Physics · Wave Optics · NEET
Two separate bulbs (or two independent lamps) are incoherent. Light from a normal source is emitted by billions of atoms in tiny random bursts of about 10 to the power minus 9 seconds. The phase of each burst is random, so the phase difference between the two bulbs changes millions of times per second. The eye and screen only see a time-averaged intensity, so no steady bright and dark fringes appear. This is why you never see interference fringes from two room lamps.
For a stable interference pattern the phase difference at every point must stay fixed in time. Two independent sources keep switching their phase randomly and very fast. So the bright and dark positions shift so rapidly that they blur into a uniform average. The condition for visible interference is a constant phase difference, which independent sources cannot maintain. This is the core reason NCERT gives for using ONE source split into two.
It means that if you look at the two waves at any point, the amount by which one wave leads or lags the other does not change as time passes. It does not mean the phase difference is zero. It just has to be steady, for example a fixed 90 degrees or a fixed 180 degrees. Because it is steady, each point on the screen keeps the same resultant intensity, so bright stays bright and dark stays dark. Same frequency is required for this, but same frequency alone is not enough; the phase relation must also be locked.
Yes. In YDSE a single source is placed behind one slit, and its light then falls on two slits S1 and S2. Since both slits are lit by the same original wavefront, any phase change in the source reaches both slits together, so their phase difference stays constant. That is why the two slits behave as coherent sources and give clear fringes. The trick of splitting one source is the standard way to make coherent sources in the lab.
No. Two sources can have exactly the same frequency and still be incoherent if their phase difference keeps changing randomly. Coherence needs BOTH the same frequency AND a constant (locked) phase difference. Same frequency alone is a necessary but not sufficient condition. This is a common NEET trap in the definition of coherence.
For two coherent sources of equal intensity I0, the resultant intensity varies point to point as I = 4 I0 cos squared (phi/2). At bright points it is 4 I0 and at dark points it is 0. For two incoherent sources the cos squared term averages to one half over the random phases, so every point just gets I = 2 I0, a uniform sum with no fringes. Note that in both cases the average energy is conserved (average of 4 I0 cos squared is 2 I0).
Ordinarily no. Even two lasers usually do not keep a locked phase relation over time, so they behave as incoherent for simple interference. A single laser split by a beam splitter or two slits gives coherent beams. Lasers ARE much more coherent than bulbs (long coherence length), which is why one laser split into two shows very clear interference, but two independent lasers are not automatically coherent.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Coherent sources keep a constant phase difference with time and give a stable interference pattern; incoherent sources have a randomly changing phase difference and give only a uniform, time-averaged intensity with no fringes.
Yes. Coherent sources must have the same frequency (and hence same wavelength in a given medium) along with a constant phase difference. Different wavelengths cannot keep a fixed phase relation.
For two incoherent sources of intensity I0 each, the intensities simply add: I = 2 I0 at every point, because the interference term averages to zero over random phases.
By splitting the light from a single source into two, for example using two slits (Young's experiment), a biprism, or mirrors. Both parts then share the same phase changes, so their phase difference stays constant.
Because a fixed pattern needs the phase difference at each point to stay constant in time. Only coherent sources keep that constant phase difference, so the bright and dark positions do not move.
