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Define a wavefront. Using Huygen's principle, verifyi the laws of reflection at a plane surface. |
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Answer» Solution :A wavefront is defined as a surface of contant phase. Let us consider a plane wavefront AB incident obliquely on a plane reflecting surface MM.. Let one end A of wavefront strikes the mirror at an angle `i` but the other end B has still to cover DISTANCE BC. The required for thie will be `l=(BC)/(c)`, where c is the speed of light. According to Huygen.s principle point A starts emitting secondary wavelents and in time t, these will cover a distance `ct=c.(BC)/(c)=BC` and as radius, draw a circular arc. draw tangent CD on this arc from the point C. OBVIOUSLY, CD is the reflected INCLINED at an angle r. Obviously the incident and reflected wavefront both are in the plane of paper.  Again in `DeltaABC` and `DeltaADC`, we have `BC=AD` (by construction) and AC is common. So, the two triangles are congruent and, HENCE `angleBAC=angleDCA` or `anglei=angler` i.e., the angle of reflection is EQUAL to the angle of incidence. |
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