
• Measured an apparent refractive index of 4.52 for flaky hematite crystals, aligning with theoretical expectations.

Conclusions:

• The apparent refractive index measured with white light is influenced by the material's dispersion and relates to group velocity.
• True refractive index measurements require careful consideration of illumination and material properties to avoid overestimation.
• The study confirms theoretical models and highlights the importance of distinguishing between apparent and true refractive indices in optical measurements.