Question

What happens to the optical absorption spectrum of a nanoparticle as its size is reduced?

• A. It shifts toward longer wavelengths (red shift)
• B. It shifts toward shorter wavelengths (blue shift)
• C. It remains unchanged
• D. It disappears completely

Hint:

Smaller nanoparticle → Larger band gap → Blue shift in absorption spectrum

Answer 1

The Correct Option is B

Understanding the Question: We need to determine how the light-absorption properties of a material change as its particle size enters the nanoscale.
Key Formulas and approach: Use the energy-wavelength relationship: \[ E = \frac{hc}{\lambda} \] According to quantum confinement, smaller particles have a larger band gap ($E_g$).
Detailed Solution:
Step 1: Size and Energy Gap. As a nanoparticle gets smaller, the electrons are more confined, which increases the energy required to excite them (the band gap increases).
Step 2: Energy to Wavelength. Higher energy ($E$) corresponds to a smaller wavelength ($\lambda$).
Step 3: Defining the shift. A shift toward shorter wavelengths is known in physics as a "blue shift" (since blue is at the short-wavelength end of the visible spectrum).
Conclusion: Size reduction leads to a blue shift in the absorption spectrum.