The law of photochemical equivalence was given by:

Options

A. Drapper
B. Grauths
C. Einstein
D. Lambert

Answer

To answer the question regarding the law of photochemical equivalence, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Photochemical Equivalence**: The law of photochemical equivalence states that each molecule of a substance that absorbs light (or photons) will undergo a photochemical change. This means that for every photon absorbed, one molecule of the absorbing substance is affected.

2. **Identifying the Origin**: This law was formulated by Albert Einstein, who established the relationship between the energy of photons and their wavelength. The law is often referred to as the Stark-Einstein law of photochemical equivalence.

3. **Energy of a Photon**: The energy (E) of a photon can be expressed using the equation:
\[ E = \frac{hc}{\lambda} \]
where:
- \(E\) is the energy of the photon,
- \(h\) is Planck's constant,
- \(c\) is the speed of light, and
- \(\lambda\) is the wavelength of the light.

4. **Conclusion**: The law of photochemical equivalence indicates that the amount of light absorbed by a substance is directly proportional to the number of molecules that undergo a photochemical reaction. Thus, the correct answer to the question is that the law of photochemical equivalence was given by Albert Einstein.

### Final Answer:
The law of photochemical equivalence was given by Albert Einstein.

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