Spectroscope:
A simple type of prism spectroscope is shown in Figure 6. It is an optical instrument
designed to observe the effect of optical dispersion and to permit analysis of spectra.
Light emitted by the source, and limited by the slit, is transformed into parallel rays by
the collimating lens (A collimating lens is a converging lens that focuses a diverging
light beam into a parallel beam). After being dispersed by the prism, initially parallel
beams emerge, each at a different angle according to its wavelength. A telescope
focuses the rays and allows the observer to see the image of the slit. Most
spectroscopes have a calibrated circular scale, which enables the observer to
measure the angle of the emerging light for each image. Each image of the slit
Figure 5: Symmetrical path of the ray
trough a prism in the case of
minimum deviation.
(corresponding to the different values of the wavelength) is called a spectral line of
the light coming from the source. This is an origin of the term "line spectra". If the slit
is narrow enough only thin strip of the source is observed trough a prism, the band of
the images has very little overlap and bright spectral colors across the band are
visible.
Most modern spectrometers are made with diffraction grating in place of prism
because the dispersion of the grating can be made much greater than that of the
prism.