# Measurement of speed using redshift or blueshift

• mugen715
In summary, the solution on the textbook shows a velocity of 13000m/s towards Earth, which causes the light to be blue-shifted. This is indicated by the star's H lines being at a shorter wavelength than observed in the laboratory, suggesting that the star is moving away from Earth and is therefore red shifted. The concept of wavelength and frequency can be compared to the sound of a train, where shorter wavelengths (higher frequency) indicate the train is coming towards you, while longer wavelengths (lower frequency) indicate it is moving away. However, there seems to be some confusion as the statement contradicts itself regarding the train's sound and the light's wavelength.

#### mugen715

Homework Statement
One wavelength in the hydrogen spectrum of light from Ursa Majoris is 486.112 nm. In the laboratory, this spectral line is found to have a wavelength of 486.133nm. Determine the velocity of Ursa Majoris relative to Earth. (speed of light = 3.0 x 10^8 m/s)
Relevant Equations
doppler effect equation
The solution on my textbook is 13000m/s toward Earth as the light is blue-shifted

I'm able to calculate the magnitude of velocity (13000m/s), but i don't understand why thus is blue-shifted? Since in the lab, the light's wavelength observed is slightly higher than light from Ursa Majoris. So my thought is red-shift and move away from earth. Any any explanation?

Your statement says that one of the star's H lines is at a shorter wavelength (ie: higher frequency) than the same line achieved in the lab (which we can take to be the reference result).

When you're standing next to the tracks and the train zooms by, does the wavelength of the sound of its horn from the cab decrease ? or increase.

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When the train coming towards me, the wavelength of sound is shorter (higher frequency) but when the train moves away from me it's wavelength is longer (low frequency)

The star's H lines is at a shorter wavelength than observed in the laboratory (low frequency is observed), that means the star must be moving away from the Earth and is red shifted

I double checked I didn't read the question wrong

mugen715 said:
When the train coming towards me, the wavelength of sound is shorter (higher frequency) but when the train moves away from me it's wavelength is longer (low frequency)

The star's H lines is at a shorter wavelength than observed in the laboratory (low frequency is observed), that means the star must be moving away from the Earth and is red shifted

I double checked I didn't read the question wrong
Red is longer wavelength than blue.

wavelength is the inverse of frequency.

mugen715 said:
When the train coming towards me, the wavelength of sound is shorter (higher frequency) but when the train moves away from me it's wavelength is longer (low frequency)

The star's H lines is at a shorter wavelength than observed in the laboratory (low frequency is observed), that means the star must be moving away from the Earth and is red shifted

I double checked I didn't read the question wrong
You are contradicting yourself. Yes, sound that you hear that is higher than normal = coming toward you. Light that is shorter wavelength (bluer) also means coming toward you.