To express \( 29.4 \times 10^{-5} \) in scientific notation, we need to move the decimal point of \( 29.4 \) one place to the left, which changes it to \( 2.94 \). This adjustment requires us to increase the exponent of \( 10 \) by 1.
So, we have:
\[ 29.4 \times 10^{-5} = 2.94 \times 10^{-4} \]
Thus, the expression in scientific notation is:
\[ 2.94 \times 10^{-4} \]