Question

A mineral displays magenta interference colour (retardation, \( \Delta m = 550 \, \text{nm} \)) under crossed polarized light. If an accessory plate adds retardation of 100 nm, the interference colour observed is:

• A. 1st order yellow
• B. 1st order green
• C. 2nd order blue
• D. 2nd order red

Hint:

When an accessory plate adds retardation to a mineral, the total retardation determines the order of the interference colour observed.

Answer 1

The Correct Option is C

Step 1: Understand Interference Colours.
Interference colours are observed when minerals are viewed under crossed polarized light, and they depend on the retardation of light passing through the mineral. The order of the interference colours corresponds to the retardation values:
- 1st order yellow corresponds to a retardation around 550 nm.
- 1st order green corresponds to slightly lower retardation values.
- 2nd order blue corresponds to a total retardation value around 650 nm.
Step 2: Calculate the Total Retardation.
The mineral displays magenta interference colour at \( \Delta m = 550 \, \text{nm} \). When an accessory plate with 100 nm of retardation is added, the total retardation becomes \( 550 \, \text{nm} + 100 \, \text{nm} = 650 \, \text{nm} \). This corresponds to the 2nd order blue interference colour.
Thus, the correct answer is (C) 2nd order blue. Final Answer: \[ \boxed{\text{2nd order blue}} \]