A star, which is emitting radiation at a wavelength of 5000 Å, is approaching the earth with a velocity of 1.5 × 10⁶ m/s. The change in wavelength of the radiation as received on the earth is (a) 25 Å (b) 100 Å (c) zero (d) 2.5 Å

Answer: (a). Doppler shift for light ( v c): Δλ = /c. Δλ = 5000 × 1.5×10⁶3×10⁸ = 5000 × 5×10⁻³ Δλ = 25 Å (a decrease, since the star approaches). So the answer is (a) 25 Å.

A star, which is emitting radiation at a wavelength of 5000 Å, is… | NEET 1995 (AIPMT)

Q26NEET 1995 · AIPMTEasy

Wave Theory, Wavefronts & Doppler Effect

A star, which is emitting radiation at a wavelength of 5000 Å, is approaching the earth with a velocity of $1.5 \times 10^{6}\text{ m/s}$ . The change in wavelength of the radiation as received on the earth is

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Answer: (a)

Doppler shift for light ( $v \ll c$ ): $\Delta\lambda = \lambda\dfrac{v}{c}$ .
$\Delta\lambda = 5000 \times \dfrac{1.5\times10^{6}}{3\times10^{8}} = 5000 \times 5\times10^{-3}$
$\Delta\lambda = 25$ Å (a decrease, since the star approaches).
So the answer is (a) 25 Å.

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