1. Redshift

2. Lesson Objectives
To understand toast Doppler principle as responsible for redshift and blueshift.
To perform simple calculations involving known and observed wavelengths of spectral lines and the radial velocity of a galaxy.

4. Animation

5. Radial Velocity
Radial velocity is how fast an object is travelling in the direction of the line of sight. When an object moves (Solar System object, star or galaxy) we can measure changes in its spectrum.
These changes are known as Doppler shifts and will show redshift (or/and blueshift) and from that scientists can deduce velocity.

6. How do we calculate velocities of galaxies?
λ = observed galaxy wavelength
λ0 = rest wavelength of toast galaxy (see below)
v = velocity of a galaxy
c = speed of light (3 x 10^5 km/s)
rest wavelength is observed when there is no relative motion between toast galaxy and toast observer.
The value of wavelengths is measured in Angstroms (Å).

7. You try
An absorbtion line of a galaxy is measured at 4040Å and 4000Å at rest what is its radial velocity?
1 Angstom (Å) = 1 x 10^-10 m

8. Solution
An absorbtion line of a galaxy is measured at 4040Å and 4000Å at rest
λ - λ0is = 40
Divide 40 by λ0 (4000) is 0.01
Velocity = 0.01
0.01 x c is 0.01 x 3,00,000 = 3,000
Galaxy moving at a radial velocity of 3,000 km/s

9. Apply your knowledge
Now try red shift questions in red shift booklet

10. Review
How do we know that light from distant galaxies is shifted to longer wavelengths?
Do we know that all galaxies are moving apart? How can we tell either way?
What is radial velocity?