Two light rays will interfere only if:

1. They are both of the same wavelengths
2. They are both traveling in the same direction
3. They are traveling in opposite directions
4. They are both from the same source
5. They are both of the same polarization
6. They both have the same phase
7. Something else

A rainbow appears on an oil slick or soap bubble due to:

1. Refraction of light
2. Transmission of light
3. Dispersion of light
4. Interference of light
5. Scattering of light
6. Something else