Page 190 - Physics_Form_2
P. 190
Physics for Secondary Schools
that is identical to the first prism? If the second “primary colours” separated by
prism is arranged similarly to the first prism, no the three “secondary colours”, as
further splitting of white light can be observed. shown in Figure 5.20. This colour
But if the second prism is inverted with respect to wheel is famously known as the
the first prism, a beam of white light is observed Newton colour wheel or disc.
to emerge from the other side of the second prism. When the disc is rotated about its
FOR ONLINE READING ONLY
This means that the dispersed colours of white axis at high speed, all the colours
light can be recombined to form white light, as blend to form a white colour.
However, as the disc slows down,
illustrated by Figure 5.19. This phenomenon is individual colours are seen again.
known as the white colour recombination.
GREEN Cyan
Yellow BLUE
RED Magenta
Figure 5.19: Dispersion and recombination of white light
Figure 5.20: A Newton’s colour wheel
The experiment of recombination of the
spectrum of light can be extended to observe the Since magenta was not a part of the
recombination of two or more individual colours. light spectrum, its origin posed a
Sir Isaac Newton did this for the first time using mystery. This mystery was resolved
prisms and mirrors. He discovered that when the by Hermann von Helmholtz, who
light from the red, green, and blue regions of the established that the human eye
consists of three types of colour
spectrum are recombined, they regenerate white receptors. One of the primary
light. Therefore, he called red, green, and blue colours (red, blue and green)
the primary colours. can stimulate only one receptor
type. Moreover, the eye perceives
Newton also observed that when two primary all of the variations in colour by
colours are combined, other colours are formed. internally combining the signals
For example, when blue and green lights are from these receptors. Therefore,
combined, cyan light is formed. Furthermore, white light is seen when red, blue
green and red lights combine to give yellow and green light enters the eye.
light, while red and blue lights combine to form However, when both red and blue
magenta light. Newton named the colours that light but no green light enters the
resulted from recombining two primary colours eye, magenta is seen even though
as secondary colours. Secondary colours include the light is not magenta. Similarly,
yellow, cyan and magenta. He finally organised when red and green light enters
his findings in a colour wheel showing the three the eye, a yellow light is seen, and
184
Student’s Book Form Two
Physics Form 2 Final.indd 184 25/10/2025 10:28
