Answers
Year 9 Science
13
1. A SPECTRUM
Reverse the lamp bracket in the ray box so that the light shines out though the back of the box. Place the
ray box and the spherical lens (in plasticine) on the bench so that an image of the filament of the globe is
projected onto a white screen 1 metre away.
Insert the prism and move the screen around in an arc until the spectrum is formed on it. Rotate the prism
and adjust the position of the screen (and the lens if necessary) until the clearest spectrum is produced.
a) Name as many colours as you can detect in the spectrum.
Red, Orange, Yellow, Green, Blue, Indigo and Violet. (R O Y G B I V)
In the adjacent rectangle sketch in the spectrum formed on the screen. Use
coloured pencils and make the spectrum fill the rectangle. Make the width of
each colour band in the rectangle proportional to the width of each colour in
the spectrum.
(V I B G Y O R)
b) Which colour is refracted most by the prism?
Violet
c) Which colour is refracted least by the prism?
Red
13 White Light Page 1 of 2
The white light of the ray box has been split up (or dispersed) into a number of different colours. Place a
second prism (borrowed from a near-by group) between the first prism and the screen.
d) Describe and explain the nature of the image formed.
The image is of white light.
2. EFFECT OF FILTERS
Set up the apparatus again so that a spectrum is formed on the white screen. Hold a red filter in front of
the top half of the spectrum on the screen. Compare the light which passes through the filter onto the
screen with that which does not pass through the filter.
Observe which colours are transmitted and absorbed. Enter your observations on the table.
FILTER
RED
COLOURS TRANSMITTED
COLOURS ABSORBED
Red
OYGBIV
GREEN
Green
ROY BIV
BLUE
Blue
ROYG IV
YELLOW
Yellow
RO GBIV
ORANGE
Orange
R YGBIV
CYAN
Green/ Blue
ROY
IV
EXERCISES
1. What colour would white paper appear when viewed through a red filter? Why?
The paper would appear red since red light will be transmitted and all other colours
are absorbed.
2. The earth's atmosphere acts as a filter. Find out which radiations are absorbed.
The earth’s atmosphere absorbs most of the ultra-violet light from the Sun.
3. Why does a blue filter appear to be blue when white shines on it?
Blue light is reflected and all other colours are absorbed.
4. What colour does chlorophyll reflect best? What colour does chlorophyll absorb best? What happens
to the light energy, which is absorbed by a green plant? Would green plants grow well under pure
green light? Explain.
The colour that chlorophyll reflects best is it own colour, Green. It would absorb all
other colours.
Green plants would only reflect and not absorb green light so then it would not absorb
very much light energy.
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