Double Slit Experiment

Physics 101 LO9

Double Slit Experiment

• Thomas Young demonstrated the wave-like properties of light.

Interference Pattern on the Screen

Wave Interference

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

Geometry of the Experiment

dsinθ = mλWhere m is an integer, often called the spectral order

ExampleIf 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 = mλA. 4.09°B. 6.08°C. 4.50°D. 5.10°

ExampleIf 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 = mλA. 4.09°B. 6.08°C. 4.50°D. 5.10°

SolutionFirst, 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 = mλsinθ = mλ/d

θ = sin-1(mλ/d)

Plugging in the values, we get:Answer = 4.09°

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.

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.

SolutionThe 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.