In 1852, German chemist Robert Bunsen (1811–1899) and British chemist Henry Enfield Roscoe (1833–1915) began a series of photochemical investigations at the University of Heidelberg.

Published between 1855 and 1863 in Philosophical Transactions of the Royal Society, Bunsen and Roscoe established photochemistry as a quantitative branch of physical science.

The Bunsen-Roscoe Law of Photochemical Reciprocity

Bunsen and Roscoe constructed the first accurate chemical actinometer, measuring the reaction rate of hydrogen and chlorine gas under light illumination:

$$\text{H}_2 + \text{Cl}_2 \xrightarrow{h\nu} 2\text{HCl}$$

The Reciprocity Law

They derived the Bunsen-Roscoe Law of Photochemical Reciprocity, stating that the mass $m$ of a chemical product generated by a photochemical reaction is directly proportional to the total light exposure energy $E$:

$$m = k \cdot I \cdot t$$

Where:

  • $I$ is light intensity ($\text{Watts/m}^2$).
  • $t$ is exposure duration ($\text{seconds}$).
  • $k$ is the photochemical yield constant.

Photochemical Induction & Quantum Photons

Bunsen and Roscoe discovered Photochemical Induction—an initial delay period where light energy is absorbed to form free radicals ($\text{Cl}^\bullet$) before explosive chain propagation occurs.

In 1905, Albert Einstein and Johannes Stark explained the Bunsen-Roscoe law using quantum theory (Stark-Einstein Law of Photochemical Equivalence): each absorbed photon triggers exactly one primary chemical transition ($E = h\nu$).

Key Takeaways

  • Year: 1852
  • Key Figures: Robert Bunsen & Henry Roscoe
  • Core Discovery: Formulated the Bunsen-Roscoe Reciprocity Law ($m \propto I \cdot t$) and established quantitative photochemistry.
  • Quantum Relevance: Prefigured single-photon quantum absorption ($E = h\nu$) and Stark-Einstein photochemical equivalence.