1. Hydrogen atoms excited in a discharge tube only emit four different discrete wavelengths of visible photons.

In a semi-darkened room, a single slit is placed in front of the discharge tube. A student holds a diffraction grating which has 300 lines per millimetre.

The student looks through the grating at a 15cm plastic ruler placed 0.50m away, as shown in Fig. 5.1.

The paths of the different colours of light from the slit to the student’s eye are shown in Fig. 5.2.

Fig. 5.1 (not to scale)                                           Fig. 5.2 (not to scale)

Four first order images of the slit, one at each photon wavelength, are observed as vertical lines against the background of the plastic ruler, as shown in Fig. 5.3.

Fig. 5.3

The student decides to determine the wavelength of the photons which form the red line observed at x = 10 cm on the ruler.

• Describe how the information that has been given can be used to determine the wavelength of the red photons.

• Estimate the percentage uncertainty in the measured value of the wavelength.