A student sets up apparatus to observe the double-slit interference of monochromatic light, as shown.

Question 10

A student sets up apparatus to observe the double-slit interference of monochromatic light, as shown.

Interference fringes are formed on the screen.

Which change would increase the distance between adjacent fringes?

A Decrease the distance between the two slits.

B Decrease the width of each slit.

C Move the screen closer to the double-slit.

D Use light of a higher frequency.

Reference: Past Exam Paper – November 2013 Paper 13 Q29

Solution:

Answer: A.

For double-slit interference:

x = λD / a

where  x is the fringe separation
            λ is the wavelength of the light
            D is the distance between the screen and the slits
            a is slit separation


Choice A (decrease a): The fringe separation would increase. [A is correct]

Choice B: will not affect fringe separation directly

Choice C (decrease D): The fringe separation would decrease.
 
Choice D (decrease λ): The fringe separation would decrease.