wavelength of the spectral lines in nanometres. To observe the spectrum of any light
source, the diffraction grid and the slit must be aligned with the source. The spectrum
is seen on the scale, which gives the wavelengths of the spectral lines.
To calibrate the spectroscope, we analyse the diffraction phenomenon produced
by the grid. When a wavefront reaches a diffraction grid, each of the slits in the grid
becomes a new source of new wavefronts. On each point of a screen located at a
distance L from the diffraction grid (as shown in Fig. 21.3), there will be an overlap
of these secondary wavefronts. At those points on the screen where the waves arrive
in phase, constructive interference will occur and a maximum intensity will be
detected. In fact, if we point a laser (monochromatic light) through the diffraction
grid we observe a series of light points on the screen where the interference is
constructive. If we change the colour of the laser, the light points are placed in
different positions, because of the different laser’s wavelength.
Considering the waves that reach point X on the screen (as shown in Fig. 21.3)
from two consecutive slits in the diffraction grid, the paths’ length difference for
both waves corresponds to the length of the segment Δ. We can then set the
relationships:
sin α ¼
Δ
d
Fig. 21.2 On the left it is shown the spectroscope once mounted. At the bottom we can see the slit
behind which the light source is located and to the right we appreciate the calibrated scale for
measuring the wavelength. On the front, we see the ocular, covered by a diffraction grid. Next to the
spectroscope there is a 2-cent euro coin which serves as a reference for appreciating its small size.
On the right side of the image we show two spectra, corresponding to a white light emitting LED
(top) and a low-energy light bulb (bottom)
266
F. Savall-Alemany et al.
source, the diffraction grid and the slit must be aligned with the source. The spectrum
is seen on the scale, which gives the wavelengths of the spectral lines.
To calibrate the spectroscope, we analyse the diffraction phenomenon produced
by the grid. When a wavefront reaches a diffraction grid, each of the slits in the grid
becomes a new source of new wavefronts. On each point of a screen located at a
distance L from the diffraction grid (as shown in Fig. 21.3), there will be an overlap
of these secondary wavefronts. At those points on the screen where the waves arrive
in phase, constructive interference will occur and a maximum intensity will be
detected. In fact, if we point a laser (monochromatic light) through the diffraction
grid we observe a series of light points on the screen where the interference is
constructive. If we change the colour of the laser, the light points are placed in
different positions, because of the different laser’s wavelength.
Considering the waves that reach point X on the screen (as shown in Fig. 21.3)
from two consecutive slits in the diffraction grid, the paths’ length difference for
both waves corresponds to the length of the segment Δ. We can then set the
relationships:
sin α ¼
Δ
d
Fig. 21.2 On the left it is shown the spectroscope once mounted. At the bottom we can see the slit
behind which the light source is located and to the right we appreciate the calibrated scale for
measuring the wavelength. On the front, we see the ocular, covered by a diffraction grid. Next to the
spectroscope there is a 2-cent euro coin which serves as a reference for appreciating its small size.
On the right side of the image we show two spectra, corresponding to a white light emitting LED
(top) and a low-energy light bulb (bottom)
266
F. Savall-Alemany et al.