Page 167 - Science Class 08
P. 167
the incident ray (black in colour) mee ng the DISPERSION OF LIGHT
face AB of the prism at E. The word dispersion means sca ering or being
A L split up. When white light passes through a
Angle of devia on prism, it undergoes two processes:
Angle of incidence v It gets refracted, i.e., it comes out of the
d
M Q prism a er deflec on as explained above.
P d₁ d₂
i₁ r₁ r₂ i₂ v It gets split up (dispersed) into the different
Incident ray Refracted ray Emergent ray colours that it is made up of.
N
O B C R Sunlight is white to look at ; so it is called white
Angle of Refrac on
light as well. When white light passes through a
Refrac on Through a Prism
prism, it gets dispersed into its colours out of
Look through the prism from the other side, i.e.
which we can see seven colours clearly. These
from the face AC. On that side fix two pins L, M
seven colours are violet, indigo, blue, green,
such that they are in a straight line with the
yellow, orange and red. You can remember the
images of the pins P and Q. The line LM will be names and order of these colours by remem-
the emergent ray (blue in colour).
bering the acronym VIBGYOR.
Remove the prism and the pins. Join LM and
If these colours are made to fall on a screen, a
produce it to meet the prism at F. Also, join EF wide patch of coloured light shall be obtained.
that points the refracted ray (red in colour).
This patch is known as spectrum.
Draw lines at E and F and mark i as the angle of
In order to understand the spectrum, we have
incidence and r, as the angles of refrac on and got to follow the nature of the light at first.
d as the angle of emergence.
Remember that light is the point of energy that
Now observe the following facts: consists of various waves. Each of these waves
v The incident ray and the emergent ray are has a wave-length. A wave-length is the
not parallel. If both of them are produced distance between any point on one wave and
in one side, they will meet at a point m (in the corresponding point on the next wave.
the diagram). So d at m, is the angle Different wave-lengths of light appear to the
through which, the incident ray deflects to eye as different colours. The light that contains
come out of the prism as the emergent ray. all wave-lengths each in the same propor on
v While entering the prism, the incident ray appears to be white. Clearly, sunlight is also
deflects towards the normal. white light.
v While emerging from the prism, the When a beam of sunlight passes through a
emergent ray deflects away from the prism, the rays of different wave-lengths refract
normal. at various angles. Then, different deflec ons
break up the white-light into a beau ful patch
v But, the deflec on in the both the above
(band) of colours just as we see in the rainbow.
cases is the same.
This band of seven colours is called the visible
v Angle d is called the angle of devia on. spectrum.
v A light ray passing through a prism always The violet colour has the shortest wave-length
deflects towards the base. which goes on increasing in length towards the
Science-8 167
