Given below are two statements:one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Finest gold is red in colour, as the size of the particles increases, it appears purple then blue and finally gold.
Reason R: The colour of the colloidal solution depends on the wavelength of light scattered by the dispersed particles.
In the light of the above statements, choose the most appropriate answer from the options given below

We need to evaluate the assertion and reason about colloidal gold.

Assertion A: Finest gold is red in colour, as the size of the particles increases, it appears purple then blue and finally gold.

This is correct. Colloidal gold nanoparticles exhibit different colours depending on their size. Very fine particles ($$\sim$$20 nm) appear red, medium-sized particles appear purple/blue, and bulk gold has its characteristic golden colour.

Reason R: The colour of the colloidal solution depends on the wavelength of light scattered by the dispersed particles.

This is correct. The colour of colloidal solutions depends on the wavelength of scattered light. As particle size changes, the wavelength of light scattered changes, which results in different observed colours. This phenomenon is related to the Tyndall effect and surface plasmon resonance in metal nanoparticles.

Is R the correct explanation of A?

Yes. The reason gold colloids of different sizes appear as different colours (Assertion A) is precisely because the wavelength of scattered light depends on the particle size (Reason R). Smaller particles scatter shorter wavelengths, while larger particles scatter longer wavelengths, producing the observed colour changes from red to purple to blue to gold.

Therefore, the correct answer is Option A: Both A and R are true and R is the correct explanation of A.