Some currency notes (to avoid counterfelts) change their colour as you till them. This is due to

Options

A. Diffraction
B. Polarization
C. Interference
D. Refraction

Answer

### Step-by-Step Solution:

1. **Understanding the Phenomenon**: The color change in currency notes when tilted is a visual effect that occurs due to the interaction of light with the surface of the note.

2. **Light Interaction**: When light (such as sunlight or artificial light) hits the surface of the currency note, it reflects off the surface. The surface of the note is designed to have specific features that can manipulate light.

3. **Interference of Light**: The reflected light waves can interfere with each other. This interference can be constructive (where waves add together) or destructive (where waves cancel each other out).

4. **Phase Difference**: As the angle of the note changes (when tilted), the path length of the reflected light changes, leading to a variation in phase difference between the waves. This change in phase difference causes different colors to be seen at different angles.

5. **White Light Composition**: White light is made up of multiple wavelengths (colors). The interference of these different wavelengths results in the colorful appearance of the note when viewed from different angles.

6. **Conclusion**: The color change observed in currency notes when tilted is primarily due to the phenomenon of interference of light, which is influenced by the surface structure of the notes.

### Final Answer:
The color change in currency notes when tilted is due to interference of light.

---