Coherent waves have the same frequency and a constant phase difference.The double slits are illuminated with the same light source. As long as the light source is a laser light or a narrow slit if the light is not a laser light the double slits will emit waves which are coherent.A laser light is a narrow beam of monochromatic light (helium-neon λ = 633nm.)The light is collimated - almost perfectly parallel.Fringe separation depends on colour.
Fringe separation is greater for red light.Red light has a longer wavelength. Light sources are vapour lamps, filament lamps, the Sun and laser lights.Vapour lamps and discharge tubes produce light with a dominant colour.Sodium vapour lamps produce yellow/orange light.590nm is dominant (most intense) so it is effectively monochromatic.Light from a filament lamp or the Sun gives out a continuous range of wavelengths ~400nm to 700nm.The light can be passed through colour filter to obtain a narrower range of wavelengths.
| w | = | λD s |
Fringe separation is greater for red light.Red light has a longer wavelength. Light sources are vapour lamps, filament lamps, the Sun and laser lights.Vapour lamps and discharge tubes produce light with a dominant colour.Sodium vapour lamps produce yellow/orange light.590nm is dominant (most intense) so it is effectively monochromatic.Light from a filament lamp or the Sun gives out a continuous range of wavelengths ~400nm to 700nm.The light can be passed through colour filter to obtain a narrower range of wavelengths.