1. In Fresnel diffraction, the relative phase difference between the curved wavefront is ___________
a) Constant
b) Zero
c) Linearly increasing
d) Non-constant
Explanation: Since the radii of each half period zone are different, the distance traveled by each wavefront is different. Thus, the relative phase difference turns out to be non-constant.
2. In Fresnel Diffraction, the incident wavefront is _________
a) Hyperbolic
b) Linear
c) Spherical
d) Elliptical
Explanation: In Fresnel Diffraction, the interference takes place between the light waves reaching a point from different parts of the same wavefront. Thus, the incident wavefront is spherical or cylindrical.
3. The radius of the half period zone is proportional to __________
a) The wavelength of light
b) The square root of the frequency of light
c) The square root of the wavelength light
d) The frequency of light
Explanation: We know that the formula for the radius of half period zone = \(\sqrt{nb\lambda}\), where n is a natural number. Thus, it is proportional to the square root of wavelength light and inversely proportional to the square root of the frequency of light.
4. In Double Slit Fraunhofer Diffraction, some orders of interference pattern are missing. It is called ____________
a) Missing Spectra
b) Absent Spectra
c) End Spectra
d) Emission Spectra
Explanation: 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
5. Light of 5000 Å is incident on a circular hole of radius 1 cm. How many half period zones are contained in the circle if the screen is placed at a distance of 1 m?
a) 20
b) 200
c) 2000
d) 20000
Explanation: In this case, λ = 5000 Å = 5 X 10-5 cm, b = 1 m = 100 cm
Therefore, Number of half period zones = \(\frac{1}{λ}\)
= 1/5 X 10-5
= 20000.
6. Light of 6000 Å is incident on a circular hole and is received on a screen 50 cm away. What is the radius of the hole, if the intensity of light on the screen is 4 times the intensity without the hole?
a) 0.025 cm
b) 0.047 cm
c) 0.054 cm
d) 0.089 cm
Explanation: The intensity will be 4 times than in its absence if the radius of the hole is equal to that of the first half period zone.
Therefore, radius, r = \(\sqrt{b\lambda}\)
Here, b = 50 cm and λ = 6000 Å = 6 X 10-5 cm
r = 0.0548cm.
7. The zone plate behaves like a ___________
a) Concave Lens with multiple foci
b) Convex Lens with multiple foci
c) Convex Lens with single foci
d) Concave Lens with single foci
Explanation: In a zone plate, a much brighter image of an object is obtained at the screen, which shows the converging action of a zone plate. Also, it’s equation resembles that of a lens. Thus, the zone plate behaves like a convex lens with multiple foci.
8. Find the missing order for a double-slit Fraunhofer Diffraction pattern if the slit widths are 0.2 mm separated by 0.6 mm.
a) 1st, 5th, 9th, ….
b) 2nd, 6th, 10th, …
c) 3rd, 7th, 11th, ….
d) 4th, 8th, 12th, …
Explanation: We know, for interference maxima (e + d) sin θ = ±nλ and diffraction minima e sinθ = ±mλ.
Therefore, (a+b)/a=n/m
(0.2+0.6)/0.2=n/m
n = 4 m
As m = 1, 2, 3, …. Therefore, 4th, 8th, 12th, …. order interference maximum will be missing.
9. A screen is placed 2m away from the lens to obtain the diffraction pattern in the focal plane of the lens in a single slit diffraction experiment. What will be the slit width if the first minimum lies 5 mm on either side of the central maximum when plane light waves of wavelength 4000 Å are incident on the slit?
a) 0.16 mm
b) 0.26 mm
c) 0.36 mm
d) 0.46 mm
Explanation: Given: f = 2 m, x = 5 X 10-3 m, λ = 4 X 10-7 m, n=1
sin θ = \(\frac{n\lambda}{a}\), we have
a = \(\frac{n\lambda}{sin \theta}\)
= 1.6 X 10-4 m
= 0.16 mm.
Location for first maximum, x1 = \(\sqrt{\frac{3b(a+b)}{a}}\)
= 1.01 cm.
10. If a is the width of the slits and b the distance between the slits, then a + b is called as _________
a) Opacities
b) Transparency
c) Grating Constant
d) Grating value
Explanation: The sum of the width of the slits and the distance between the slits is called the grating constant of the grating. It can also be described as the separation between the corresponding point of two adjacent transparencies