Wave Optics- 1 Flashcards
nature of light
Light is a form of energy which travels through a medium in the form of transverse wave motion, the speed of light in a medium depends upon the medium.
what is a wavefront
It is defined as the continuous locus of all the particles in the medium, which are vibrating in the same phase at a given instant.
The speed with which the wavefront moves outwards from the
source is called the speed of the wave. The energy of the wave travels
in a direction perpendicular to the wavefront.
types of wave fronts
i) spherical wave front:
whenthe source of light is a point source, then the wavefront is a sphere with the source at centre
ii)Cylindrical wave front:
when the source of light is linear, eg slit: a slit. All pts equidistant from the source lie on a cylinder,ie, cylindrical wave front
iii)Plane wave front:
When a point/line source is at a very large distance, a small portion of the spherical or cylindrical wave front appears to be a plane.
There can also be converging or diverging wavefront.
State huygen’s principle
each point of the
wavefront is the source of a secondary disturbance and the wavelets
emanating from these points spread out in all directions with the speed
of the wave. These wavelets emanating from the wavefront are usually
referred to as secondary wavelets and if we draw a common tangent
to all these spheres, we obtain the new position of the wavefront at a
later time.
«-0-»
each point on a given wave front( primary wavefront) act as a source for a new secondary distrubance called secondary wavelets.
the secondary wavelets travel spread out in all directions in the medium, with the speed of the wave in the medium
the newshape and position of the wavefront at any instant is given by the forward envelope of the secondary wavelets.
shortcoming of huygen’s principle
Huygens argued that the amplitude of the
secondary wavelets is maximum in the forward direction and zero in the
backward direction; by making this adhoc assumption, Huygens could
explain the absence of the backwave. However, this adhoc assumption is
not satisfactory and the absence of the backwave is really justified from
more rigorous wave theory
behaviour of wavefront on passing through a prism
different parts of a prism travel different thickness/distance through the prism, maximum distance at the bottom and minimum at the top.
The lower portion slowed down the most while the upper portion bends the least.
As the speed of light is less in glass than in air, the refracted wavefront is tilted towards the base of the prism.
behaviour of wavefont on passing through a convex lens
i) infinte
ii)source at focus
i) When the source is an infinity, it produces a plane wavefront. The different parts of the wavefront travel through different thicknesses of the lens, maximum in the middle and the minimum at the edges. Since, light travels slower in air, the refracted wavefront is a converging spherical wavefront
ii) When the source is at focus, it produces a diverging spherical wavefront. The refracted rays are parallel to principal axis, so refratcted wavefront is plane wavefront
what are superposition principles
At a particular
point in the medium, the resultant
displacement produced by a number of
waves is the vector sum of the displacements produced by each of the waves
reflection of wavefront by a concave mirror
if a planewavefront is incident, the the reflected wavefront is converging spherical
if source is at focus, and the wavefront is spherical diverging, the relfected wavefront is plane wavefront
