As we know that when light falls on an object, it bends and move through the material, this is what refraction is. Also when the light bounces off the medium it is called a reflection. Let us know study reflection and refraction of waves by Huygen’s principle.
Reflection using Huygens Principle
We can see a ray of light is incident on this surface and another ray which is parallel to this ray is also incident on this surface. Plane AB is incident at an angle ‘ i ‘ on the reflecting surface MN. As these rays are incident from the surface, so we call it incident ray. If we draw a perpendicular from point ‘A’ to this ray of light, Point A, and point B will have a line joining them and this is called as wavefront and this wavefront is incident on the surface.
These incident wavefront is carrying two points, point A and point B, so we can say that from point B to point C light is travelling a distance. If ‘ v ‘ represents the speed of the wave in the medium and if ‘ r ‘ represents the time taken by the wavefront from the point B to C then the distance
BC = vr
In order the construct the reflected wavefront we draw a sphere of radius vr from the point A. Let CE represent the tangent plane drawn from the point C to this sphere. So,
AE = BC = vr
If we now consider the triangles EAC and BAC we will find that they are congruent and therefore, the angles ‘ i ‘ and ‘r ‘ would be equal. This is the law of reflection
We know that when a light travels from one transparent medium to another transparent medium its path changes. So the laws of refraction state that the angle of incidence is the angle between the incident ray and the normal and the angle of refraction is the angle between the refracted ray and the normal.
The incident ray, reflected ray and the normal, to the interface of any two given mediums all lie in the same plane. We also know that the ratio of the sine of the angle of incidence and sine of the angle of refraction is constant.
A plane wave AB is incident at an angle i on the surface PP' separating medium 1 and medium 2. The plane wave undergoes refraction and CE represents the refracted
wavefront. the figure corresponds to v_{2} < v_{1} so that the refracted waves bends towards the normal.
We can see a ray of light is incident on this surface and another ray which is parallel to this ray is also incident on this surface. As these rays are incident from the surface, so we call it incident ray.
Let PP’ represent the medium 1 and medium 2. The speed of the light in this medium is represented by v_{1} and v_{2}. If we draw a perpendicular from point ‘A’ to this ray of light, Point A, and point B will have a line joining them and this is called as wavefront and this wavefront is incident on the surface.
If ‘ r ‘ represents the time taken by the wavefront from the point B to C then the distance,
BC = v_{1}r
So to determine the shape of the refracted wavefront, we draw a sphere of radius v_{2}r from the point A in the second medium. Let CE represent a tangent plane drawn from the point C on to the sphere. Then, AE = v_{2}r, and CE would represent the refracted wavefront. If we now consider the triangles ABC and AEC, we readily obtain
where’ i ‘ and ‘ r ‘ are the angles of incidence and refraction, respectively. Substituting the values of v_{1} and v_{2} in terms of we get the Snell’s Law,
n_{1} sin i = n_{2} sin r
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1. What is refraction and reflection of plane waves? 
2. How does Huygens Principle explain refraction and reflection of plane waves? 
3. Can you explain how refraction and reflection affect the direction of plane waves? 
4. What factors determine the extent of refraction and reflection in plane waves? 
5. How are refraction and reflection of plane waves useful in practical applications? 

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