Two slits that paint with light

Science · Waves, Light & Sound · ages 16-18

Name ______________________ Date ____________

A bright fringe sits where the path difference is a whole number of wavelengths.
Circle one: True False
What does coherent mean?
- Same frequency with a fixed phase link
- Same brightness with random phases
- Same colour with different speeds
In w equals lambda D over s, what is s?
- The screen distance
- The slit separation
- The wavelength
Fringes are 0.006 m apart with slit gap 0.001 m and screen 2 m away. Lambda equals w s over D. Give lambda in metres.
Answer: ______________
The slit gap doubles with all else fixed. What happens to fringe spacing?
- It halves
- It doubles
- It stays the same
Where do two waves cancel on the screen?
- Where the paths are exactly equal
- Where the path gap is half a wavelength
- Where both slits are widest
A pupil lights each slit with its own bulb and expects fringes. What is wrong?
- Slits must be lit from the sides
- Bulbs are too dim for screens
- Separate bulbs fire at random, so no fixed phase link holds
A red laser is swapped for blue with all else fixed. What happens to the fringes?
- They squeeze together
- They spread wider
- They vanish

Answer key

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

True · Whole gaps arrive in phase, crest to crest.
Same frequency with a fixed phase link · Coherent waves hold their phase relationship over time.
The slit separation · Lowercase s is the gap between the two slits.
0.000003 · Multiply 0.006 by 0.001 and divide by 2 to get 0.000003.
It halves · Spacing goes as one over s, so doubling s halves w.
Where the path gap is half a wavelength · Half wavelength gaps arrive in antiphase and cancel dark.
Separate bulbs fire at random, so no fixed phase link holds · Without one parent wave the sources are incoherent and fringes wash out.
They squeeze together · Blue has a shorter wavelength, so the pattern shrinks.