To express the ratio \( 3600 \text{s} : 2 \text{ hours} \) as a fraction, we first need to convert the hours into seconds.
1 hour is equal to 3600 seconds, so \( 2 \text{ hours} \) is:
\[ 2 \text{ hours} = 2 \times 3600 \text{ seconds} = 7200 \text{ seconds} \]
Now we can express the ratio as:
\[ \frac{3600 \text{ seconds}}{7200 \text{ seconds}} \]
Next, we can simplify this fraction:
\[ \frac{3600}{7200} = \frac{1}{2} \]
Thus, the ratio \( 3600 \text{s} : 2 \text{ hours} \) expressed as a fraction is:
\[ \frac{1}{2} \]