# Intensity of an Electromagnetic Wave

What happens to the intensity of an electromagnetic wave if the amplitude of the electric field is halved?

• Intensity of an electromagnetic wave is directly proportional to the square of amplitude of the wave. – walber97 Mar 20 '19 at 6:33

The intensity if an electromagnetic wave is the square of the amplitude $$I=A^2$$, which means that if $$A_{new}=\frac{A}{2}$$ the intensity becomes $$I_{new}=\left(\frac{A}{2}\right)^2=\frac{A^2}{4}$$ and we can see that the intensity is one quarter of what it was before.
$$\frac{I_{new}}{I}=\frac{\left(\frac{A^2}{4}\right)}{A^2}=\frac{1}{4}$$ which gives us $$I_{new}=\frac{I}{4}$$