Red Fringe Count in Young's Double Slit Experiment

[somecelxis]

Homework Statement

In a Young's double slit experiment , blue monochromatic light of wavelength 450nm is used to illuminate the slits which are 0.60mm apart. A screen is placed 1.5m form the slit is used to catch the interfrence pattern .
If red light is used (700nm) , how many red light will be seen? the ans is 7

Homework Equations

The Attempt at a Solution

dsinθ = m λ
0.6x10^-3 ( sin 90 ) = m (700x10^-9)
m = 857
no fringe can be seen = 857+ 857+1 ..
What's wrong with my working?

 

[elegysix]

where does sin(90) come from?

 

[somecelxis]

Sin90 beacuse I wamt to calculate themaximum order of fringes. Is my concept wrong?

 

[NascentOxygen]

no fringe can be seen = 857+ 857+1 ..
What's wrong with my working?

I would agree with you. Perhaps the question is meant to say something like "a 1 metre square screen is placed 1.5m from the slits".

Can you work backwards, and work out what screen dimension would show just 7 bands either side of the central maximum?

 

[somecelxis]

Do you meam the fringe is only formed on 1mm square of screen?

 

[NascentOxygen]

Do you meam the fringe is only formed on 1mm square of screen?

If you were wanting to see just the first order fringe, yes, that might be about right. But didn't you say the answer is n=7?

Could you check the question, and make sure you haven't got some detail wrong.

 

[nasu]

How is the part about blue light relevant? It seems that the question is incomplete or mixed-up somehow.

 

[somecelxis]

I would agree with you. Perhaps the question is meant to say something like "a 1 metre square screen is placed 1.5m from the slits".

Can you work backwards, and work out what screen dimension would show just 7 bands either side of the central maximum?

that's the whole question i have typed above...