IB Physics HL C.5 Doppler effect questions

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Question 1

Multiple choiceEasy1 mark

A source emits light with wavelength 600 nm and moves directly away from an observer at 1.5×106 m s−11.5 \times 10^6\ \text{m s}^{-1} Take the speed of light as 3.0×108 m s−13.0 \times 10^8\ \text{m s}^{-1} What is the magnitude of the relative change in wavelength, expressed as a percentage?

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Answer: C. For light, the magnitude of the relative wavelength change is approximately v/cv/c Thus v/c=(1.5×106)/(3.0×108)=5.0×10−3=0.50%v/c=(1.5 \times 10^6)/(3.0 \times 10^8)=5.0 \times 10^{-3}=0.50\%

A: converts the fractional change to a percentage using a factor of 10 rather than 100.
B: introduces an erroneous factor of 1/21/2
D: shifts the decimal point one place too far when converting the fraction to a percentage.

Remember: multiply a fractional change by 100 to express it as a percentage.

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