# Value of theta of first minimum

1. Sep 13, 2014

### desmond iking

1. The problem statement, all variables and given/known data
A parallel beam of monochromatic light of wavelength 580nm is incidentally normal on a diffraction grating. The separation between successive lines on the grating is 0.7μm as shown in the figure.
please refer to the photo , i am having problem with part (a)

2. Relevant equations

3. The attempt at a solution
my working is d sin θ = λ
sin θ = λ /d , where d is the seperation between slits.. by plug in value of λ = 580x10^-9 and d = 1/ (5000x10^2) , i have my θ = 47 ° , the ans given is 56°
why am i wrong?

#### Attached Files:

• ###### IMG_20140912_162943[1].jpg
File size:
121.2 KB
Views:
65
2. Sep 13, 2014

### Staff: Mentor

Where are you given that value for d?

Last edited: Sep 13, 2014
3. Sep 15, 2014

### desmond iking

here's what i mean :
sin θ = λ /d , where d is the separation between slits.. by plug in value of λ = 580x10^-9 and d = 0.7x10^-6 , i have my θ = 47 ° , the ans given is 56°

4. Sep 15, 2014