Physics Waves MDCAT 2011
PMDC Verified Question 118 of 125
The spectrum of a star's light is measured and the wavelength of one of the lines as the sodium's line is found to be 589 nm. The same line has the wavelength of 497 nm when observed in the laboratory. This means the star is
A
Moving away from the earth
B
Stationary
C
Moving towards the north
D
Revolving around the planet
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: Moving away from the earth
Concept:

The Doppler Effect applies to light waves. A shift in the observed wavelength indicates relative motion.

Solution:

  • The laboratory wavelength (true wavelength) is 497 nm.


  • The observed wavelength from the star is 589 nm.


  • Since the observed wavelength is longer (\( 589 > 497 \)), the light has shifted toward the red end of the electromagnetic spectrum (Redshift).


  • A redshift strictly occurs when the source is receding, or moving away from the observer.


Why other options are incorrect:

If the star were stationary, the wavelengths would match perfectly. If it were moving towards Earth, we would observe a smaller wavelength (Blueshift).

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