Figure 4.1 shows an arrangement for demonstrating light interference. The wavelength of the light is $610\,\text{nm}$. The separation between the double slit and the screen is $2.7\,\text{m}$. A pattern of bright fringes and dark fringes is seen on the screen. The centres of the bright fringes are labelled B, while the centres of the dark fringes are labelled D. Point P is the centre of one specific dark fringe and point Q is the centre of one specific bright fringe. The distance across five bright fringes is $22\,\text{mm}$.
(a)[2]
State what the principle of superposition is.
(b(i))[1]
The light waves leaving the two slits are coherent. State the meaning of coherent.
(b(ii)1)[1]
State the phase difference of the waves meeting at Q.
(b(ii)2)[2]
Calculate the path difference, in $\text{nm}$, for the waves meeting at P.
(b(iii))[3]
Determine the slit separation $a$.
(b(iv))[1]
A greater frequency of visible light is now used. State and explain how the separation of the fringes changes.
(b(v))[2]
The intensity of the light incident on the double slit is now increased without changing its frequency. Compare the appearance of the fringes after this change with their appearance before it.
Worked solution & mark scheme
This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: “definition: when at least two waves arrive at the same point” …