Refractive index – determination and applications MCQs With Answer is an essential resource for B. Pharm students studying physicochemical properties of drugs. This introduction explores the refractive index (n), its measurement using Abbe and digital refractometers, dependence on wavelength (dispersion) and temperature, and important pharma applications such as concentration analysis, solvent identification, purity testing, and polymer/drug formulation characterization. Understanding concepts like Snell’s law, critical angle, molar refraction and the relationship between refractive index and polarizability helps in quality control and formulation development. These focused, exam-oriented MCQs reinforce theory, instrumentation, calculations and real-world pharmaceutical uses. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the refractive index of a medium?
- The ratio of the speed of light in vacuum to its speed in the medium
- The speed of light in the medium
- The angle between incident and refracted rays
- The wavelength of light in the medium
Correct Answer: The ratio of the speed of light in vacuum to its speed in the medium
Q2. Which instrument is commonly used in laboratories to measure refractive index for liquids?
- UV-Vis spectrophotometer
- Abbe refractometer
- Mass spectrometer
- Polarimeter
Correct Answer: Abbe refractometer
Q3. The refractive index is typically reported at which standard wavelength for pharma measurements?
- 488 nm (blue)
- 365 nm (UV)
- 589 nm (sodium D-line)
- 632.8 nm (He-Ne laser)
Correct Answer: 589 nm (sodium D-line)
Q4. Which law describes the relationship between angles of incidence and refraction?
- Beer’s law
- Snell’s law
- Kirchhoff’s law
- Le Chatelier’s principle
Correct Answer: Snell’s law
Q5. Refractive index is a dimensionless quantity because it is a ratio of:
- Two wavelengths
- Two speeds
- Two angles
- Two frequencies
Correct Answer: Two speeds
Q6. Which factor causes dispersion in refractive index?
- Temperature only
- Wavelength dependence of light
- Pressure only
- Concentration only
Correct Answer: Wavelength dependence of light
Q7. Which equation approximates the relationship between refractive index and dielectric constant for non-magnetic materials?
- n ≈ √ε_r
- n = ε_r × μ_r
- n = c/λ
- n = 1/ε_r
Correct Answer: n ≈ √ε_r
Q8. In quality control, refractive index measurements are often used to:
- Determine molecular weight directly
- Estimate concentration and detect impurities
- Measure pH of solutions
- Assess thermal stability
Correct Answer: Estimate concentration and detect impurities
Q9. Which refractometer feature minimizes human reading error?
- Manual prism adjustment
- Digital readout
- Glass scale
- Analog dial
Correct Answer: Digital readout
Q10. The critical angle for total internal reflection occurs when light travels from:
- A less dense to a more dense medium
- A more dense to a less dense medium
- Vacuum to glass
- Air to vacuum
Correct Answer: A more dense to a less dense medium
Q11. The Abbe refractometer determines refractive index by measuring:
- Absorbance at a wavelength
- Critical angle or minimum deviation using a prism
- Mass of a sample
- Optical rotation
Correct Answer: Critical angle or minimum deviation using a prism
Q12. Which expression defines molar refraction (R)?
- R = (n^2 – 1)/(n^2 + 2) × (M/ρ)
- R = n × M × ρ
- R = c/λ × M
- R = ε × ρ
Correct Answer: R = (n^2 – 1)/(n^2 + 2) × (M/ρ)
Q13. Gladstone–Dale relation links refractive index of a mixture to:
- Sum of refractive indices of pure components
- Weighted sum of refractive contributions of components
- Product of component densities
- Logarithm of concentrations
Correct Answer: Weighted sum of refractive contributions of components
Q14. A decrease in temperature typically causes refractive index of a liquid to:
- Increase
- Decrease
- Remain constant
- Change unpredictably
Correct Answer: Increase
Q15. Refractive index measurements can help detect adulteration in pharmaceutical excipients by:
- Measuring thermal conductivity
- Comparing measured n with reference values
- Analyzing magnetic susceptibility
- Checking electrical resistance
Correct Answer: Comparing measured n with reference values
Q16. The term nD refers to refractive index measured at:
- The D-line of sodium (589 nm)
- Deep UV region
- Infrared region
- Green light (532 nm)
Correct Answer: The D-line of sodium (589 nm)
Q17. Which relation approximates how refractive index changes with concentration for dilute solutions?
- Linear dependence: n = n0 + k·c
- Exponential: n = n0·e^{kc}
- Inverse: n = n0/c
- Quadratic only
Correct Answer: Linear dependence: n = n0 + k·c
Q18. Which concept explains the microscopic origin of refractive index in materials?
- Electron polarizability interacting with the electromagnetic field
- Nuclear decay
- Gravitational attraction between atoms
- Ionic conductivity
Correct Answer: Electron polarizability interacting with the electromagnetic field
Q19. Lorentz-Lorenz equation relates refractive index to:
- Viscosity
- Molar refractivity and density
- Melting point
- Thermal expansion coefficient
Correct Answer: Molar refractivity and density
Q20. For transparent polymers in formulations, refractive index matching is used to:
- Increase melting point
- Reduce scattering and improve clarity
- Change chemical stability
- Modify osmotic pressure
Correct Answer: Reduce scattering and improve clarity
Q21. Which measurement is directly used to determine the critical angle at an interface?
- Refractive index difference only
- Angle of total internal reflection when transmitted ray grazes the surface
- Absorbance at 280 nm
- Optical rotation of polarized light
Correct Answer: Angle of total internal reflection when transmitted ray grazes the surface
Q22. Digital refractometers often correct readings for:
- Magnetic field
- Temperature
- pH
- Humidity
Correct Answer: Temperature
Q23. Which of the following is NOT an application of refractive index in pharmaceutical analysis?
- Concentration estimation
- Identification of solvents
- Determination of optical isomerism (chirality)
- Purity assessment
Correct Answer: Determination of optical isomerism (chirality)
Q24. A refractometer measuring Brix is primarily used to estimate:
- Viscosity of solutions
- Sugar concentration (or soluble solids) in solutions
- pH of the sample
- Protein content
Correct Answer: Sugar concentration (or soluble solids) in solutions
Q25. Which property of a liquid sample can influence its refractive index the most?
- Concentration of solute
- Color
- Electrode potential
- Magnetic susceptibility
Correct Answer: Concentration of solute
Q26. In a prism refractometer, the principle of measurement is based on:
- Photoelectric effect
- Refraction and total internal reflection at the prism-sample interface
- Diffraction of light through a grating
- Thermal emission
Correct Answer: Refraction and total internal reflection at the prism-sample interface
Q27. Refractive index can be used to estimate molar mass when combined with which technique?
- NMR spectroscopy
- Light scattering (e.g., Rayleigh scattering) and refractive index increment (dn/dc)
- IR spectroscopy alone
- pH titration
Correct Answer: Light scattering (e.g., Rayleigh scattering) and refractive index increment (dn/dc)
Q28. The refractive index of air is approximately:
- 0.5
- 1.0003
- 2.0
- 1.5
Correct Answer: 1.0003
Q29. Which of the following will NOT change the refractive index of a pure liquid significantly?
- Small temperature change
- Adding a solute
- Changing wavelength measured (dispersion)
- Changing the container material
Correct Answer: Changing the container material
Q30. Specific refractivity is useful because it is:
- Dependent only on temperature
- An additive property related to molecular composition
- A measure of ionic strength
- Equal to refractive index squared
Correct Answer: An additive property related to molecular composition
Q31. Which reference value is essential when using refractive index for concentration determination?
- Known relation between n and concentration (calibration curve)
- Boiling point of solvent
- Magnetic susceptibility of solute
- Viscosity of the solution
Correct Answer: Known relation between n and concentration (calibration curve)
Q32. In refractometry, a sample with suspended particles will typically cause:
- Lower reading due to dilution only
- Scattering and unreliable refractive index readings
- No change in measurement
- Higher thermal stability
Correct Answer: Scattering and unreliable refractive index readings
Q33. Which mathematical relation describes Snell’s law?
- n1 sinθ1 = n2 sinθ2
- n1 cosθ1 = n2 cosθ2
- n1 + n2 = constant
- n1 / n2 = θ1 / θ2
Correct Answer: n1 sinθ1 = n2 sinθ2
Q34. For highly absorbing samples in the visible range, refractive index measurement is:
- Unaffected
- Challenging and possibly inaccurate due to absorption
- More accurate than for transparent samples
- Independent of wavelength
Correct Answer: Challenging and possibly inaccurate due to absorption
Q35. The refractive index of water at 20°C (nD) is approximately:
- 1.00
- 1.333
- 1.5
- 2.0
Correct Answer: 1.333
Q36. Which parameter is often reported along with refractive index to fully describe the optical behavior?
- Surface tension
- Dispersion or Abbe number
- Electrical conductivity
- Melting point
Correct Answer: Dispersion or Abbe number
Q37. In pharmaceutical formulations, refractive index measurements can monitor:
- Evaporation and concentration changes during processing
- Radioactivity
- Bacterial contamination directly
- Viscosity only
Correct Answer: Evaporation and concentration changes during processing
Q38. Which term describes change in refractive index with wavelength?
- Chromatography
- Dispersion
- Diffraction
- Reflection coefficient
Correct Answer: Dispersion
Q39. The refractive index increment (dn/dc) is important in polymer analysis because it:
- Relates refractive index to concentration for light scattering molecular weight determination
- Measures thermal expansion
- Indicates pH sensitivity
- Quantifies color changes
Correct Answer: Relates refractive index to concentration for light scattering molecular weight determination
Q40. Which calibration standard is commonly used for refractometers?
- Pure ethanol at any temperature
- Distilled water at a defined temperature
- Table salt solution
- Glycerin without reference
Correct Answer: Distilled water at a defined temperature
Q41. In emulsions, refractive index mismatch between phases causes:
- Increased clarity
- Scattering and opacity
- Reduced density
- Change in pH
Correct Answer: Scattering and opacity
Q42. Which phenomenon is used by total internal reflection refractometers?
- Transmission maximization
- Total internal reflection at angles beyond the critical angle
- Absorption peak shifts
- Fluorescence emission
Correct Answer: Total internal reflection at angles beyond the critical angle
Q43. Molar refractivity gives insight into which molecular property?
- Polarizability of molecules
- Number of stereocenters
- Boiling point only
- Magnetic moment
Correct Answer: Polarizability of molecules
Q44. When reporting refractive index for official specifications, it is important to include:
- Measurement temperature and wavelength
- Only the instrument brand
- Only the sample color
- Storage humidity
Correct Answer: Measurement temperature and wavelength
Q45. The presence of dissolved salts in water generally causes the refractive index to:
- Decrease significantly
- Increase
- Remain exactly constant
- Become negative
Correct Answer: Increase
Q46. Which optical constant is closely related to refractive index for non-magnetic materials?
- Dielectric constant (permittivity)
- Magnetic permeability
- Specific heat
- Viscosity
Correct Answer: Dielectric constant (permittivity)
Q47. In dissolution testing, refractive index can be used to:
- Directly measure osmolarity
- Monitor concentration changes of dissolved drug if optically appropriate
- Replace HPLC in all cases
- Measure tablet hardness
Correct Answer: Monitor concentration changes of dissolved drug if optically appropriate
Q48. Which sample condition is essential for accurate refractive index measurement?
- Presence of bubbles and particulates
- Homogeneous and particle-free sample
- Highly colored and opaque solution
- Very high turbidity
Correct Answer: Homogeneous and particle-free sample
Q49. The relationship between refractive index n and molecular polarizability α for gases at low density can be given by:
- Clausius-Mossotti or Lorentz-Lorenz type relation
- Beer–Lambert law
- Raoult’s law
- Henderson–Hasselbalch equation
Correct Answer: Clausius-Mossotti or Lorentz-Lorenz type relation
Q50. Why is refractive index considered a useful physico-chemical parameter in pharmaceutical compendia?
- Because it directly measures potency
- Because it is easy to measure, reproducible and indicative of purity/composition
- Because it measures microbial contamination
- Because it replaces all chromatography
Correct Answer: Because it is easy to measure, reproducible and indicative of purity/composition
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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