AS OCR Waves Question

I'm having some issues with understanding the answer to a question on waves in the OCR G482 June 2011 paper.

Past Paper - http://www.ocr.org.uk/Images/65871-question-paper-unit-g482-electrons-waves-and-photons.pdf
Mark Scheme - http://www.ocr.org.uk/Images/62150-mark-scheme-unit-g482-electrons-waves-and-photons-june.pdf

The question is 5 a) ii - "Explain how Fig. 5.1 shows that the period of the wave is 1.5 ms".

I understand that the time period is the time it takes for one complete oscillation. But I don't understand how to interpret these graphs where the x and y axes are both units of distance. What exactly does it mean when it says that the "dotted line shows the displacement at a later time, t = 0.75ms"?

The mark scheme says that the "wave has moved along 0.5 wavelengths in 0.75ms" but I can't seem to grasp why that is the case. As far as I can see from the graph, the dotted line is (3/2) wavelengths. There's one complete cycle going from left to right and an extra quarter of a cycle. But that appears to be wrong? Also how do you know the direction of the wave?

I would appreciate if someone could breakdown how to determine the time period of this graph and also to interpret these type of graphs for future questions.

Thanks.

Understanding the graph:

The graph is basically a picture of a wire. So the x-axis is the horizontal distance along the wire, and the y-axis is the vertical position of the wire.

In other-words, if you flick a wire, and take a photo of it, that is what the graph is.

Understanding the dotted line:

Now the wave in the wire is going to move, so if you take a photo of the wire, and then take another photo some time later, the two photos will look different (unless of course the second picture is taken exactly 1 period in time after the 1st picture, then they will look the same).

Now the dotted line represents the photo after time 0.75 seconds, and the solid line represents the photo taken at time 0 seconds.

Determining the Period:

If we begin with the solid line, and shift it to the right until it matches the dotted line, how far have we shifted the line? It can be seen that you must shift the solid line 0.3 units to the right in order to match it up with the dotted line. 0.3 units is also half a wavelength.

This means that 0.75seconds represented half a wavelength, and thus 1.5 seconds is a whole wavelength, and thus the period.

But we don't know if the wave is moving to the right or the left?

That is true, but it doesn't matter in this case because if we moved our solid line to the left, we would still have to move it 0.3 units to match up with the dotted line, and thus our answer would be the same. (Note when imaging shifting the waves, you pretend the wave patterns extend infinitely beyond the written page)