71.

In Young's double slit experiment, a third slit is made in between the double slits. Then

• intensity of fringes totally disappears

• only bright light is observed on the screen

• fringes of unequal width are formed

• contrast between bright and dark fringes is reduced

In a two slit experiment with monochromatic light fringes are obtained on a screen placed at some distance from the slits. If the screen is moved by 5 x 10^-2 m towards the slits, the change in fringe width is 3 x 10^-5 m. If separation between the slits is 10^-3 m, the wavelength of light used is

• $6000 \stackrel{\circ }{\mathrm{A}}$

• $5000 \stackrel{\circ }{\mathrm{A}}$

• $3000 \stackrel{\circ }{\mathrm{A}}$

• $4500 \stackrel{\circ }{\mathrm{A}}$

In a Fraunhofer diffraction experiment at a single slit using a light of wavelength 400 nm, the first minimum is formed at an angle of 30°. The direction θ of the first secondary maximum is given by

• ${\sin }^{-1} \left(\frac{2}{3}\right)$

• ${\sin }^{-1} \left(\frac{3}{4}\right)$

• ${\sin }^{-1} \left(\frac{1}{4}\right)$

• ${\tan }^{-1} \left(\frac{2}{3}\right)$

Maximum diffraction takes place in a given slit for

• γ-rays

• ultraviolet rays

• infrared light

• radiowaves

An unpolarised beam of intensity I₀ falls on a polaroid. The intensity of the emergent light is

• $\frac{{\mathrm{I}}_0}{2}$

• I₀

• $\frac{{\mathrm{I}}_0}{4}$

• Zero

Which of the following is a dichroic crystal ?

• Quartz

• Tourmaline

• Mica

• Selenite

For the constructive interference the path difference between the two interfering waves must be equal to

• (2n + 1) λ

• 2nπ

• nλ

• $\left(2\mathrm{n} + 1\right) \frac{\mathrm{\lambda }}{2}$

G P Thomson experimentally confirmed the existence of matter waves by the phenomena

• diffraction

• refraction

• polarisation

• scattering

Young's double slit experiment gives interference fringes of width 0.3 mm. A thin glass plate made of material of refractive index 1.5 is kept in the path oflight from one of the slits, then the fringe width becomes

• zero

• 0.3 mm

• 0.45 mm

• 0.15 mm

Red light of wavelength 625 nm is incident normally on an optical diffraction grating with 2 × 10⁵ lines/m. Including central principal maxima, how many maxima may be observed on a screen which is far from the grating?

• 15

• 17

• 8

• 16