Huygens' Principle and Wavefronts

State the assumption that every point on a known wavefront is in the same phase. Draw 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.

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.

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.

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.

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. 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. 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.

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. 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. Suppose a plane wavefront travels in air with speed 3.0 x 10^8 m s^-1. In 2.0 ns, distance travelled = vt = 3.0 x 10^8 x 2.0 x 10^-9 = 0.60 m. The new plane wavefront is parallel to the original wavefront and 0.60 m ahead in the direction of propagation. It is commonly asked as a statement-and-diagram question, or as the starting point for deriving reflection or refraction laws. Assertion-reason questions may test whether wavefronts are perpendicular to rays. 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.