Question

2.

a) As best you can, describe in words what the diffraction pattern from a double slit will look like.

b)For diffraction by a double slit, describe what will happen to the diffraction pattern as the separation between the two slits gets very small compared to the wavelength of the light and explain your answer.

Answer 1

Hi,

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(a)

YOUNG'S DOUBLE SLIT EXPERIMENT.

This is an experimental study of interference of light from two coherent sources of light. It consist of two slits S₁ and S₂ . S (slit) represents a pinhole illuminated by a monochromatic light source. The sunlight was incident on them through a shutter in the form of a slit. The slit of the shuter was symmetrically placed with respect the slits was observed on a screen, on which bands of coloured fringes was obtained. However if the light is coming from a Monochromatic light source, a number of dark and bright fringes can be observed on the screen. (Bright and dark spots or fringes can be seen on a screen kept at a distance(D). This is due to the interference produced by the light coming from the pinholes/slits S₁ and S_2.). When the slits S₁ and S₂ are covered with an Opaque object, the bands will disappear (instead we get a uniform illumination on the screen). From this experiment, Young concluded that, This pattern is due to the superposition of waves interference took place.

REFER THE FIGURE BELOW TO UNDERSTAND THE CONCEPT BETTER.

(b)

The separation between consecutive bright fringes or dark fringes is known as Fringe width().

We know that, Expression for finding fringe width is given by,

Where,

is the fringe width (m).

is the distance of the screen from the slit S₁ and S₂ (m).

is the wavelength of light. (m)

is the separation between the slit S₁ and S₂ (m).

IF separation between the slit S₁ and S₂ ( ) get very small as compared to wavelength of light (),

Then,

Fringe width,  

IF separation between the slit S₁ and S₂ ( ) get very small as compared to wavelength of light (), The fringe width will become greater than the distance of the screen from the slit S₁ and S_{2 .} Thus, as distance to the screen from slits increases the fringe width will increase and we can't distinguish dark and bright fringes.

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