Final answer:
When a light source is moving away from an observer at a speed of 3,000,000 m/sec, the light waves it emits are stretched, which causes an increase in wavelength and a decrease in frequency. This is an example of the Doppler effect specifically resulting in redshift for light waves.
Step-by-step explanation:
If a source of light is moving away from us at 3,000,000 m/sec, the waves are stretched. This phenomenon is known as the Doppler effect, which describes how the wavelength and frequency of waves (including light) change when the source of the waves is moving relative to an observer. In the case of light, when the source is moving away, the light waves are stretched, leading to an increased wavelength and a decreased frequency, a phenomenon also referred to as redshift. Conversely, if the light source were moving towards the observer, the wavelengths would be compressed, leading to what is known as blueshift.
Several examples help clarify these effects. When observing a light source moving away from the observer, much like the scenario described, the waves would appear to be stretched and have a longer wavelength. In contrast, when a light source is moving towards an observer, such as in the case provided for observer A, the waves appear to follow one another more closely with a decreased wavelength and thus increased frequency.