Representing waves

1. Representing waves

2. Representing waves

There are two ways we can represent a
wave in a graph;

3. Displacement/time graph

This looks at the movement of one point of
the wave over a period of time
displacement
cm
1
0.1
-1
-2
0.2
0.3
0.4
Time s

4. Displacement/time graph

This looks at the movement of one point of
the wave over a period of time
displacement
cm
1
PERIOD
0.1
-1
-2
0.2
0.3
0.4
Time s

5. Displacement/time graph

This looks at the movement of one point of
the wave over a period of time
displacement
cm
1
PERIOD
0.1
-1
-2
0.2
0.3
0.4
Time s

6. Displacement/time graph

This looks at the movement of one point of
the wave over a period of time
displacement
cm
1
PERIOD
0.1
-1
-2
0.2
0.3
0.4
Time s
IMPORTANT
NOTE: This
wave could be
either transverse
or longitudnal

7. Displacement/distance graph

This is a “snapshot” of the wave at a
particular moment
displacement
cm
1
0.4
-1
-2
0.8
1.2
1.6
Distance cm

8. Displacement/distance graph

This is a “snapshot” of the wave at a
particular moment
displacement
cm
1
WAVELENGTH
0.4
-1
-2
0.8
1.2
1.6
Distance cm

9. Displacement/distance graph

This is a “snapshot” of the wave at a
particular moment
displacement
cm
1
WAVELENGTH
0.4
-1
-2
0.8
1.2
1.6
Distance cm

10. Displacement/distance graph

This is a “snapshot” of the wave at a
particular moment
displacement
cm
1
WAVELENGTH
0.4
-1
-2
0.8
1.2
1.6
Distance cm
IMPORTANT
NOTE: This
wave could also
be either
transverse or
longitudnal

11. Wave intensity

12. Wave intensity

This is defined as the amount of energy per
unit time flowing through unit area
It is normally measured in W.m-2

13. Wave intensity

For example, imagine
a window with an
area of 1m2. If one
joule of light energy
flows through that
window every second
we say the light
intensity is 1 W.m-2.

14. Intensity at a distance from a light source

I = P/4πd2
where d is the distance from the light source
(in m) and P is the power of the light
source(in W)

15. Intensity at a distance from a light source

I=
2
P/4πd
d

16. Sound intensity

The lowest intensity that can normally be
heard by a human ear is 1 x 10-12 W.m-2
This is a sound intensity level of 0 dB

17. Intensity and amplitude

18. Intensity and amplitude

The intensity of a wave is proportional to
the square of its amplitude
I α a2
(or I =
2
ka )

19. Intensity and amplitude

This means if you double the amplitude of a wave,
its intensity quadruples!
I=
2
ka
If amplitude = 2a,
new intensity = k(2a)2
new intensity = 4ka2

20. Surfers know this!

21. Let’s try some more questions!