diff --git a/CHANGES b/CHANGES
index 50b524949..3b011a177 100644
--- a/CHANGES
+++ b/CHANGES
@@ -11,8 +11,11 @@ Pint Changelog
 - Add `dim_order` and `default_sort_func` properties to FullFormatter.
   (PR #1926, fixes Issue #1841)
 - Fix LaTeX siuntix formatting when using non_int_type=decimal.Decimal.
+- Added refractive index units.
+  (PR #1816)
 - Fix converting to offset units of higher dimension e.g. gauge pressure
   (PR #1949)
+  
 
 0.23 (2023-12-08)
 -----------------
diff --git a/pint/default_en.txt b/pint/default_en.txt
index 5fc7f8265..83ea967f6 100644
--- a/pint/default_en.txt
+++ b/pint/default_en.txt
@@ -494,6 +494,10 @@ buckingham = debye * angstrom
 bohr_magneton = e * hbar / (2 * m_e) = µ_B = mu_B
 nuclear_magneton = e * hbar / (2 * m_p) = µ_N = mu_N
 
+# Refractive index
+[refractive_index] = []
+refractive_index_unit = [] = RIU
+
 # Logaritmic Unit Definition
 #  Unit = scale; logbase; logfactor
 #  x_dB = [logfactor] * log( x_lin / [scale] ) / log( [logbase] )