Light waves projected on oil surface show seven colours due to the phenomenon of

The appearance of seven colors on an oil surface is caused by thin-film interference. When light waves strike a thin layer of oil, some light reflects off the upper air-oil interface, while the rest is transmitted and reflects off the lower oil-water interface [2]. These two reflected waves overlap and interfere with each other. The phase difference between these waves depends on the film's thickness and refractive index. For specific wavelengths (colors), the waves undergo constructive interference, making those colors visible, while others undergo destructive interference and are canceled out [3]. Because the oil film's thickness varies across the surface, different colors are reinforced at different points, creating a colorful iridescent pattern [4]. This phenomenon is distinct from polarization, which involves wave orientation, or simple refraction and reflection [2].

Sources

1. [1] https://en.wikipedia.org/wiki/Thin-film_interference
2. [2] https://www.britannica.com/science/light/Thin-film-interference
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 147
4. [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC2706478/