Help with interference question AH physics? exam tomorrow :(

[ah4p]

Could someone please explain this question to me I'll include what I thought the answer was and the actual SQA answers.

Please can you read my answer just to see what I'm doing wrong and what I don't understand

It's from the 2009 past paper AH physics

Thanks so much I really appreciate any help

exam tomorrow ;(
1. Homework Statement

Homework Equations

these are gien below in my attampt and in the SQA marking scheme included below

The Attempt at a Solution

this is the answer given by sqa

thanks very very very much

 

[BvU]

Your 2 n t looks more like 4 n t and what you write down doesn't correspond to what you calculate.

Furthermore, you and sqa both seem to know $\lambda$. We don't.

So: work a little less hasty, and you'll be just fine. Good luck tomorrow !

 

[ah4p]

Your 2 n t looks more like 4 n t and what you write down doesn't correspond to what you calculate.

Furthermore, you and sqa both seem to know $\lambda$. We don't.

So: work a little less hasty, and you'll be just fine. Good luck tomorrow !

sorry λ is 580nm

thanks so much for ur reply