State what the principle of superposition means.
Coherent light is directed normally at two identical slits X and Y. The diffracted light from the slits overlaps to form an interference pattern on a screen that is $1.9\,\text{m}$ from the slits. Fig. 4.1 shows the setup and the central region of the bright and dark fringes on the screen. The slit separation is $0.65\,\text{mm}$. The distance between the centres of neighbouring bright fringes is $1.7\,\text{mm}$. Calculate the wavelength $\lambda$ of the light.
Light waves from slits X and Y in (b) reach a point between adjacent bright fringes on the screen. Fig. 4.2 shows how displacement varies with time for the waves arriving at the point where they meet. A student says: ‘The phase difference between the waves is $90^\circ$.’ Explain why this statement is correct.
State and explain whether the claim ‘The amplitude of the resultant wave is zero.’ is correct.
The width of each slit in (b) is reduced by the same amount. There is no change in the slit separation. Describe and explain the effect, if any, of this change on the appearance of the interference pattern.