Mercury vapour lamps produce characteristic line emission of mercury atoms, giving a discrete emission spectrum.

The Poynting vector shows electromagnetic power is transmitted in the direction perpendicular to both electric and magnetic fields.

All electromagnetic waves travel at the same speed in vacuum but differ in wavelength, so option B is correct.

X-rays are especially useful because they can penetrate materials and reveal internal structures, i.e., they have high penetrating power.

Wavelength in water is less than in air, so fringe width (proportional to wavelength) decreases when experiment is in water.

Path difference = nλ − (n−2)λ = 2λ, which corresponds to the second-order constructive (bright) fringe.

Thin-film interference between reflections from the top and bottom of the oil layer produces the observed colors.

The block's explanatory answer gives B; using Ay = λL/d with λ=500 nm, L=1 m, d=1×10^-3 m yields Ay = 5×10^-4 m = 0.5 mm.

Mirror movement m changes path difference by 2m; for m = λ/4 the path difference change is 2(λ/4) = λ/2.

Adding mass (wax) to the prongs lowers their frequency, so the frequency decreases.