wave optics Flashcards
Define wavefront
Wavefront is the locus of points, which oscillate in phase or a wavefront is
defined as a surface of constant phase.
.Explain Huygens Principle
According to Huygens principle, each point of the wavefront acts as a
source secondary wavelets and if we draw a common tangent to all these
secondary wavelets, we obtain the new position of the wavefront at a later
time.
What are coherent sources?
Two sources are said to be coherent if they emit light waves of same
frequency and same phase or constant phase difference.
Define Interference.
Interference is the phenomenon in which two waves superpose to form a
resultant wave of greater or lower amplitude
Write the Condition for constructive interference
If the path difference at a point is an integral multiple of
λ, there will be constructive interference and a bright
fringe is formed at that point
S2P – S1P = nλ where (n = 0, 1, 2, 3,…)
.Write the condition for destructive interference
If the path difference at a point is an odd integral multiple
of λ/2 , there will be destructive interference and a dark
fringe is formed at that point
S2 – S1P = = (n+ 𝟏/𝟐 ) λ where (n = 0, 1, 2, 3,…)
Two sodium lamps illuminating two pinholes cannot produce
interference fringes. Why?
The light waves coming out from two independent sources of light(like a
sodium lamp) will not have any fixed phase relationship and would be
incoherent and cannot produce interference pattern.
Explain Young’s double slit experiment to produce interference Pattern.
Thomas Young made two pinholes S1 and S2 (very close to each other) on
an opaque screen. These were illuminated by another pinholes which is
illuminated by a bright source. Light waves spread out from S and fall on
both S1 and S2. S1 and S2 then behave like two coherent sources and
interference pattern with alternate bright and dark fringes is formed on
the screen. The central fringe will be bright.
Define band width(fringe width) of interference pattern.
Write the expression for the fringe width (Band width) of interference pattern.
The distance between two consecutive bright bands or dark bands
is called bandwidth (β).
β = λ𝐷/d
Write the condition to produce good interference pattern
▪ Sources must be coherent
▪ The screen must be placed at a large distance from coherent
sources(D large)
▪ The coherent sources must be close to each other.(d small)
What is Diffraction?
Diffraction is the phenomenon of bending of light around the corners of an
obstacle , into the region of geometrical shadow of the obstacle
Write the differences between interference and diffraction
interference:
1. has equally spaced bright and dark fringes
2.all bands are of same intensity
3.we calculate the interfernce patter by superposing 2 waves originating from the 2 narrow slits
4. fro a single slit of width a we get the first null of the interference pattern occurs at an angle λ/a
diffraction:
1.has a central bright maximum which is twice as wide as other maxima
2.the intensity falls as we go to successive maxima away from the centre on both sides
3.diffreaction patter is superposition of waves originating from each point on a single slit.
4.at the same angle λ/a we get the maximum for 2 narrow slits separated by width a
