Diffraction MCQ Level -2

10 Questions MCQ Test Topic wise Tests for IIT JAM Physics | Diffraction MCQ Level -2

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Red light of wavelength 644 nm, from a point source, passes through two parallel and narrow slits which are 1.00 mm apart. Determine the distance between the central bright fringe and the third dark interference fringe formed on a screen parallel to the plane of the slits and 1.00 m away.


In 2-slit interference, the interference fringe spacing y is given by

y = λD/d (e.g. see 1st link).

where D is screen distance and d = slit separation

That's a standard formula, though you may have been taught it using different symbols.

In this question, D = 1.00m and d = 1.00mm = 0.00100m

y = 644x10⁻⁹ x 1.00 / 0.00100

. .= 6.44x10⁻⁴ m

. .= 0.644mm

The bright and dark fringes (B and D)are arranged outwards from each side of the centre like this



The first dark fringe is therefore y/2 from the centre and the third dark fringe is y/2 + y + y = 2.5y from the centre.

From the centre to the third dark interference fringe is 2.5 x 0.644 = 1.61mm.


Which of the following effects could not be observed for sound waves in air?


The correct answer is: polarization


In Double Slit Fraunhofer Diffraction, some orders of interference pattern are missing. It is called _____ 


In Double Slit Fraunhofer Diffraction, there are certain angles where the interference maxima and Diffraction minima overlap. These orders of interference pattern are missing in the pattern. It is known as Absent Spectra.


Determine the ratio of the wavelengths of two spectral lines if the second-order image of one line coincides with the third-order image of the other line, both lines being examined by means of the same grating.


The correct answer is: 3 : 2


A thin slit produces a fuzzy shadow because of.


The correct answer is: diffraction


A double-slit experiment is done in the usual way with 480-nm light and narrow slits that are 0.050 cm apart. At what angle to the straight-through beam will one observe (A) the third-order bright spot and (B) the second minimum from the central maximum?


The correct answer is: A- 0.17°, B - 0.083°


How shall a diffraction pattern change when white light is used instead of a monochromatic light? 


When white light is used instead of monochromatic light, then the central maximum remains white as all seven wavelengths meet there in the same phase. The first minimum and second maximum will be formed by violet color due to its shortest wavelength while the last is due to the red color as it has the longest wavelength. Thus, a colored pattern is observed. However, after the first few colored bands, the clarity of the band is lost, due to overlapping.


A spectrum of the Sun’s radiation in the infrared region is produced by a grating. What is the wavelength being studied, if the infrared line in the first order occurs at an angle of 25.0° with the normal, and the fourth-order image of the hydrogen line of wavelength 656.3 nm occurs at 30.0°?


The correct answer is: 2.22 x10-6 m


Green light of wavelength 500 nm is incident normally on a grating, and the second- order is diffracted 32.0° from the normal. How many lines/cm are marked on the grating?


The correct answer is: 5.30 x 10lines/cm


What is the effective distance between the source of light and the screen in Fraunhofer Diffraction? 


In Fraunhofer Diffraction, the source of light and the screen are effectively placed at infinite distance. Two convex lenses are used for achieving such a condition. Thus, the incident waveform is plane and the secondary wavelets are in the same phase at every point in the plane of the aperture.

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