TEST
Diffraction & Interference
Name:
Level:
Year:
Duration:
________________________________
Intermediate Level Physics
1st Year
55 minutes
Group: ______
Grade:
Answer all questions. Calculators may be used. All necessary working must be shown.
(1)
A monochromatic source of light is placed in front of a double slit as shown.
(a)
Explain the meaning of monochromatic.
A monochromatic source of light is one which produces light waves of a
single wavelength and a single colour.
(b)
Slits S1 and S2 may be considered as coherent sources of light in order to
observe an interference pattern. What do you understand by the term
coherent?
Two sources are said to be coherent if they can produce identical waves with
a constant phase relationship.
(c)
(3)
Briefly describe what happens to light as it passes through slits S 1 and S2.
What is the name to this phenomenon?
At the two slits light diffracts. Closer to the screen, wavefronts originating
from the two slits overlap each other and produce a pattern of alternate
bright and dark fringes. This phenomenon is known as interference of light.
(d)
(3)
(3)
A series of bright and dark fringes are observed on the screen. What changes
would you observe if:
Test – Diffraction & Interference
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(i)
S1 and S2 are moved closer together,
More fringes with a wider fringe spacing are observed.
(ii)
(2)
the slits are kept the same distance apart, but made wider,
The number of observable fringes decreases and fringes appear brighter.
(2)
(iii) white light replaces the monochromatic source?
All colours of the visible spectrum show up on all bright bands except for the
central bright fringe. Violet is deviated least so it appears closer to the
central bright band.
(2)
(2)
Microwaves pass through two slits as shown. A detector is moved along the
line XY and detects a series of maxima and minima.
(a)
Explain, in terms of phase difference, the condition which must be satisfied
in order for the detector to detect:
(i)
a maximum,
For a maximum, constructive interference should take place with waves
meeting in phase. This happens if the path difference between the two
interfering waves equals an integral multiple of wavelengths.
(ii)
a minimum along XOY.
For a minimum, destructive interference should take place with waves
meeting out of phase. This happens if the path difference between the two
interfering waves equals half multiple of wavelengths.
(b)
(3)
(3)
Assuming that microwaves of wavelength 3cm are incident on a double-slit
with slit separation 0.1cm, calculate how far should a screen be placed from
the double-slit if two cosecutive bright fringes are to be separated by 2cm.
𝑦=
𝐷
𝑑
3𝐷
2=
0.1
𝐷 = 0.067𝑐𝑚
Test – Diffraction & Interference
(4)
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