Question
Two coherent electromagnetic waves are travelling in a vacuum. The two waves meet at a point. At this point, the two waves have different intensities.
Which statement about the waves is not correct?
(A) They have a constant phase difference at the point.
(B) They have the same amplitude at the point.
(C) They have the same frequency.
(D) They travel at the same speed.
Answer/Explanation
Ans: B
Question
The diagram shows a view from above of a double-slit interference demonstration.
L is a monochromatic light source with a vertical filament. B is a barrier with two narrow vertical
slits and S is a screen upon which interference fringes form.
The intensity is I at the point on the screen where the centre of the fringe pattern forms.
When one of the slits is covered, what is the intensity at the same point on the screen?
(A) $\frac{I}{\sqrt{2}}$
(B) $\frac{I}{2}$
(C) $\frac{I}{2\sqrt{2}}$
(D) $\frac{I}{4}$
▶️Answer/Explanation
Ans: D
Question
A laser produces a beam of light of wavelength 650 nm. The beam is incident normally on two slits that are a distance of 0.12 mm apart.
A screen is placed parallel to the slits. The bright interference fringes on the screen have a separation of 7.5 cm.
What is the distance between the screen and the two slits?
(A) 1.4 m
(B) 2.8 m
(C) 7.0 m
(D) 14 m
▶️Answer/Explanation
Ans: D
Question
A beam of light with power P has an area of cross-section A. The amplitude of the light waves in the beam is X. The beam of light is then changed to one with the same frequency but with an increased amplitude of 4X and an area of cross-section reduced to \(\frac{A}{3 }\). What is the power of the new beam?
A 1.3P
B 5.3P
C 12P
D 48P
▶️Answer/Explanation
Ans B