Q.Explain Doppler effect in light. Distinguish between red shift and blue shift.

Just as sound pitch changes with relative motion of source and listener, the observed frequency (colour) of light shifts when the source and observer move relative to each other - a receding source causes a redshift, an approaching source causes a blueshift.

Doppler Effect in light

When there is relative motion between a light source and an observer along the line joining them, the frequency (and hence wavelength) of light received by the observer is different from the frequency actually emitted by the source. This apparent change in frequency/wavelength due to relative motion is called the Doppler effect.

For speeds much smaller than the speed of light ($v \ll c$), the fractional change in wavelength is given by:

$\frac{\Delta\lambda}{\lambda} = \frac{v_r}{c}$

where $v_r$ is the radial (line-of-sight) component of the relative velocity between source and observer, and $c$ is the speed of light. $\Delta\lambda$ is taken positive if the source is receding (moving away).

Red shift

When the source of light moves away from the observer, the observed wavelength increases - the spectral lines shift toward the longer-wavelength (red) end of the spectrum. This is called a red shift. It is the observational basis for concluding that distant galaxies are receding from us, and hence that the universe is expanding.

Blue shift …

Just as with sound, the observed frequency of light shifts when the source and observer move relative to each other; a receding source causes a redshift (longer wavelength), an approaching source causes a blueshift (shorter wavelength).

Doppler Effect in light

When there is relative motion between a source of light and an observer along the line joining them, the frequency (and hence wavelength) of light received by the observer appears different from the frequency emitted by the source. This is called the Doppler effect in light.

For a source moving with radial velocity $v$ relative to the observer (small compared to speed of light $c$), the fractional change in frequency (or wavelength) is given approximately by:

$\dfrac{\Delta\nu}{\nu} \approx -\dfrac{v}{c} \quad \text{or equivalently} \quad \dfrac{\Delta\lambda}{\lambda} \approx \dfrac{v}{c}$

where $v$ is taken positive when the source is receding.

Red shift: When the source of light moves away from the observer, the observed wavelength increases (shifts towards the red/longer-wavelength end of the spectrum), i.e., the frequency decreases. This is observed, for example, in the spectra of distant galaxies receding from Earth due to the expansion of the universe.

…

Doppler Effect in light is the apparent shift in frequency/wavelength due to relative source-observer motion — red shift for a receding source, blue shift for an approaching one — and is central to measuring astronomical velocities.

Doppler Effect in light: Just as with sound, when there is relative motion between a light source and an observer, the observed frequency of light differs from the frequency emitted by the source. For light, the fractional change in frequency (for speeds much less than c) is approximately:

$\frac{\Delta \nu}{\nu} = \frac{\Delta \lambda}{\lambda} \approx \frac{v}{c}$

where $v$ is the relative radial velocity between source and observer, and $c$ is the speed of light.

Red Shift: When the source of light (e.g. a star or galaxy) is moving away from the observer, the observed wavelength increases — it shifts toward the longer-wavelength (red) end of the visible spectrum. This is called a red shift.

Blue Shift: When the source is moving toward the observer, the observed wavelength decreases — it shifts toward the shorter-wavelength (blue/violet) end of the spectrum. This is called a blue shift.

Importance: …