## Saturday, May 17, 2008

### Past JEE OQ - Wave Motion - Light Waves-1

Past JEE Objective Questions Wave Motion - Light Waves

Questions on light waves (Young’s double slit experiment)
1982
1. In Young’s double slit experiment the interference pattern is found to have an intensity ratio between bright and black fringes as 9. This implies that

a. the intensities due to the two slits are 5 units and 4 units respectively.
b. the intensities at the screen due to two slits are 4 units and 1 unit respectively.
c. the amplitude ratio is three.
d. the amplitude ratio is two.

1984
1. White light is used to illuminate the two slits in a Young’s double slit experiment. The separation between the slits is b and the screen is at a distance d (d>>b) from the slits. At a point on the screen directly in front of one of the slits, certain wavelengths are missing. Some of these missing wavelengths are

a. λ = b²/d
b. λ = 2b²/d
c. λ= b²/3d
d. λ= 2b²/3d

1986
1. In Young’s double slit experiment, the two slits act as coherent sources of equal amplitude A and of wavelength λ. In another experiment with the same set p the two slits are sources of equal amplitude A and wavelength λ, but are incoherent. The ratio of the intensity of light at the mid point of the screen in the first case to that in the second case is --------------.

1987
1. In a Young’s double slit experiment performed with a source of white light, only black and white fringes are observed.

1994
1. A beam of light of wavelength 600 nm from a distant source falls on a single slit of 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away. The distance between the first dark fringes on either side of the central bright fringe is

a. 1.2 cm
b. 1.2 mm
c. 2.4 cm
d. 2.4 mm

1995
1. In an interference arrangement similar to Young’s double-slit experiment, the slits S1 and S2 are illuminated with coherent microwave sources, each of frequency 10^6 Hz. The sources are synchronized to have zero phase difference. The slits are separated by a distance of d = 150 m. The intensity of I(θ) is measured as a function of θ, where θ is defined as shown in the figure*. If I0 is the maximum intensity, the I(θ) for 0≤ θ ≤90° is given by

a. I(θ) = I0/2 for θ = 30°
b. I(θ) = I0/4 for θ = 90°
c. I(θ) = I0 for θ = 0°
d. I(θ) is constant for all values of θ

*(θ is the angle between the line joining the mid point of slits and its image on the screen and the line joining the image on the screen with mid pint of the slits)

1998
1. A parallel monochromatic beam of light is incident normally on a narrow slit. A diffraction pattern is formed on a screen placed perpendicular to the direction of the incident beam. At the first minimum of the diffraction pattern, the phase difference between the rays coming from the two edges of the slit is

a. 0
b. π/2
c. π
d. 2 π

1999.
1. Yellow light is used in a single slit diffraction experiment with slit width of 0.6 mm. If yellow light is replaced by X-rays, then the observed pattern will reveal

a. that the central maximum is narrower.
b. more number of fringes
c. less number of fringes
d. no diffraction pattern

2000
1. In a doble slit experiment, instead of taking slits of equal widths, one slit is made twice as wide as the other. Then, in the interference pattern

a. the intensities of both the maxima and minima increase.
b. the intensity of the maxima increases and the minima has zero intensity.
c. the intensity of the maxima decreases and that of the minima increases.
d. the intensity of the maxima decreases and the minima has zero intensity.

2001
1. In a Young’s double slit experiment, 12 fringes are observed to be formed in a certain segment of the screen when light of wavelength 600 nm is used. If the wavelength is changed to 400 nm, number of fringes observed in the same segment of the screen is given by

a. 12
b. 18
c. 24
d. 30

2002
1. In an ideal double slit experiment, when a glass plate (refractive index 1.5) of thickness t is introduced in the path of one of the interfering beams (wavelength λ), the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass plate is
a. 2 λ
b. 2 λ/3
c. λ/3
d. λ

(optical path concept is to be used for this problem)

2004
1. In a Young’s double slit experiment, bichromatic light of wavelengths 400 nm and 560 nm are used. The distance between the slits is 0.1 mm and the distance between the slits and the screen is 1 m. The minimum distance between two minima is

a. 4 mm
b. 5.6 mm
c. 14 mm
d. 28 mm

2005
1. In a Young’s double slit experiment an electron beam is used to obtain interference pattern. If the speed of electrons decreased then

a. no interference pattern is observed.
b. distance between two consecutive fringes decreases
c. distance between two consecutive fringes increases.
d. distance between two consecutive fringes remains the same.

2. In a Young’s double slit experiment, the angular position θ of a bright fringe having intensity one fourth of the maximum intensity is given by

a. sin-1 (λ/4d)
b sin-1(λ/3d)
c. sin-1(λ/2d)
d. sin-1(λ/d)

(θ is the angle between the line joining the mid point of slits and its image on the screen and the line joining the image on the screen with mid pint of the slits)