# Thread: Variable conditions of double slit interference

1. ## Variable conditions of double slit interference

Hi
Can anyone help on this. The questions is as follows:

In a double slits experiment using red light, a fringe pattern is observed on a screen at a fixed distance from the double slits. How would the finge pattern change if:

a) The screen is moved closer to the slits
b)one of the double slits is blocked completely.

I surmised the following:
a) The standard formula
W= (DX Wavelength)/s
W= Fringe seperation
D= distance of slit to the screen
s= distance between centre of the slits
Since D Represents the distance of the slit from the the screen, it is a proportional relationship to the fringe seperation(W). So, if the distance is decreased less spaced.

b)I am not sure of this. Blocking one of the double slits seems to imply a reduced wavelength. Red has one of the longest wavelengths. From the above formula, it seems to suggest the same effect as above. But a part of me is suggesting there is an increased wavelength.

2. a) The formula can be written as

$sW = D\lambda$

Keeping s, and lambda constant, as D decreases, W decreases, correct.

b) When you have only one diffraction, there is no more interference and the resultant is a single slit diffraction pattern.

3. Originally Posted by Unknown008
a) The formula can be written as

$sW = D\lambda$

Keeping s, and lambda constant, as D decreases, W decreases, correct.

b) When you have only one diffraction, there is no more interference and the resultant is a single slit diffraction pattern.
Thanks a lot