Learning Object 9

1. Double-Slit Interference
By: Ria Tiwary
Student Number: 34003146
Physics 101

2. Thomas Young’s Experiment
• Thomas Young’s double-slit experiment was
conducted to prove that light has wave-like
properties, as well as having photon particles.

3. • The condition for a bright fringe to appear on the
optical screen: dsinθ=nλ
Where d is the distance between the two slits and n is
an integer.
• The distance between 2 successive fringes (on the
optical screen): s=λD/d
Where D is the distance between the screen and the
slits.

4. How to Make Sense of the Formulae
• An increase in the wavelength of the light results
in: increase in number of fringes on screen and
decrease in distance between fringes
• An increase in the distance between slits results
in: increase in number of fringes on screen and
decrease in distance between fringes
• An increase in the distance between the screen
and the slits results in: increase in number of
fringes on screen and decrease in distance
between fringes

5. Application of Double-Slit Interference
• Double-slit interference allows us to measure
the wavelength of lights using Young’s
equation: λ = y d / (m L)
• d: distance between slits
• L: distance from slits to screen
• y: distance from central antinode to 4th
antinode
• m: order value

6. Sample Question
• In a two-slit apparatus the slits are 0.3 mm
apart. Fringes in sodium are observed at a
distance of 1.2 m from the slits. The
separation of the fringes is 2.4 mm.
• What is the wavelength of sodium light?

7. Solution
m106
m1.2/m)103.0104.2(
*1/
L)(m/dy
7
43
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