# Wavelength of cosine-squared

I am confused. Usually, the wavelength is the x-distance between the tops of two consecutive waves. Here is the graph.

There is only 0.1 m between 2 crests. But the answer counts the wavelength as 0.2 m

This is puzzling. But use the identity $$\cos^2(\theta)=\frac{\cos(2\theta)+1}{2}.$$ The period of $$\cos(2\theta)$$ is $$\pi$$, so the period of $$\cos^2(\theta)$$ is also $$\pi.$$ I think I disagree with the answer in your post. The period is $$0.1$$.
• Yes, there should be more context involved on the question. Still, we should note that the "oscillations" in your consideration would be about the value of $1/2$, and not about zero. Subtle, indeed. – Bruno Anghinoni May 30 at 23:49
• The "oscillations" in the power graph are about the value $R_s/2$. – user52817 May 31 at 0:10