When light travels across the vastness of space, its wavelength can stretch. This stretching makes the light appear more red than it was when it left the galaxy. The amount of stretching is called redshift.
Imagine a 🚗 moving away from you while you shout a sound. The sound waves get stretched, so the pitch drops. Light behaves the same way: as a galaxy moves away, its light waves stretch, shifting toward the red end of the spectrum.
The redshift \(z\) is calculated with:
\$\$
z = \frac{λ{\text{obs}} - λ{\text{emit}}}{λ_{\text{emit}}}
\$\$
where \(λ{\text{obs}}\) is the wavelength we observe and \(λ{\text{emit}}\) is the wavelength the light originally had.
| Galaxy | Emitted Wavelength \(λ_{\text{emit}}\) (nm) | Observed Wavelength \(λ_{\text{obs}}\) (nm) | Redshift \(z\) |
|---|---|---|---|
| Galaxy A | 500 | 550 | 0.10 |
| Galaxy B | 400 | 520 | 0.30 |
Picture a balloon with dots painted on it. As you inflate the balloon, the dots move apart. Light from a galaxy is like a dot on the balloon – as the universe expands, the dots (light waves) get further apart, shifting toward red.
Tip 1: Always identify which wavelength is observed and which is emitted before plugging into the formula.
Tip 2: Remember that a positive \(z\) indicates redshift (moving away), while a negative \(z\) would mean blueshift (moving closer).
Tip 3: Use the definition of redshift to solve for missing values, e.g. \(λ{\text{obs}} = λ{\text{emit}} (1 + z)\).
Tip 4: Practice converting between wavelength and frequency using \(c = λν\) if the question involves frequency shift.
Good luck! 🌌