Everything Science Grade 12

11.6 CHAPTER 11. 2D AND3D WAVEFRONTS

Step 2 : Applicable principles

We know that there is arelationship between theslit width, wave-

length and interferenceminimum angles:

sin θ =

mλ

a

We can use this relationship to find the width bysubstituting what

we know and solving for the width.

Step 3 : Substitution

sin θ =

532 × 10 −^9 m

a

sin 20. 77 o =

532 × 10 −^9

a

a =

532 × 10 −^9

0. 354666667

a = 1500× 10 −^9

a = 1500 nm

The slit width is 1500 nm.

See simulation: VPpil atwww.everythingscience.co.za)

11.6 Shock Waves andSonic Booms

ESCFF

Now we know that the waves move away from the source at the speed ofsound. What

happens if the source moves at the same timeas emitting sounds? Once a sound

wave has been emittedit is no longer connected to the source so if thesource moves

it doesn’t change the way the sound wave is propagating through themedium. This

means a source can actually catch up to the sound waves it has emitted.

The speed of sound is very fast in air, about 340 m· s−^1 , so if we want to talk about a

source catching up to sound waves then the source has to be able to move very fast. A

good source of sound waves to discuss is a jet aircraft. Fighter jets can move very fast

and they are very noisyso they are a good source of sound for our discussion. Here

are the speeds for a selection of aircraft that canfly faster than the speedof sound.