Huygens' Principle and Wavefronts
Huygens' principle states that every point on a wavefront acts as a source of secondary wavelets, and the common tangent or envelope to these wavelets gives the position of the new wavefront after a short time.
In one line: Same phase makes the old wavefront; equal-radius wavelets make the construction; the forward envelope is the final new wavefront.
Concept Practice Coming SoonUnderstand it
- A wavefront is a surface on which all points of a wave are in the same phase.
- For a plane wave, the wavefront is plane; for a point source, it is spherical.
- Huygens' construction tracks how the wavefront moves forward without following individual light rays.
- Rays are drawn perpendicular to the wavefront and show the direction of energy propagation.
Example
Formula and rule
Assumption: all points on the given wavefront vibrate in the same phase and secondary wavelets travel with the same speed v in that medium.
Key steps: choose equal time t, draw wavelets of radius vt from several points on the old wavefront, and draw the common forward tangent.
Final result cue: this forward envelope is the next wavefront; rays are normal to it.
Backward wavelets are not used in the school-level construction because the observed propagation is forward.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
Suppose a plane wavefront travels in air with speed 3.0 × 108 m s−1.
- 1In 2.0 ns, distance travelled = vt = 3.0 × 108 × 2.0 × 10−9 = 0.60 m.
- 2The new plane wavefront is parallel to the original wavefront and 0.60 m ahead in the direction of propagation.
Method you can reuse
- 1State the assumption that every point on a known wavefront is in the same phase.
- 2Draw the initial wavefront, choose several points on it, draw secondary wavelets of equal radius vt for the same time interval, draw only the common forward envelope, and finally draw rays normal to the new wavefront to show the direction of propagation.
Sketch it in your answer
Draw a plane wavefront AB, secondary wavelets from several points on AB, and the new envelope A'B'. Mark rays perpendicular to AB and A'B'. For a spherical wavefront, show concentric surfaces centered at the source.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why are rays drawn perpendicular to a wavefront in Huygens' construction?
Think first, then show answerHide answer
Common wrong answer
A common wrong answer says that only the edge points of a wavefront produce secondary wavelets. The correct idea is that every point on the wavefront acts as a source.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
Huygens' principle states that every point on a wavefront acts as a source of secondary wavelets, and the common tangent or envelope to these wavelets gives the position of the new wavefront after a short time.
- 2
Add for 2 marks
Example: Light from a distant source reaching a small aperture can be treated as a plane wavefront because the source is very far away and the curvature of the wavefront is negligible over the aperture size.
- 3
Add for 3 marks
A wavefront is a surface on which all points of a wave are in the same phase.
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