Find Distance for 1.2cm Circular Diffraction Pattern

[Plasmosis1]

Homework Statement

You want to photograph a circular diffraction pattern whose central maximum has a diameter of 1.2cm . You have a helium-neon laser (λ=633nm) and a 0.13-mm-diameter pinhole. How far behind the pinhole should you place the viewing screen?

Homework Equations

Y = λx/b
Y=radius of central maximum
b= slit diameter
x=distance from screen

The Attempt at a Solution

.012/2=633e-9*x/0.00013
x=1.23m <--- This answer is wrong. I don't know why.

 

[ehild]

Homework Statement

You want to photograph a circular diffraction pattern whose central maximum has a diameter of 1.2cm . You have a helium-neon laser (λ=633nm) and a 0.13-mm-diameter pinhole. How far behind the pinhole should you place the viewing screen?

Homework Equations

Y = λx/b
Y=radius of central maximum
b= slit diameter
x=distance from screen

The Attempt at a Solution

.012/2=633e-9*x/0.00013
x=1.23m <--- This answer is wrong. I don't know why.

The equation does not hold for a circular aperture. See http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/cirapp2.html#c2 or check your lecture notes.

Spoiler

Y =1.22 λx/b

ehild