**CBSE NOTES CLASS 12 PHYSICS **

**CHAPTER 10 WAVE OPTICS**

**The Doppler effect**

When the source moves away from the observer the frequency as measured by the observer will be smaller. Similarly when the source moves towards the observer, the frequency measured by the observer will be greater. This is known as the Doppler Effect. The increase in wavelength is called **red shift** and decrease in wavelength is called **blue shift.**

The fractional change in frequency or Doppler shift is given by

$$\frac{\mathrm{\Delta}\mathrm{\nu}}{\mathrm{\nu}}=\mathrm{}\u2013\frac{{\mathrm{v}}_{\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{l}}}{\mathrm{c}}$$

Where, v_{radial} is the component of the **source velocity relative to the observer** along the line joining the observer to the source.

v_{radial} is considered **positive** when the **source moves away from the observer**.