As we know few currency notes change their color as you tilt them to avoid counterfeits.

This is because of __________ of light

CONCEPT:

Thin-film interference:

• Thin-film interference is the phenomenon that is a result of lightwave being reflected off two surfaces that are at a distance comparable to its wavelength.
• When light waves that reflect off the top and bottom surfaces interfere with one another we see different coloured patterns.
• During this, the light reaches the boundary between two media, and part of it gets reflected and some part gets transmitted.
• In the thin film, there are two reflections occurring close together at the top and bottom boundary surfaces of the thin film.
• Thus, there are two waves emerging from a thin film – one wave reflected off the top surface of the film and the other reflected off the bottom surface.

• Meanwhile, the type of interference that can take place is dependent upon these factors.

  • Wavelength and angle of the incident light.

  • The thickness of the film.

  • Refractive index of the material on either side of the film.

• In thin-film the interference occurs in two ways:

  1. Interference in reflected light

  2. Interference in refracted light​

EXPLANATION:

• The currency note act as a thin film for light.
• Therefore when a currency note is tilted an interference occurs and it changes its colour. Hence, option 3 is correct.