# Why doesn't a backward wave exist? (Huygens principle) [duplicate]

Since every point on a wavefront act as a source of secondary wave (wavelets) then why do we get only forward wavefront not backward. Huygens principal says that amplitude of the backward wave is zero, but why and how it happens?

• Welcome New contributor Aakanksha sharma! I've flagged your question for moderator attention for the "very low quality" reason. Apr 25, 2020 at 15:17
• Which "previous answer"?
– user258881
Apr 25, 2020 at 15:19
• ... and in what way are not your satisfied? Apr 25, 2020 at 15:38
• @DavidWhite When we apply Huygen's principle, there are two directions where the light wavefront can propagate, one in the direction where it is going and the other in the direction where it came from. The OP is asking the explanation for the absence of the latter way of propagation.
– user258881
Apr 25, 2020 at 17:41
• Does this answer your question? How does Huygens Principle incorporate the unidirectional property of a traveling wave? Apr 29, 2020 at 1:59

Today the principle could be explained as follows.

Every medium has an elasticity and a viscosity. In simple words, the first describes how deeply a body can penetrate or shift the medium over time. The second describes how the medium around is displaced over time.

Imagine a hammer falling lightly on a metal block. The hammer deforms the metal elastically at this point and the metal gives way. In which direction? In all directions. Where the metal is homogeneous, at the same rate. What you get is Huygens (semi)spherical wave.

Note that this wave has a longitudinal and a transverse component. In the direction of the hammer blow, the transverse component predominates (as with sound), and perpendicular to it all around the surface, the longitudinal component predominates.

Absolutely important is that the initial point of the disturbance determines the propagation direction. For a slit, the edges and the wall are the two disturbances. The edges bend the wave behind the edges of the slit, the wall reverse the movement direction.

• Please give some relevant information about the formation of the backward wave.
– user262060
Apr 30, 2020 at 4:57
• By backward wave I mean a wave travelling in the backward Direction just like it goes in the forward direction ( without any obstacle).
– user262060
Apr 30, 2020 at 5:05
• That's why I asked why it doesn't happen
– user262060
Apr 30, 2020 at 5:07
• Yes you are right but my question was about the Huygens principle which says that every point on the wavefront act as a source of secondary wave so why doesn't the secondary wave goes in backward direction for example think of a sphere expanding (same as a secondary wave) , the sphere expands in all the directions equally .So why doesn't it happen to the secondary wave???
– user262060
Apr 30, 2020 at 5:23
• Because than you have to take every point around and conclude it into the calculations. They cancel each other out. Apr 30, 2020 at 5:28

Huygens original description of wave propagation did not adequately explain the backward wave. Elimination of the backward wave is the reason for the 'obliquity factor' or 'inclination factor' that was added by Fresnel/Kirchhoff. It adjusts the strength of the Huygen's wavelets as a function of the direction of propagation of each segment of the wavelet so that there is no backward wave. It is:

$$1 + \cos \theta,$$

where $$\theta$$ is the angle between the normal to the original wavefront and the normal to the secondary wavefront.

Google "obliquity factor" and also see this:

Intuition of inclination factor in Kirchhoff's diffraction law

However the obliquity factor is often considered arbitrary and may not be necessary--see my

"Huygens' Principle geometric derivation and elimination of the wake and backward wave"

https://www.researchgate.net/publication/340085346

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