Question

Statement-1: In Young’s experiment, the fringe width for dark fringes is the same as that for white fringes.
Statement-2: In Young’s double slit experiment performed with a source of white light, only black and bright fringes are observed.
A. Statement-1 is True, Statement-2 is True, Statement-2 is a correct explanation for Statement-1.
B. Statement-1 is True, Statement-2 is True, Statement-2 is NOT a correct explanation for Statement-1.
C. Statement -1 is False, Statement-2 is True.
D. Statement -1 is True, Statement-2 is False.

Answer 1

Hint: Use concept of Young’s Double Slit experiment. Ratio of bright and dark fringes is one. Fringe width is the distance between consecutive dark and bright fringes. For statement 2, use the concept of experiment on prisms when white light passes through it.

Complete step-by-step answer:

First understand what the question wants to convey.
Statement 1 explanation: In young’s experiment, when light passes through a slit an bright and dark fringes are obtained. So it is said that the width for both dark and bright are the same.
Statement 2 explanation: It says that in young people's experiments, experiments are done by using white light and white light has passed through the slit so by using this source only bright and dark fringes are seen.
We know that fringe width is given by,
$width=\dfrac{\lambda D}{d}$
Where,
$\lambda $= wavelength of source used
D = distance between screen and slit
d = distance between slits
From this we can conclude that our fringe width is dependent on wavelength, distance between screen and slit and distance between slit. Colour of the fringe does not depend upon these three factors.
We know that, width of dark and bright are the same and therefore the distance between two successive dark and bright will also be the same.
The Right width distance between two successive maxima or two successive minima is the same. Therefore the fringe width of dark and bright bands will be the same.
Therefore our statement 1 will be correct.

Now we know that, white light contains 7 colours. When we use white light as a source in the young’s double slit experiment then at the centre, a bright fringe has formed and coloured fringe will be formed on either side. So I will get a fringe of not dark and bright but fringes of the whole seven colours.
Therefore, Statement 2 is false.

Note: Alternate word for fringe is band. White fringe resembles a bright one. Fringe width will remain constant throughout on screen. White light contains seven colours. Monochromatic light must be used to get sharp dark and bright fringes.