Define the term 'wavefront of light'. A plane wave front $A B$ propagating from denser medium (1) into a rarer medium (2) is incident on the surface $P_1 P_2$ separating the two media as shown in figure.
Using Huygens' principle, draw the secondary wavelets and obtain the refracted wavefront in the diagram.
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Updated On 18th June 2025
Expert Explanation :
A source of light sends the disturbance in all the directions and continuous locus of all the particles vibrating in same phase at any instant is called as wavefront. $P_1 P_2$ represents the surface separating medium 1 and medium 2 as shown in figure.
.png)
Let $v_1$ and $v_2$ represents the speed of light in medium 1 and medium 2 respectively. We assume a plane wavefront $A B$ propagating in the direction $A^{\prime} A$ incident on the interface at an angle $i$. Let $t$ be the time taken by the wavefront to travel the distance $B C$.
$
\therefore \quad B C=v_1 t \quad[\because \text { distance }=\text { speed } \times \text { time }]
$
In order to determine the shape of the refracted wavefront, we draw a sphere of radius $v_2 t$ from the point $A$ in the second medium (the speed of the wave in second medium is $v_2$ ).
Let $C D$ represents a tangent plane drawn from the point $C$. Then
$
A E=v_2 t
$
$\therefore \quad C D$ would represent the refracted wavefront.
Concept Applied
- Huygens' principle tells us that each point on a wavefront is a source of secondary waves called secondary wavelets. A surface touching these secondary wavelets tangentially in the forward direction at any instant gives the new (secondary) wavefront at that instant.
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