Red Shift/Blue Shift and the Doppler Effect

Spectroscopy and Motion

What is the Doppler Effect?

What is the Doppler Effect?

• The apparent change in frequency due to the motion of the source (or receiver)
• The greater the speed of the source, the greater the effect

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Doppler Shift in Sound

If the source of sound is moving, the pitch changes!

Doppler Effect and the Electromagnetic Spectrum

In which direction does the wavelength increase? The red side

In which direction does the frequency increase? The blue side

Stationary Wave Sources

L

R

Q: Does either eye (left or right) observe a different wavelength light?

A: No. Both eyes observe the same wavelength, yellow, light.

Q: Does either eye (left or right) observe a different wavelength light?

Moving Wave Sources

L

R

Q: Does either eye (left or right) observe a different wavelength light?

A: Yes. The left eye will see longer wavelength light and the right eye will see shorter wavelength light.

Q: Does either eye (left or right) observe a different wavelength light?

Red Shift / Blue Shift

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R

• Wave source moving away (red shift)
1. wave fronts are spread apart
2. wavelength is longer, frequency is lower
3. so it appears redder

• Wave source approaching (blue shift)
1. wave fronts are squished together
2. wavelength is shorter, frequency is higher
3. so it appears bluer

The Sun & The Stars

• The sun is a star
• The sun and the stars are all made of hydrogen
• Light from hydrogen leaves a distinctive pattern of black lines (an absorption spectrum)

Red Shift/Blue Shift in Spectra

Blue shift shows object is moving toward the observer

This shows the normal position or no motion

This shows a red shift meaning that the object is moving away from the observer

Compare the dark lines in the middle spectra to the dark lines in the other two spectra.

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

This is the absorption spectrum (pattern of black lines) from a close star like our sun.

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

If the nearby galaxy wasn’t moving, the absorption spectrum

(the pattern of black lines) would be in the same place.

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

Is the pattern red shifted or blue shifted?

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

The pattern is red shifted

since it is shifted towards the red side of the spectrum.

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

The pattern is red shifted.

— So —

Is the nearby galaxy moving towards us or away from us?

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What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

The pattern is red shifted.

— So —

Is the nearby galaxy moving towards us or away from us?

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What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

The bigger the shift, the faster the light source is moving.

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

Which is moving away faster:

the nearby galaxy or the faraway galaxy?

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

Which is moving away faster:

the nearby galaxy or the faraway galaxy?

What We See

Close

Star

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Nearby Galaxy

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Faraway Galaxy

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Really Faraway Galaxy

Your Turn

faster

slower

The further away a galaxy is,

the bigger the ______ shift.

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So far away galaxies are moving away _______.

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red

blue

Your Turn

red

blue

The further away a galaxy is,

the bigger the ______ shift.

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So far away galaxies are moving away _______.

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slower

faster

red

Your Turn

slower

red

blue

The further away a galaxy is,

the bigger the ______ shift.

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So far away galaxies are moving away _______.

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faster

red

faster

Summary

• An observed increase in wavelength is called a redshift
• An observed decrease in wavelength is called a blueshift (regardless of whether or not the waves are visible)
• Doppler shift is used to determine an object’s velocity