# Homework Help: N slit diffraction

1. Oct 15, 2015

### Sharlom

• This thread lacks the homework template because it was originally posted in a non-homework forum.
if I'm given grating width=5cm ,slits of width=0.0001cm separated by a distance of 0.0002cm
how many order would be observable at λ=5.5 x 10-5cm?
calculate the width of principle maximum?

please tell me the approach to do this problem

2. Oct 15, 2015

### Sharlom

to find observable orders,
In the websites it said use the formula dsinθ=mλ
then keep θ=90 and find m.
but why θ is put 90 degrees i'm not able understand?

3. Oct 15, 2015

### Sharlom

here is one more similar question..please clear these doubts.
How many orders will be visible if the wavelength of the incident light is 5000 Å and the number of lines on the grating is 2620 per inch?
Ans: First of all I will find distance between each slit
d=1/2620 inch=2.54/2620 cm=9.69 x 10-4cm
keep θ=90 and find m (this i did to find the number of visible orders)
substituting values,
using the formula dsinθ=mλ
(9.69 x 10-4 cm)sin90=m(5000 x 10-8 cm)
I got m=19.38
# here it is coming to be a decimal number,the thing I know is that m should not come in decmals so i have a doubt that is my answer correct?
#let's say i round off 19.38 to 19 then it seems that 19 orders are visible, but imagine if m=19.6 or 19.8 etc. if this i round it off to 20,will my answer be right?
or still 19 orders are only visible ,since 0.6 or 0.8 are not an integer?

4. Oct 16, 2015

### Sharlom

even if m comes to be 19.8 or 19.6 or 19.9 etc. it should be taken as m=19 only.thus 19 orders will be visible.

5. Oct 16, 2015

### Staff: Mentor

Imagine that you are standing right next to the slits. 0° means you are looking directly away from the slits, perpendicular to the screen that the slits are cut into. If you now turn through an angle greater than 90°, where are you looking? Can the light diffracted through the slits actually travel in that direction? (in the diagram below, the light travels from left to right)

6. Oct 16, 2015

Thank you..