CBSE Class 12 Physics 2023: question 26(b)

Hints

Open one hint at a time and try again before the next.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

1. Construction (diagram described): A plane wavefront $AB$ is incident at angle $i$ on the reflecting surface $MN$, with $A$ on the surface. Let $\tau$ be the time for the wavefront to advance from $B$ to $C$, so $BC=v\tau$. With centre $A$, draw a secondary wavelet of radius $AE=v\tau$ in the same medium. Draw the tangent plane $CE$ from $C$ to this wavelet. $CE$ is the reflected wavefront. Show the incident and reflected rays normal to the wavefronts.

   1 mark

2. Compare triangles $ABC$ and $AEC$: $AE=BC=v\tau$, since both are covered in the same time in the same medium.

   ½ mark

3. $\angle ABC=\angle CEA=90^{\circ }$, because a wavefront is perpendicular to its ray and the tangent plane is perpendicular to the radius.

   ½ mark

4. $AC$ is common to both triangles, so $△ABC\cong △CEA$ by RHS congruence.

   ½ mark

5. Therefore $\angle BAC=\angle ECA$, i.e. $i=r$. This is the law of reflection.

   ½ mark

Answer: Reflected wavefront CE; congruent triangles ABC and CEA give i = r

Revise the chapter

• Chapter: Wave Optics (notes, formulas, where marks are lost and every past question)
• Topic: Huygens Principle and Wavefronts