#### phys-lexic

i just have the two mixed up, and need an example to set it straight...

lets say you're source is a red light (hight wavelength, low frequency) to green light (lower wavelenght, higher frequency)... that would be a blue shift?

redshift = wavelength (H->L)
frequency (L->H)
blueshift = wavelength (L->H)
frequency (H->L)

... is that right or do i have it backwards?

Related Other Physics Topics News on Phys.org

#### Danger

Gold Member
I'm not quite sure what you're asking. If a source is approaching you, it's blue-shifted. If it's receding, it's red-shifted. Think of it as compressing or stretching the waves relative to your position. Simply changing the colour of something doesn't involve a Doppler shift at all.

#### cjl

You are correct though that if a red laser appears as green due to doppler shift, that would be a blueshift.

#### cragar

red shift = wavelength (H->L)
frequency (L->H)
blue shift = wavelength (L->H)
frequency (H->L)

... is that right or do i have it backwards?

in a red shift Wavelength would be shorter to longer wavelength
and f higher frequency to lower frequency
red shift higher energy to lower energy photon.

#### Danger

Gold Member
You are correct though that if a red laser appears as green due to doppler shift, that would be a blueshift.
I'm not a chromatologist, so I'm not sure about this. I'm fairly certain, though, that green would not appear in a Doppler shift. It's not a normal part of the visible spectrum; it is rather a combination of blue and yellow. I can't see that a red source approaching at a significant percentage of light speed would produce it.

(I'm not sure that 'chromatologist' is an actual job title; I just mean someone who specializes in colour.)

#### Doc Al

Mentor
I'm fairly certain, though, that green would not appear in a Doppler shift. It's not a normal part of the visible spectrum; it is rather a combination of blue and yellow.
Green (λ ≈ 530 nm) is most definitely part of the "normal" visible spectrum.

#### Danger

Gold Member
I stand corrected. Thanks, Doc.

#### phys-lexic

I'm not quite sure what you're asking.
I know the concept behind the shifts, i just am not sure what they mean by an increase/decrease in wavelength.... do they mean from the source to the observed or from the observed to the source... which is why i gave the example from a source it is 630nm (red) to being observed as 530nm (green), would that be considered an increase in wavelenghth or decrease in wavelenght (it depends on what you consider the change to be from and to)

-i agree with doc, green light is most definitely in the "normal" spectrum

#### Doc Al

Mentor
I know the concept behind the shifts, i just am not sure what they mean by an increase/decrease in wavelength.... do they mean from the source to the observed or from the observed to the source... which is why i gave the example from a source it is 630nm (red) to being observed as 530nm (green), would that be considered an increase in wavelenghth or decrease in wavelenght (it depends on what you consider the change to be from and to)
That example is a decrease in wavelength (compared to what the source is emitting) and thus is a "blue" shift.

If the source is emitting red light (630nm)--which means that someone traveling along with the source would measure the wavelength to be 630nm--and that source is moving towards you fast enough that you observe the light to have a wavelength of 530nm (green), that is called a "blue" shift, because the observed wavelength is shortened by the relative motion.

#### cragar

ya i was wondering if it would work 2 cause if u put a red laser through a prism u get red out of the other side , but i think with the doppler shift u could still have it look green.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving