Equipment

○ Laser

○ Diffraction grating

○ White screen

○ Metre ruler

Method

○ Ensuring that the laboratory is partially darkened, set up the apparatus as shown in the diagram.

○ Adjust the apparatus so that D is 1.000m, measured using the metre ruler.

○ Measure the distances and on either side of the central maximum using the h1 h2 ruler and find the mean of each of them.

Graphs and calculations

○ Work out the slit separation by finding the reciprocal of the number of slits per metre quoted on the diffraction grating.

○ Find and by using θ1 θ2 by using θn = arctan( hn/D)

○ Find λ from both of these angles using λ=dsin(θ)/n and then calculate the mean.

Safety

○ Shining a laser into someone’s eyes can be dangerous, so ensure that the lasers are pointed away from them.

Improvements and notes

○ Make sure that the screen and double slit are aligned perfectly normal to the laser to avoid parallax error in the measurement of the fringe width. A set square can be used in conjunction with the ruler.

Diagram