Page 188 - Physics_Form_2

Physics for Secondary Schools

Formation of rainbow
The rainbow phenomenon can be observed when there are water droplets in the
atmosphere as the sun shines from behind the observer at a low altitude. The rainbow’s
appearance is caused by the dispersion of sunlight as it is refracted by nearly circular
raindrops. The white light from the sun is first refracted as it enters the surface of the
raindrops.
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When light travels from a less dense medium to a light denser medium, it bends towards
the normal. However, not all colours bend equally. As in a glass prism, red light bends
the least while violet bends the most. At the back of the raindrops, some of the dispersed
light is reflected back over a wide range of angles and again refracted as it leaves the
raindrop. In most cases, the violet light leaves the raindrop at a smaller angle relative
to incident sunlight compared to red light as shown in Figure 5.17 (a). As a result, the
observer sees an image of well-arranged coloured bands in the atmosphere called a
rainbow, with red light appearing on top and violet at the bottom of the spectrum as
shown in Figure 5.17 (b).

White light

Violet
Indigo Raindrop
Blue
Green
Yellow

Orange
Red
(a) (b)
Figure 5.17: Formation of a rainbow

Activity 5.8
2. Open the water tap so that the hose

Aim: To observe the formation sprays water into the air.
of a rainbow by spraying 3. Stand near the spray of water drops
water into the air with your back directed towards the

Materials: water, a hose that can sun.
spray water, a sprinkler or 4. Move around until you locate your
a fountain shadow.
Procedure 5. Look for a rainbow in the spray of
1. Bring the water hose to an open water.
space.

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Student’s Book Form Two

Physics Form 2 Final.indd 182 25/10/2025 10:28