# Homework Help: The Huygens–Fresnel principle

1. Oct 28, 2016

### Hamal_Arietis

1. The problem statement, all variables and given/known data

I don't understand about some words of this problem. Does "over time" mean? Why constructive interference (CI) appear?
2. Relevant equations
I think we use The Huygens–Fresnel principle

3. The attempt at a solution

But I don't know the physical phenomenon.

2. Oct 28, 2016

### andrevdh

The incident wave and reflected wave interferes with each other and produces regions (crests) of inteference that travels in a certain direction.
The question wants you to answer in which direction these constructive interference regions would travel.

3. Oct 28, 2016

### Hamal_Arietis

4. Oct 28, 2016

### andrevdh

Have a look at the concept of reflection in a mirror and combine this with what you know about the interference pattern of a double slit.

5. Oct 28, 2016

### Hamal_Arietis

There are many double slits not one . And the interference image is total of many double slits. How to know the direction of interference?

6. Oct 28, 2016

### Hamal_Arietis

Help me ??? Thanks

7. Oct 28, 2016

### TSny

Do any of the marked points below represent a point of constructive interference between the incident and reflected waves? If so, which?

8. Oct 28, 2016

### Hamal_Arietis

I think a, b and c. because they are in constructive interference area between the incident and reflected waves.

9. Oct 28, 2016

### Hamal_Arietis

If I seem the souce S is very far from wall. The constructive interference area between the incident and reflected waves will be rectilineal. And I take the S' image, we have a double slits SS'. The waves go to P before going to Q. So 1-a is the right direction. Is OK ?

Last edited: Oct 28, 2016
10. Oct 29, 2016

### TSny

The question asks about crests of constructive interference. In post #7 I should have been more specific and asked which of the 4 points is located on a crest of constructive interference. Only one of the points a, b, c, d is such a point.

11. Oct 29, 2016

### Hamal_Arietis

i think it is b. I dont know how the constructive interference appears? Because if constructive interference appears. Must have double slits or d2-d1=k lambda. I dont see any double slits. And cant calculate d2-d1. Does my idea in #9 correct?

12. Oct 29, 2016

### TSny

Interference occurs whenever two or more waves overlap. The double slit experiment is just one example of a case where waves interfere. In the situation that you are dealing with here, the reflected waves overlap with the incoming waves. So, they will interfere.

A crest of constructive interference occurs where the crest of one wave overlaps the crest of another wave. So, if you look at the point a, b, c, d in post #7, only one of those points is a place where the crest of an incoming wave overlaps the crest of a reflected wave. (The blue lines represent wave crests.)

13. Oct 29, 2016

### Hamal_Arietis

Is c right ?Intially I think your blue lines is the direction of the incident. so I choose b

14. Oct 29, 2016

### TSny

The blues lines are the wave crests. The lighter blue lines are wave crests of the incident wave. The green arrows show the direction of propagation of these incident wave crests. The darker blue lines are wave crests of the reflected wave. The orange arrows show the direction of propagation of these reflected wave crests.

At which of the points a b c d does a wave crest of the incident wave overlap with a wave crest of the reflected wave?

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15. Oct 29, 2016

### Hamal_Arietis

I think that is c. But how this connect with the direction of constructive interference appear ?

16. Oct 29, 2016

### Hamal_Arietis

The red points will be the crest of the incident wave overlap with a wave crest of the reflected wave.

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17. Oct 29, 2016

### TSny

Yes, point c is a point where crests are constructively interfering. You want to find the direction in which point c moves as the waves propagate. Draw the figure as shown. On the same figure, draw the waves as they would appear a little later in time.

18. Oct 29, 2016

### Hamal_Arietis

I understanded. Thanks

19. Oct 29, 2016

OK. Good.