Chapter 33 - Wave Optics - Exercises and Problems - Page 956: 37

$\lambda = 500~nm$

Note that $y_4 = 2.0~cm$ We can find the wavelength of the light: $y_m = \frac{m~\lambda~L}{d}$ $\lambda = \frac{y_m~d}{m~L}$ $\lambda = \frac{(0.020~m)~(0.20\times 10^{-3}~m)}{(4)~(2.0~m)}$ $\lambda = 5.0\times 10^{-7}~m$ $\lambda = 500~nm$