P = 1/f(m) ⇒ f = 1/P = 1/(-4) = -0.25 m = -25 cm. Negative focal length indicates diverging (concave) lens. Memory aid: 'Negative power = concave lens; f(cm) = 100/P(D)'. Direct lens power application frequently tested in competitive exams with sign convention awareness.
Explanation:
Spectrometer uses diffraction grating/prism to disperse light and measure wavelengths via angular positions. Memory tip: 'Spectrometer = wavelength via dispersion; used in spectroscopy'. Instrument application frequently tested in competitive exams.
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