Photochemical Researches. Part V. On The Measurement Of The Chemical Action Of Direct And Diffuse Sunlight Bunsen & Roscoe

Photochemical Researches. Part V. On The Measurement Of The Chemical Action Of Direct And Diffuse Sunlight Bunsen & Roscoe

Author: R. Bunsen and H. E. Roscoe

Year: 1863

Paper Summary Bunsen and Roscoe were trying to answer the question: Can a piece of silver-chloride paper be used as a reliable scientific instrument for measuring the chemical intensity of daylight? The entire paper is essentially a sequence of experiments designed to answer that question. 1. The problem Bunsen and Roscoe wanted to measure the "chemical action" (actinic power) of sunlight and daylight. Earlier investigators had suggested using the darkening of photographic paper, but nobody had demonstrated that the method was reliable. The key difficulty was that the darkness of a photographic image is not directly proportional to the intensity of the light that produced it. So they asked: Can equal shades of darkening be used as a measure of equal quantities of light? 2. Their key discovery They found that the important quantity is not intensity alone, but: Intensity × Time In modern terms: Photochemical Effect ∝ Light Intensity × Exposure Time Thus: bright light for a short time weak light for a long time produce the same darkening if the product is equal. This is the famous Bunsen–Roscoe Reciprocity Law . Everything else in the paper follows from proving and exploiting this pri