State Huygen’s principle and verify laws of reflection using suitable diagram.
Ans. Huygens’ Principle:
Huygens’ principle states that every point on a wavefront can be considered as a source of secondary spherical wavelets that spread out in all directions with a speed equal to the speed of propagation of the wave. The new wavefront at any later time is the envelope tangent to these secondary wavelets.
Verification of the Laws of Reflection:
The laws of reflection state:
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The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
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The angle of incidence is equal to the angle of reflection.
Let’s verify these laws using Huygens’ principle:
Consider a plane mirror with an incident ray striking its surface. According to Huygens’ principle, each point on the incident wavefront generates secondary wavelets that spread out spherically. The diagram below illustrates this process:
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<- Incident Ray
\
\
\
\ Normal
\
\ Reflected Ray
\
According to Huygens’ principle, each point on the incident wavefront acts as a source of secondary wavelets. These wavelets propagate outward in all directions, reaching the plane mirror. The reflected wavefront is formed by constructing a tangent to the secondary wavelets at the mirror’s surface.
Now, let’s verify the laws of reflection:
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Plane of Incidence: Since each point on the incident wavefront generates secondary wavelets in all directions, the reflected wavefront is formed by the envelope tangent to these wavelets. This means that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane, thus verifying the first law of reflection.
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Angle of Incidence Equals Angle of Reflection: By construction, the angles between the incident ray and the normal (angle of incidence) and between the reflected ray and the normal (angle of reflection) are equal. This verifies the second law of reflection.
