Sharp lines measure light

Science · Waves, Light & Sound · ages 17-18

Name ______________________ Date ____________

What is the grating equation?
- d equals n lambda
- d sin theta equals n lambda
- sin theta equals n over d
A grating has 500 lines per millimetre. What is the slit spacing d?
- 2 times ten to the minus 6 metres
- 500 millimetres
- 600 nanometres
Orders that need sin theta above one can still be seen past 90 degrees.
Circle one: True False
With d of 2 times ten to the minus 6 metres and 600 nm light, what is the highest visible order?
Answer: ______________
With d of 2 times ten to the minus 6 metres and 600 nm light at order 2, what is sin theta?
Answer: ______________
Why are grating maxima sharper than double slit fringes?
- The screen sits closer
- The slits are wider
- Many slits cancel almost everywhere except exact angles
Ava plugs 500 straight in as d for a 500 lines per mm grating. What is her error?
- She forgot the order
- She squared the wavelength
- She used lines per mm as spacing
Sin theta is 0.6 at order 2 with d of 2 times ten to the minus 6 metres. What is lambda in metres?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

d sin theta equals n lambda · Path difference between neighbours must be a whole number of wavelengths.
2 times ten to the minus 6 metres · Divide one millimetre by 500 and convert: 0.002 mm is 2 times ten to the minus 6 metres.
False · Sine never passes one, so those orders vanish and cap the pattern.
3 · Push sin theta to one: d over lambda is 3.33, and rounding down gives 3.
0.6 · Sin theta is n lambda over d: 2 times 6 times ten to the minus 7 over d is 0.6.
Many slits cancel almost everywhere except exact angles · Many slits reinforce only at exact angles and wipe out the rest, splitting close lines.
She used lines per mm as spacing · 500 counts lines per millimetre, while d is one millimetre split 500 ways.
6e-7 · Rearrange to lambda equals d sin theta over n: 2 times ten to the minus 6 times 0.6 over 2.