$ \lambda_1 $ and $ \lambda_2 $ are used to illuminate the slits. $ \beta_1 $ and $ \beta_2 $ are the corresponding fringe widths. The wavelength $ \lambda_1 $ can produce the photoelectric effect when incident on a metal, but the wavelength $ \lambda_2 $ cannot. The correct relation between $ \beta_1 $ and $ \beta_2 $ is