The double slit experiment demonstrates the wave-like properties of light. When light passes through two slits, it exhibits an interference pattern of light and dark fringes on a screen due to the constructive and destructive interference of light waves. The spacing and position of the fringes can be calculated using the equation dsinθ = mλ, where d is the slit separation, θ is the angle from the central maxima, m is the spectral order, and λ is the wavelength of light. Increasing the slit separation or wavelength increases the fringe spacing.

1. Double Slit Experiment
Physics 101 LO9

2. Double Slit Experiment
• Thomas Young demonstrated the wave-like
properties of light.

3. Interference Pattern
on the Screen

4. Wave Interference
• Light and dark bands correspond to constructive
and destructive interference respectively.

5. Geometry of the Experiment
dsinθ = mλ
Where m is an integer, often
called the spectral order

6. Example
If a blue light (wavelength = 475nm) illuminates on a
double slit, and the slits are cut 0.0200mm apart.
Determine what angle you should look away from
the central fringe to see the third order fringe?
dsinθ = mλ
A. 4.09°
B. 6.08°
C. 4.50°
D. 5.10°

7. Example
If a blue light (wavelength = 475nm) illuminates on a
double slit, and the slits are cut 0.0200mm apart.
Determine what angle you should look away from
the central fringe to see the third order fringe?
dsinθ = mλ
A. 4.09°
B. 6.08°
C. 4.50°
D. 5.10°

8. Solution
First, make sure all of the values’ units match.
λ = 475nm = 4.75x10-7m
slit separation (d) = 0.0200mm = 2.00x10-5m
“Third order fringe” = 3rd spectral order:
m=3
Rearrange the equation:
dsinθ = mλ
sinθ = mλ/d
θ = sin-1(mλ/d)
Plugging in the values, we get:
Answer = 4.09°

9. Question
You observe the interference pattern from two slits, but
the fringes seem to be too close together to be
measured. What could you do to increase the spacing
between the fringes?
Move the slits FARTHER/CLOSER to the screen.
INCREASE/DECREASE the wavelength of light.
INCREASE/DECREASE the spacing of the slits.

10. Question
You observe the interference pattern from two slits, but
the fringes seem to be too close together to be
measured. What could you do to increase the spacing
between the fringes?
Move the slits FARTHER/CLOSER to the screen.
INCREASE/DECREASE the wavelength of light.
INCREASE/DECREASE the spacing of the slits.

11. Solution
The simplest way to increase the spacing between the fringes is to
move the slits farther from the screen.
Otherwise, you have to increase the angle:
dsinθ = mλ
From the equation above, the angle is directly proportional to the
wavelength of light, λ.
Increasing the wavelength of light increases the angle.
Also, the angle is inversely proportional to the distance at which the
slits are cut and spaced apart, d.
Decreasing this distance increases the angle.