Step 1: Understanding Huygens' Wave Theory
Huygens' wave theory posits that each point on a wavefront acts as a source of secondary wavelets, enabling the derivation of the laws of: Reflection, Refraction, and Diffraction.
Step 2: Analyzing the Given Options
Reflection: Huygens' principle explains reflection by how secondary wavelets from the incident wavefront form the reflected wavefront.
Refraction: Huygens' principle explains refraction by the differential speeds of wavefront parts traversing media with varying refractive indices.
Diffraction: Huygens' principle accounts for diffraction; each wavefront point is a secondary wavelet source, allowing light to bend around obstructions.
Step 3: Limitations of Huygens' Principle Regarding Spectra
Spectral lines originate from atomic photon emission and absorption, a phenomenon explained by quantum mechanics. Huygens' wave theory omits the particle nature of light and energy quantization, crucial for understanding:
Atomic emission spectra, Blackbody radiation, and the Photoelectric effect.
As Huygens' theory solely addresses the wave nature of light, excluding its quantum properties, it cannot elucidate the origin of spectra.
Therefore, the correct option is: \(\mathbf{D}\)
A metal plate of area 10-2m2 rests on a layer of castor oil, 2 × 10-3m thick, whose viscosity coefficient is 1.55 Ns/m2. The approximate horizontal force required to move the plate with a uniform speed of 3 × 10-2ms-1 is: