Physics · Wave Optics · NEET
The single slit S makes the light look like it comes from one point source. That single wave then reaches both slits S1 and S2 together, so S1 and S2 send out waves that stay in step. Without S, light from different parts of the source would hit the slits with random phases and no steady fringes would form. So the single slit is what makes S1 and S2 coherent.
Coherent means the two slits have the same frequency and a fixed (unchanging) phase difference. Only then is the path difference at each screen point constant in time, so a bright point stays bright and a dark point stays dark. If the phase link kept changing, the bright and dark spots would jump around too fast to see and the screen would just look uniformly lit.
The path difference, which is the extra distance one wave travels compared to the other to reach that point. If path difference = nλ (n = 0, 1, 2...), the two waves arrive crest-on-crest and add up: bright fringe. If path difference = (2n − 1)λ/2 (half-odd multiple of λ), they arrive crest-on-trough and cancel: dark fringe. The central point (path difference zero) is always bright.
Two separate bulbs are independent sources. The phase difference between them changes randomly billions of times per second, so they are incoherent. The bright and dark positions would shift far too fast for the eye or screen to record, giving smooth uniform light instead of stripes. That is why YDSE splits ONE source into two, rather than using two sources.
Waves from S1 and S2 spread out (diffract) and overlap in the region beyond the slits. On the screen you get a row of equally spaced bright fringes separated by dark fringes. The middle one is the central bright fringe (zero path difference), and bright fringes appear on both sides wherever path difference is a whole number of wavelengths.
In Young's double slit experiment, using monochromatic light of wavelength λ, the intensity of light at a point on the screen where the path difference is λ is K units. The intensity of light at a point where the path difference is λ/3 will be
In a Young's double slit experiment, a student observes 8 fringes in a certain segment of screen when a monochromatic light of 600 nm wavelength is used. If the wavelength of light is changed to 400 nm, then the number of fringes he would observe in the same region of the screen is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is an experiment that proves light behaves like a wave. Light from one source passes through two very close slits, and the two overlapping waves form a pattern of bright and dark bands (fringes) on a screen.
It is the base of the whole wave optics chapter. Understanding coherent sources and path difference here lets you solve fringe width, bright and dark fringe conditions, intensity, and medium-shift problems, which are asked almost every year.
Coherent sources have the same frequency and a constant phase difference. In YDSE the two slits are coherent because they come from the same single source, so they always stay in step.
A bright fringe forms where the path difference between the two waves equals a whole number of wavelengths: path difference = nλ, with n = 0, 1, 2, 3 and so on. The central fringe (n = 0) is bright.
A dark fringe forms where the path difference is a half-odd multiple of the wavelength: path difference = (2n − 1)λ/2, with n = 1, 2, 3 and so on. Here the two waves cancel each other.
No, two independent lasers are usually incoherent, so their phase difference keeps changing and no steady fringes appear. YDSE works by splitting one source into two so they remain coherent.
