Page 172 - NCERT Science Class 10 English Medium

The prism has probably split the incident
white light into a band of colours. Note the
colours that appear at the two ends of the
colour band. What is the sequence of colours
that you see on the screen? The various
colours seen are Violet, Indigo, Blue, Green,
Yellow, Orange and Red, as shown in
Fig. 10.5. The acronym VIBGYOR will help
you to remember the sequence of colours.
The band of the coloured components of a
Figure 10.5
Figure 10.5
Figure 10.5
Figure 10.5 Dispersion of white light by the glass prism
light beam is called its spectrum. You might Figure 10.5
not be able to see all the colours separately.
Yet something makes each colour distinct from the other. The splitting
of light into its component colours is called dispersion.
You have seen that white light is dispersed
into its seven-colour components by a prism.
Why do we get these colours? Different colours
of light bend through different angles with
respect to the incident ray, as they pass
through a prism. The red light bends the least
while the violet the most. Thus the rays of each
colour emerge along different paths and thus Figure 10.6
Figure
Figure
10.6
Figure 10.610.6
Figure 10.6 Recombination of the spectrum of
become distinct. It is the band of distinct white light
colours that we see in a spectrum.
Isaac Newton was the first to use a glass prism to obtain
the spectrum of sunlight. He tried to split the colours of
the spectrum of white light further by using another similar
prism. However, he could not get any more colours. He
then placed a second identical prism in an inverted position
with respect to the first prism, as shown in Fig. 10.6. This
allowed all the colours of the spectrum to pass through
the second prism. He found a beam of white light emerging
from the other side of the second prism. This observation
gave Newton the idea that the sunlight is made up of seven
colours.
Any light that gives a spectrum similar to that of Figure 10.7
Figure
Figure 10.710.7
Figure 10.7 Rainbow in the sky
10.7
Figure
sunlight is often referred to as white light.
A rainbow is a natural spectrum appearing in the sky
after a rain shower (Fig. 10.7). It is caused by dispersion Raindrop
of sunlight by tiny water droplets, present in the
Sunlight
atmosphere. A rainbow is always formed in a direction
opposite to that of the Sun. The water droplets act like
small prisms. They refract and disperse the incident
sunlight, then reflect it internally, and finally refract it
again when it comes out of the raindrop (Fig. 10.8). Due
to the dispersion of light and internal reflection, different
colours reach the observer’s eye.
You can also see a rainbow on a sunny day when
you look at the sky through a waterfall or through a water
fountain, with the Sun behind you. Figure 10.8
Figure 10.8 Rainbow formation
Figure 10.810.8
10.8
Figure
Figure
The Human Eye and the Colourful World 167

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