Transparent photographic films gradually change their colour when exposed to light due to.

Options

A. combination reaction
B. thermal decomposition
C. oxidation reaction
D. displacement reaction

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks why transparent photographic films change color when exposed to light.

2. **Identifying the Process**: The process responsible for this color change is known as thermal decomposition.

3. **Defining Thermal Decomposition**: Thermal decomposition is a chemical reaction where a compound breaks down into simpler substances when exposed to heat or light.

4. **Example of Thermal Decomposition**: A common example in photography is the decomposition of silver bromide (AgBr), which is used in photographic films.

5. **Chemical Reaction**: When silver bromide is exposed to light, it decomposes into silver (Ag) and bromine (Br2). The reaction can be represented as:
\[ 2 \text{AgBr} \xrightarrow{\text{light}} 2 \text{Ag} + \text{Br}_2 \]

6. **Observing Color Change**: Silver bromide is yellow in color. Upon decomposition, silver is metallic (gray) and bromine is a brown gas. The combination of these products results in a grayish-brown color, which is the observed change in the photographic film.

7. **Conclusion**: Therefore, the gradual color change in transparent photographic films when exposed to light is due to thermal decomposition of compounds like silver bromide.

### Final Answer:
The correct answer is **Thermal Decomposition**.

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