Explain what is meant by coherence and interference.
Use Fig. 6.1 to determine the waves' frequency.
State the phase difference between the waves.
Use the principle of superposition to sketch, on Fig. 6.1, the resultant wave.
Fig. 6.2 shows the arrangement used to generate an interference pattern. Laser light of wavelength $\lambda = 546\,\text{nm}$ falls on slits $S_1$ and $S_2$. The separation of the slits is $0.13\,\text{mm}$. The screen is $85\,\text{cm}$ from the slits. Two points on the screen are marked $A$ and $B$. The path difference between $S_1A$ and $S_2A$ is zero. The path difference between $S_1B$ and $S_2B$ is $2.5\lambda$. The screen shows maxima and minima of light intensity.
Calculate the distance $AB$.
The laser is swapped for one that emits blue light. State and explain how the spacing between the maxima seen on the screen changes.