# Questions and Solutions part 2

11.  Angular width of central maximum in the Fraunhoffer's diffraction pattern is measured. Slit is illuminated by the light of another wavelength, angular width decreases by 30%. Wavelength of light used is
(a) 3500 Å    (b) 4200 Å    (c) 4700 Å        (d) 6000 Å

12. A parallel beam of white light falls on a thin film whose refractive index is 1.33. If angle of incidence is 52 then thickness of the film for the reflected light to be coloured yellow (λ = 6000 Å) most intensively must be
(a) 14 (2n + 1) μm        (b) 1.4 (2n + 1) μm
(c) 0.14 (2n + 1) μm        (d) 142 (2n + 1) μm

13.  A two slit Young's experiment is done with monochromatic light of wavelength 6000 Å. Slits are 2 mm apart and fringes are observed on a screen placed 10 cm away from the slits. If a transparent plate of thickness 0.5 mm is placed in front of one of the slit, interference pattern shifts by 5 mm. Then refractive index of transparent plate should be
(a) 1.1    (b) 1.2    (c) 1.3        (d) 1.5

14. In Young's double slit experiment, using monochromatic light, fringe pattern shifts by a certain distance on the screen when a mica sheet of refractive index 1.6 and thichness 1.964 mm is introduced in the path of one of the two waves. If now mica sheet is removed and distance between slit and screen is doubled, distance between successive max. or min. remains unchanged. The wavelength of the monochromatic light used in the experiment is
(a) 4000 Å    (b) 5500 Å    (c) 5892 Å        (d) 6071 Å

15. In a two slit experiment with monochromatic light, fringes are obtained on a screen placed at some distance from the slits. If screen is moved by 5 × 10–2 m towards the slits, then change in fringe width is 3 × 10–5 m. If the distance between slits is 10–3 m then wavelength of the light used will be
(a) 4000 Å    (b) 6000 Å    (c) 5890 Å        (d) 8000 Å

16.    White light is used to illuminate two slits in Young's double slit experiment. Separation between the slits is b and the screen is at a distance d(>>b) from the slits. Then wavelengths missing at a point on the screen directly in front of one of the slit are

17.  In a Young's double slit experiment, angula width of a fringe formed on a distant screen is 0.1o. If wavelength of light used is 6000 Å , then distance between the slits will be
(a) 0.241 mm    (b) 0.344 mm    (c) 0.519 mm        (d) 0.413 mm

18. Monochromatic light of wavelength 5000 Å is incident on two slits separated by a distance of 5 × 10–4 m. Interference pattern is seen on the screen placed at a distance of 1 m from the slits. A thin glass plate of thickness 1.5 × 10–6 m and refractive index 1.5 is laced between one of the slits and the screen. If intensity in the absence of plate was Io then new intensity at the centre of the screen will be

19. In a Young's interference experimental arrangement incident yellow light is composed of two wavelength 5890 Å and 5895 Å. between the slits is 1 mm and the screen is placed 1 m away. Order upto which fringes can be seen on the screen will be

(a) 384               (b) 486            (c) 512                        (d) 589

20. Ratio of intensities between a point A and that of central fringe is 0.853. Then path difference between two waves at point A will be

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