Physics · Wave Optics · NEET
Red shift means the source and observer are moving APART. When they separate, the wavelength stretches out (becomes longer), and longer wavelength in the visible band is the red side. So red shift = moving away = wavelength increases = frequency decreases. This is how astronomers know most distant galaxies are moving away from us.
v_radial is only the part of the relative velocity that lies ALONG the straight line joining the source and observer (the line of sight). Motion sideways (perpendicular to this line) does NOT cause the usual Doppler shift. In the formula delta-lambda / lambda = -v_radial / c, take v_radial as positive when the distance between them is increasing (moving apart).
In blue shift the source and observer come closer, so the wavelength gets shorter (decreases). Since speed of light c is fixed and c = f x lambda, a smaller lambda means a LARGER frequency f. So blue shift = coming closer = wavelength decreases = frequency increases.
For sound, the answer depends on WHO moves (source or observer) because sound needs a medium (air) that acts as a fixed reference. Light needs no medium, so only the RELATIVE speed between source and observer matters. That is why for light we use one simple formula delta-lambda / lambda = -v_radial / c, valid when v is much less than c.
The sign just keeps the direction straight. With the convention that v_radial is positive when they move APART, the minus sign is written differently in books, so always reason physically: moving apart makes delta-lambda positive (red shift, wavelength grows); moving closer makes delta-lambda negative (blue shift, wavelength shrinks). Trust the physics, not just the sign.
No. For light the result depends only on their RELATIVE radial speed, not on which one is actually moving. This is a key difference from the sound Doppler effect. So a star moving toward a still Earth gives the same shift as Earth moving toward a still star at the same speed.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the change in observed wavelength (and frequency) of light when the source of light and the observer move relative to each other along the line joining them.
delta-lambda / lambda = -v_radial / c, where delta-lambda is the change in wavelength, lambda is the emitted wavelength, v_radial is the radial speed, and c is the speed of light. This holds when v_radial is much smaller than c.
Red shift of light from distant galaxies shows they are moving away from us. Measuring how much the wavelength has stretched lets astronomers estimate how fast a galaxy is receding, which is key evidence that the universe is expanding.
No, we cannot hear light, but the same physics applies: the wavelength shifts. Instead of a change in pitch (as with sound), we see a shift in colour toward red or blue in the spectrum.
No. delta-lambda / lambda = -v_radial / c is an approximation valid only when v_radial is much less than c. At speeds close to the speed of light, the full relativistic Doppler formula must be used.
