AH physics exam tomorrow - help with waves question? thanks

[ah4p]

1. Homework Statement
If the optical path diff between two light waves is lambda/2 will they always interfere destructively? if this in true then wherever one wave undergoes aphase change of pi at one surface and the second wave does not then these two will always interfere destructively?

that's what the 2007 past paper question 10 suggests but i didnt think this was true
SQA answers:

https://us-mg6.mail.yahoo.com/ya/download?mid=2%5f0%5f0%5f2%5f4065%5fAGtL2kIAAA9%2bVUfn%2fgyiYFWt%2bYk&m=YaDownload&pid=3&fid=Sent&inline=1&appid=yahoomail

Homework Equations

In the attached question and SQA marking scheme

The Attempt at a Solution

If the optical path diff between two light waves is lambda/2 will they always interfere destructively? if this in true then wherever one wave undergoes aphase change of pi at one surface and the second wave does not then these two will always interfere destructively?[/B]thanks so much for any help - exam tomorrow :(

 

[andrevdh]

What other parameter(s) of the waves would make this statement untrue?