Young's double slit experiment

1. Sep 6, 2015

libbytam

1. The problem statement, all variables and given/known data
In a Young's double slits experiment, a monochromatic light source of wavelength 700nm is used and the separation of the slits is 0.1mm. If 15 bright fringes are observed, what is the angle subtended by those fringes at the centre of the double slit?
A. 6.0°
B. 5.6°
C. 5.2°
D. 4.8°

the correct answer is B, but why..??
2. Relevant equations
λ = y • d / (m • L)

3. The attempt at a solution

λ = 7 x 10^-7 m
y = fringe separation = ?
d = slit separation = 1 x 10^-4 m
m = order value = 15
L = distance from slit to screen = ?

I've drawn a triangle which represents the double slit experiment and I know the final step will be using tan Θ. There'll be only 7 bright fringes on the one side so m = 7 (not sure if it's correct or not...)
However, I've tried to get the equation with two unknowns (y and L).
I got: (7 x 10^-7) = (y)(1 x 10^-4) / (7)L
(4.9 x 10^-6)L = (1 x 10^-4)y
as tanΘ = y/L = (4.9 x 10^-6)/(1 x 10^-4) = 0.049
Θ = 2.81° (correct to 3 sig. fig.)

....which is obviously not the correct answer... so if it's multiplied by 2 , Θ = 2.81 x 2 = 5.62, which is answer B!?

Is there a better way? Can someone please help me out?

2. Sep 7, 2015

blue_leaf77

The angle subtended means the angle formed between the 7th fringes on both sides and the center of the slit, so, multiplying by two is indeed the correct way.

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