# No of red fringe will be seen

1. Sep 12, 2014

### somecelxis

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. 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?

2. Sep 12, 2014

### elegysix

where does sin(90) come from?

3. Sep 13, 2014

### somecelxis

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

4. Sep 13, 2014

### Staff: Mentor

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?

5. Sep 13, 2014

### somecelxis

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

6. Sep 13, 2014

### Staff: Mentor

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.

Last edited: Sep 13, 2014
7. Sep 13, 2014

### nasu

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

8. Sep 13, 2014

### somecelxis

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