A power point that explains the concept of interference patterns.

1. NTERFERENCE (15.
5)
1 1 3 5 0 1 4 8
L 2 H
P H Y S 1 0 1

2. HAT IS INTERFERENC
When you combine 2 or more electromagnetic waves to form a resultant wave
Image from
Physics for Scientists and Engineers
An interactive Approach
The image shows the crests 2 waves travelling
in slightly different directions but with the same
wavelength and frequency
Indicates moments when the waves are out of phase
Destructive interference
Indicates moments when the waves are in phase
Instructive inference

3. CONSTRUCTIVE
Constructive interference occurs when the 2 waves (same frequency and
wavelength) produced by 2 point sources are in phase
In terms of Path difference for
constructive interference to occur:
Path difference between 2 sources must
be an integer multiple of wavelength
In terms of radians or degrees,
Path difference : integer of 360o or 2π
Wave 1
Wave 2

4. DESTRUCTIVE
Destructive interference occurs when the 2 waves (same frequency and
wavelength) produced by 2 point sources are out of phase
In terms of Path difference for destructive
interference to occur:
Path difference for one of the source must
be an integer multiple of wavelength and
the other half an integer of wavelength
In terms of radians or degrees,
Path difference : integer of 180o or π
Wave 2
Wave 1

5. STRUCTIVE VS CONSTRUCT

6. PATH DIFFERENCE
Path difference is the difference in length between two waves
Can be measured in wave length, radians and degrees
Constructive path difference Destructive path difference
In terms of :
Radians : 2π
Degrees : 360o
Wave length : λ
In terms of :
Radians : π
Degrees : 180o
Wave length : λ/2
Path difference equation : d2-d1 =√[ (x2- x1)2 + (y2-y1)2] – √[(x2- x1)2 – (y2-y1)2]
______________ _____________
Where x1 and y1 are the coordinates of wave 1 and x2 and y2 are the
coordinates of wave 2

7. TEST YOURSELF
A person stands at a point
where 2 waves
with the same wave lengths
and frequency interfere. What is
the new path difference for the
observer if he was located at
x=4 and y=5. Could we say
there is constructive or
destructive interference?
(0,+1)
(0,-1)

8. TEST YOURSELF
1. Write out the path difference equation
_____________ ____________
Hint: Use the equation for path difference
2. Plug in the x and y values, then solve
∆d =√[(4+0) 2 + (5-1)2] – √[4+0)2 – (5-1))2]
∆d = √41
∆d = 6.4
______________ _____________
As the value is neither an integer value of wave length, or an integer and a
half value, there is neither destructive or constructive interference at the point
where the observer stands
d2-d1 =√[ (x2- x1)2 + (y2-y1)2] – √[(x2- x1)2 – (y2-y1)(0,+1)
(0,-1)

9. BIBLIOGRAPHY
Physics for Scientists and Engineers: An interactive approach, Hawkes, Iqbal et
al., Nelson Education Ltd. 2015
 PHYS 101 Text Book