diff --git a/Physlib/QuantumMechanics/Blackbody/PlancksLaw.lean b/Physlib/QuantumMechanics/Blackbody/PlancksLaw.lean
index fb3c5c03e..5a6863bab 100644
--- a/Physlib/QuantumMechanics/Blackbody/PlancksLaw.lean
+++ b/Physlib/QuantumMechanics/Blackbody/PlancksLaw.lean
@@ -1,13 +1,14 @@
 /-
-Copyright (c) 2026 Samyak Rai. All rights reserved.
+Copyright (c) 2026 Samyak Rai, Dwanith C. Jayanth. All rights reserved.
 Released under Apache 2.0 license as described in the file LICENSE.
-Authors: Samyak Rai
+Authors: Samyak Rai, Dwanith C. Jayanth
 -/
 module
 
 public import Physlib.Thermodynamics.Temperature.Basic
 public import Physlib.Relativity.SpeedOfLight
 public import Physlib.QuantumMechanics.PlanckConstant
+public import Mathlib.Analysis.SpecialFunctions.Exponential
 
 /-!
 
@@ -22,32 +23,49 @@ its environment.
 
 According to Planck's distribution law, the spectral energy radiance
 (per unit frequency) for a black body at given temperature `T` as a function of frequency `ν`
- is given by
+is given by
 
     `B(ν, T) = 2 h ν³ / c² · 1 / (e^{h ν / (k_B T)} - 1)`
 
 where `h` is Planck's constant, `c` the speed of light, and `k_B` the Boltzmann constant.
 
+and per unit wavelength `λ`, given by
+
+    `B(λ, T) = 2 h c² / λ⁵ · 1 / (e^{h c / (λ k_B T)} - 1)`
+
+The two forms are related by `B(λ, T) = (c / λ²) B(ν = c/λ, T)`
+(see `spectralRadianceWave_eq_freq`).
+
 ## ii. Key results
 
 - `spectralRadiance` : The spectral radiance per unit frequency of blackbody radiation.
 - `spectralRadiance_pos` : The spectral radiance is positive for positive frequency
   and temperature.
 - `spectralRadiance_absZero` : The spectral radiance is 0 at absolute zero.
+- `spectralRadianceFreq` : Parametrized spectral radiance per unit frequency.
+- `spectralRadianceWave` : Parametrized spectral radiance per unit wavelength.
+- `spectralRadianceWave_eq_freq` : Correspondence between the two forms.
+- `firstRadiationConstant` / `secondRadiationConstant` : Radiation constants `c₁L` and `c₂`.
+- `spectralRadianceWave_eq_constants` : Planck's law in terms of radiation constants.
 
 ## iii. Table of contents
 
 - A. The spectral radiance
+- B. Parametrized spectral radiance per unit frequency
+- C. Spectral radiance per unit wavelength
+- D. Correspondence between the two forms
+- E. First and second radiation constants
 
 ## iv. References
 
 * https://en.wikipedia.org/wiki/Planck%27s_law
+* M. Planck, "Ueber das Gesetz der Energieverteilung im Normalspectrum",
+  Ann. Phys. 309 (3), 553–563 (1901).
 
 -/
 
 @[expose] public section
 
-
 namespace Blackbody
 
 /-!
@@ -68,28 +86,235 @@ open Constants
     Extended by zero outside the physical domain; zero is the unique continuous
     extension since the Rayleigh–Jeans limit vanishes -/
 noncomputable def spectralRadiance (c : SpeedOfLight) (ν : ℝ) (T : Temperature) : ℝ :=
-    if 0 < ν ∧ 0 < (T : ℝ) then
-      2 * h * ν ^ 3 / ((c : ℝ) ^ 2 * (Real.exp (h * ν / (kB * (T : ℝ))) - 1))
-    else 0
+  if 0 < ν ∧ 0 < (T : ℝ) then
+    2 * h * ν ^ 3 / ((c : ℝ) ^ 2 * (Real.exp (h * ν / (kB * (T : ℝ))) - 1))
+  else 0
 
 /-- The spectral radiance of blackbody radiation is positive for positive frequency
     and positive temperature. -/
 lemma spectralRadiance_pos (c : SpeedOfLight) (ν : ℝ) (T : Temperature)
     (ν_pos : 0 < ν) (T_pos : 0 < T.val) : 0 < spectralRadiance c ν T := by
-    have if_cond : 0 < ν ∧ 0 < (T : ℝ) := ⟨ν_pos, by exact_mod_cast T_pos⟩
-    rw [spectralRadiance, ite_eq_left if_cond]
-    refine div_pos ?numerator ?denominator
-    · exact mul_pos (mul_pos (by norm_num) h_pos) (pow_pos ν_pos 3)
-    · have expo_term : 0 < h * ν / (kB * (T : ℝ)) :=
-        div_pos (mul_pos h_pos ν_pos) (mul_pos kB_pos (by exact_mod_cast T_pos))
-      exact mul_pos (pow_pos c.val_pos 2)
-       (sub_pos.mpr (by simpa using Real.exp_strictMono expo_term))
+  have if_cond : 0 < ν ∧ 0 < (T : ℝ) := ⟨ν_pos, by exact_mod_cast T_pos⟩
+  rw [spectralRadiance, ite_eq_left if_cond]
+  refine div_pos ?numerator ?denominator
+  · exact mul_pos (mul_pos (by norm_num) h_pos) (pow_pos ν_pos 3)
+  · have expo_term : 0 < h * ν / (kB * (T : ℝ)) :=
+      div_pos (mul_pos h_pos ν_pos) (mul_pos kB_pos (by exact_mod_cast T_pos))
+    exact mul_pos (pow_pos c.val_pos 2)
+      (sub_pos.mpr (by simpa using Real.exp_strictMono expo_term))
 
 /-- Explicit promise for Spectral Radiance vanishing at absolute zero Temperature. -/
 lemma spectralRadiance_absZero (c : SpeedOfLight) (ν : ℝ) :
     spectralRadiance c ν ⟨0⟩ = 0 := by
-    rw [spectralRadiance, ite_eq_right]
-    rintro ⟨ν_pos, T_zero⟩
-    exact lt_irrefl _ T_zero
+  rw [spectralRadiance, ite_eq_right]
+  rintro ⟨ν_pos, T_zero⟩
+  exact lt_irrefl _ T_zero
+
+/-!
+## B. Parametrized spectral radiance per unit frequency
+-/
+
+/-- Spectral radiance per unit frequency of blackbody radiation at frequency `ν`
+  and temperature `T`:
+
+    `B(ν, T) = 2 h ν³ / c² · 1 / (e ^ (h ν / (kB T)) - 1)`,
+
+  extended by zero outside the physical domain. -/
+noncomputable def spectralRadianceFreq (h c kB ν T : ℝ) : ℝ :=
+  if 0 < ν ∧ 0 < T then
+    2 * h * ν ^ 3 / (c ^ 2 * (Real.exp (h * ν / (kB * T)) - 1))
+  else 0
+
+/-- Correspondence between `spectralRadiance` and `spectralRadianceFreq`. -/
+lemma spectralRadiance_eq_spectralRadianceFreq (c : SpeedOfLight) (ν : ℝ) (T : Temperature) :
+    spectralRadiance c ν T = spectralRadianceFreq h (c : ℝ) kB ν (T : ℝ) := rfl
+
+/-- The spectral radiance per unit frequency is positive for positive frequency
+  and positive temperature. -/
+lemma spectralRadianceFreq_pos (h c kB ν T : ℝ) (hh : 0 < h) (hc : 0 < c)
+    (hk : 0 < kB) (hν : 0 < ν) (hT : 0 < T) :
+    0 < spectralRadianceFreq h c kB ν T := by
+  unfold spectralRadianceFreq
+  rw [if_pos ⟨hν, hT⟩]
+  apply div_pos
+  · exact mul_pos (mul_pos zero_lt_two hh) (pow_pos hν 3)
+  · apply mul_pos (pow_pos hc 2)
+    have harg : 0 < h * ν / (kB * T) :=
+      div_pos (mul_pos hh hν) (mul_pos hk hT)
+    have h1e : 1 < Real.exp (h * ν / (kB * T)) := Real.one_lt_exp_iff.mpr harg
+    linarith
+
+/-- The spectral radiance per unit frequency vanishes at absolute zero. -/
+@[simp]
+lemma spectralRadianceFreq_absZero (h c kB ν : ℝ) :
+    spectralRadianceFreq h c kB ν 0 = 0 := by
+  unfold spectralRadianceFreq
+  simp
+
+/-- The spectral radiance per unit frequency vanishes at zero frequency. -/
+@[simp]
+lemma spectralRadianceFreq_zeroFreq (h c kB T : ℝ) :
+    spectralRadianceFreq h c kB 0 T = 0 := by
+  unfold spectralRadianceFreq
+  simp
+
+/-!
+## C. Spectral radiance per unit wavelength
+-/
+
+/-- Spectral radiance per unit wavelength of blackbody radiation at wavelength
+  `λ` and temperature `T`:
+
+    `B(λ, T) = 2 h c² / λ⁵ · 1 / (e ^ (h c / (λ kB T)) - 1)`,
+
+  extended by zero outside the physical domain. -/
+noncomputable def spectralRadianceWave (h c kB lam T : ℝ) : ℝ :=
+  if 0 < lam ∧ 0 < T then
+    2 * h * c ^ 2 / lam ^ 5 / (Real.exp (h * c / (lam * kB * T)) - 1)
+  else 0
+
+/-- The spectral radiance per unit wavelength is positive for positive wavelength
+  and positive temperature. -/
+lemma spectralRadianceWave_pos (h c kB lam T : ℝ) (hh : 0 < h) (hc : 0 < c)
+    (hk : 0 < kB) (hlam : 0 < lam) (hT : 0 < T) :
+    0 < spectralRadianceWave h c kB lam T := by
+  unfold spectralRadianceWave
+  rw [if_pos ⟨hlam, hT⟩]
+  have harg : 0 < h * c / (lam * kB * T) :=
+    div_pos (mul_pos hh hc) (mul_pos (mul_pos hlam hk) hT)
+  have h1e : 1 < Real.exp (h * c / (lam * kB * T)) := Real.one_lt_exp_iff.mpr harg
+  have hE : 0 < Real.exp (h * c / (lam * kB * T)) - 1 := sub_pos.mpr h1e
+  have hnum : 0 < 2 * h * c ^ 2 / lam ^ 5 :=
+    div_pos (mul_pos (mul_pos zero_lt_two hh) (pow_pos hc 2)) (pow_pos hlam 5)
+  exact div_pos hnum hE
+
+/-- The spectral radiance per unit wavelength vanishes at absolute zero. -/
+@[simp]
+lemma spectralRadianceWave_absZero (h c kB lam : ℝ) :
+    spectralRadianceWave h c kB lam 0 = 0 := by
+  unfold spectralRadianceWave
+  simp
+
+/-- The spectral radiance per unit wavelength vanishes at zero wavelength. -/
+@[simp]
+lemma spectralRadianceWave_zeroWave (h c kB T : ℝ) :
+    spectralRadianceWave h c kB 0 T = 0 := by
+  unfold spectralRadianceWave
+  simp
+
+/-!
+## D. Correspondence between the two forms
+
+Since `B(λ, T) dλ = -B(ν(λ), T) dν` with `ν = c / λ` and `|dν / dλ| = c / λ²`,
+the wavelength form equals `c / λ²` times the frequency form evaluated at
+`ν = c / λ`.
+-/
+
+/-- Correspondence between the wavelength and frequency forms of Planck's law:
+  `B(λ, T) = (c / λ²) B(ν = c / λ, T)`. -/
+lemma spectralRadianceWave_eq_freq (h c kB lam T : ℝ) (hh : 0 < h) (hc : 0 < c)
+    (hk : 0 < kB) (hlam : 0 < lam) (hT : 0 < T) :
+    spectralRadianceWave h c kB lam T
+      = (c / lam ^ 2) * spectralRadianceFreq h c kB (c / lam) T := by
+  have h1 : 0 < lam ∧ 0 < T := ⟨hlam, hT⟩
+  have h2 : 0 < c / lam ∧ 0 < T := ⟨div_pos hc hlam, hT⟩
+  unfold spectralRadianceWave spectralRadianceFreq
+  rw [if_pos h1, if_pos h2]
+  have hlam' : lam ≠ 0 := ne_of_gt hlam
+  have hc' : c ≠ 0 := ne_of_gt hc
+  have hkT : kB * T ≠ 0 := mul_ne_zero (ne_of_gt hk) (ne_of_gt hT)
+  have hE : Real.exp (h * c / (lam * kB * T)) - 1 ≠ 0 := by
+    have harg : 0 < h * c / (lam * kB * T) :=
+      div_pos (mul_pos hh hc) (mul_pos (mul_pos hlam hk) hT)
+    have h1e : 1 < Real.exp (h * c / (lam * kB * T)) := Real.one_lt_exp_iff.mpr harg
+    exact ne_of_gt (sub_pos.mpr h1e)
+  have hexp : h * (c / lam) / (kB * T) = h * c / (lam * kB * T) := by
+    field_simp
+  rw [hexp]
+  field_simp
+
+/-!
+## E. First and second radiation constants
+
+The wavelength variant uses only the combinations `2 h c²` and `h c / kB`,
+called the first and second radiation constants.
+-/
+
+/-- The first radiation constant `c₁L = 2 h c²`. -/
+noncomputable def firstRadiationConstant (h c : ℝ) : ℝ := 2 * h * c ^ 2
+
+/-- The second radiation constant `c₂ = h c / kB`. -/
+noncomputable def secondRadiationConstant (h c kB : ℝ) : ℝ := h * c / kB
+
+/-- The first radiation constant is positive. -/
+lemma firstRadiationConstant_pos (h c : ℝ) (hh : 0 < h) (hc : 0 < c) :
+    0 < firstRadiationConstant h c := by
+  unfold firstRadiationConstant
+  exact mul_pos (mul_pos zero_lt_two hh) (pow_pos hc 2)
+
+/-- The second radiation constant is positive. -/
+lemma secondRadiationConstant_pos (h c kB : ℝ) (hh : 0 < h) (hc : 0 < c)
+    (hk : 0 < kB) :
+    0 < secondRadiationConstant h c kB := by
+  unfold secondRadiationConstant
+  exact div_pos (mul_pos hh hc) hk
+
+/-- Planck's law per unit wavelength in terms of the radiation constants:
+  `B(λ, T) = (c₁L / λ⁵) / (e ^ (c₂ / (λ T)) - 1)`. -/
+lemma spectralRadianceWave_eq_constants (h c kB lam T : ℝ) (hh : 0 < h)
+    (hc : 0 < c) (hk : 0 < kB) (hlam : 0 < lam) (hT : 0 < T) :
+    spectralRadianceWave h c kB lam T
+      = firstRadiationConstant h c / lam ^ 5
+        / (Real.exp (secondRadiationConstant h c kB / (lam * T)) - 1) := by
+  have h1 : 0 < lam ∧ 0 < T := ⟨hlam, hT⟩
+  unfold spectralRadianceWave firstRadiationConstant secondRadiationConstant
+  rw [if_pos h1]
+  have hlam' : lam ≠ 0 := ne_of_gt hlam
+  have hk' : kB ≠ 0 := ne_of_gt hk
+  have hT' : T ≠ 0 := ne_of_gt hT
+  have hE : Real.exp (h * c / (lam * kB * T)) - 1 ≠ 0 := by
+    have harg : 0 < h * c / (lam * kB * T) :=
+      div_pos (mul_pos hh hc) (mul_pos (mul_pos hlam hk) hT)
+    have h1e : 1 < Real.exp (h * c / (lam * kB * T)) := Real.one_lt_exp_iff.mpr harg
+    exact ne_of_gt (sub_pos.mpr h1e)
+  have hexp : h * c / kB / (lam * T) = h * c / (lam * kB * T) := by
+    field_simp
+  rw [hexp]
+
+/-- The spectral radiance per unit frequency is non-negative on the physical domain. -/
+lemma spectralRadianceFreq_nonneg (h c kB ν T : ℝ) (hh : 0 < h) (hc : 0 < c)
+    (hk : 0 < kB) (hν : 0 < ν) (hT : 0 < T) :
+    0 ≤ spectralRadianceFreq h c kB ν T :=
+  le_of_lt (spectralRadianceFreq_pos h c kB ν T hh hc hk hν hT)
+
+/-- The spectral radiance per unit wavelength is non-negative on the physical domain. -/
+lemma spectralRadianceWave_nonneg (h c kB lam T : ℝ) (hh : 0 < h) (hc : 0 < c)
+    (hk : 0 < kB) (hlam : 0 < lam) (hT : 0 < T) :
+    0 ≤ spectralRadianceWave h c kB lam T :=
+  le_of_lt (spectralRadianceWave_pos h c kB lam T hh hc hk hlam hT)
+
+/-- The spectral radiance per unit frequency vanishes when frequency is
+  non-positive (the if-guard `0 < ν ∧ 0 < T` fails). -/
+lemma spectralRadianceFreq_eq_zero_of_nonpos_freq (h c kB ν T : ℝ) (hν : ν ≤ 0) :
+    spectralRadianceFreq h c kB ν T = 0 := by
+  unfold spectralRadianceFreq
+  rw [if_neg (not_and_of_not_left _ (not_lt.mpr hν))]
+
+/-- The spectral radiance per unit wavelength vanishes when wavelength is
+  non-positive (the if-guard `0 < lam ∧ 0 < T` fails). -/
+lemma spectralRadianceWave_eq_zero_of_nonpos_wave (h c kB lam T : ℝ) (hlam : lam ≤ 0) :
+    spectralRadianceWave h c kB lam T = 0 := by
+  unfold spectralRadianceWave
+  rw [if_neg (not_and_of_not_left _ (not_lt.mpr hlam))]
+
+/-- The first radiation constant equals `2 * h * c ^ 2`. -/
+@[simp]
+lemma firstRadiationConstant_eq (h c : ℝ) :
+    firstRadiationConstant h c = 2 * h * c ^ 2 := rfl
+
+/-- The second radiation constant equals `h * c / kB`. -/
+@[simp]
+lemma secondRadiationConstant_eq (h c kB : ℝ) :
+    secondRadiationConstant h c kB = h * c / kB := rfl
 
 end Blackbody