In Young's double-slit experiment, when red and blue lights are used, interference fringes will still appear on the screen. However, the characteristics of the fringes may differ due to the different wavelengths of red and blue light.
The interference fringes in Young's double-slit experiment arise from the interference of light waves that pass through two closely spaced slits and then overlap on a screen. When coherent light from a single source, such as a laser, is used, the resulting interference pattern consists of alternating bright and dark fringes.
The position of the fringes depends on the wavelength of the light used. Red light has a longer wavelength than blue light, so the fringes produced by red light will be spaced farther apart compared to those produced by blue light.
Specifically, the fringe spacing (the distance between adjacent bright or dark fringes) in Young's double-slit experiment is given by the formula:
d * sin(θ) = m * λ,
where d is the slit separation, θ is the angle between the screen and the line connecting the slits and the point on the screen, m is an integer representing the order of the fringe, and λ is the wavelength of light.
Since the wavelengths of red light are longer than those of blue light, the value of λ will be larger for red light. As a result, for the same values of d, θ, and m, the equation indicates that the fringe spacing will be larger for red light compared to blue light.
Therefore, in the interference pattern observed on the screen, the fringes produced by red light will be wider apart (with larger spacing) than the fringes produced by blue light. This difference in fringe spacing is a consequence of the different wavelengths of the two colors.