The OCR shows a selection mark beside 'Invisible Infra-red', indicating it as the correct choice in the block.

Among the given choices, microwaves have the longest wavelengths (longer than infrared, ultraviolet, and X-rays).

Wilhelm Roentgen discovered X-rays in 1895 and is credited with their discovery.

X-rays ionize air, allowing the electroscope to discharge and causing the leaves to collapse (lose divergence).

Radio telescopes are used to detect radio waves from astronomical objects.

The OCR indicates option B (600 seconds) as the selected choice in the source text.

Johann Balmer studied the visible spectral lines of hydrogen (the Balmer series), so Balmer is correct.

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

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.

Fringe width ∝ wavelength; in water λ' = λ/n, so new width = 0.4 × (3/4) = 0.3 mm, option A.