In an experiment, white light is shone vertically down upon a convex spherical l
ID: 1793028 • Letter: I
Question
In an experiment, white light is shone vertically down upon a convex spherical lens that is resting (curved side down) on a flat piece of glass. Which of the following statements correctly explain why is there a dark ring at the center of the resulting interference pattern?(select all that apply.)
1)Destructive interference of two reflected rays occurs
2)Destructive interference of the reflected ray and the refracted ray occurs
3)Constructive interference of two reflected rays occurs
4)The thickness of the air wedge is equal to one-half wavelength at that location
5)The thickness of the air wedge is negligible at that location
6)There is a 180 degree phase difference (due to reflection) between the two reflected rays
Explanation / Answer
The arrangement of the convex lens and the flat surfaces forms Newton’s rings when a white light is shone vertically.
In the Newton’s rings experiment, the rings form due to interference of waves reflected from the top surface of the flat surface and bottom surface of the curved surface of the lens.
The dark fringes in the Newton’s rings pattern are formed due to the destructive interference of the reflected waves.
Hence, the option (1) is correct.
From the above discussion, the Newton’s rings form only due to interference of waves reflected from the top surface of the flat surface and bottom surface of the curved surface of the lens. Hence, the option (2) is incorrect.
The constructive interference between the waves from top surface of the flat surface and bottom surface of the curved surface of the lens results the bright fringe. Hence, the option (3) is incorrect.
At the point of contact of lens and the glass plate, the thickness of the air film is very small than the wavelength of the wave. Therefore, the path length between the reflected waves is zero and the waves will be opposite in phase. So, these two waves form destructive interference and forms dark fringe at center of the interference pattern. Thus, the option (4) is in correct and the option (5) is correct.
The gap between the surfaces is constant throughout a fringe. The separation between two adjacent bright or dark fringes is one wavelength. Hence, the separation between the surfaces that is the thickness of the air wedge is equal to the one-half of the wavelength. Thus, the option (4) is correct and the option (5) is incorrect.
The destructive interference forms if the phase difference between two waves is 180 degree that is out of phase. Hence, the option (6) is correct.
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