5.4 Double slit interference Flashcards
Who first suggested the wave nature of light
Christiaan Huygens in the 17th century but it was rejected in favour of Isaac Newton’s corpuscular theory of light
What was Newton’s corpuscular theory of light
He considered that light was composed of tiny particles called corpuscles
His theory remained unchallenged for over a century until 1803 when Thomas Young demonstrated interference of light
How can you observe the interference of light
Can illuminate two closely spaced parallel slits using a suitable light source. The two slits act as coherent sources of waves which means they emit light waves with a constant phase difference and the same frequency. Alternate bright and dark fringes (‘Young’s fringes’) can be seen on a white screen placed where the diffracted light from the double slits overlaps. Evenly spaced and parallel to double slits
What happens if the single slit is too wide
Each part of it produces a fringe pattern which is displaced slightly from the pattern due to adjacent parts of the single slit. As a result, the dark fringes of the double slit pattern become narrower than the bright fringes, and contrast is lost between the dark and the bright fringes
The fringes are formed due to interference of light from the two slits:
Where a bright fringe is formed, the light from one slit reinforces the light from the other slit. Ie. The light waves from each slit arrive in phase with each other
Where a dark fringe is formed, the light from one slit cancels the light from the other slit.. In other words, the light waves from the two slits arrive 180 degrees out of phase
What is fringe separation
The distance from the centre of a bright fringe to the centre of the next bright fringe
Fringe separation equation
w = λD /s
Fringes become more widely spaced if:
The distance D from the slits to the screen is increased
The wavelength λ of the light used is increased
The slit spacing s is reduced. Note that the slit spacing is the distance between the centres of the slits
What is path difference
The difference between distances S1P and S2P
For reinforcement at P, the path difference (equation)
S1P - S2P = mλ
For cancellation at P, the path difference (equation)
S1P - S2P = (m+1/2) λ
Distance between the centres of adjacent bright fringes w =
λD /s
Where
D is the slit-screen distance
s is the fringe separation
Two loudspeakers connected to the same signal generator can be used to demonstrate interference as…
…they are coherent sources of sound waves. You can detect points of cancellation and reinforcement by ear as you move across in front of the speakers
Slit equation can then be used to estimate wavelength of sound
