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P. 216
Physics for Secondary Schools
8. An object 5 cm high is placed 25 12. Rays of light from luminous objects
cm from a convex lens with a focal are brought to focus at point Q. A
length of 20 cm. Using the lens convex lens of 24 cm focal length
formula, determine the position, size, is then placed 12 cm from Q to
and nature of the formed image. intercept the rays before they meet
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9. When an object is placed 30 cm from at Q. If now they meet at P, find the
a lens, a virtual and erect image, distance QP.
which is half the size of the object
is formed. 13. A glass block whose refractive index
is 1.564 for sodium light is to be
(a) Explain whether the lens is a
concave or a convex lens. used to construct a prism such that
(b) What is the focal length of the the angle of minimum deviation for
such light shall be equal to the angle
lens?
10. When an object is placed 25 cm from of the prism. What is the angle of the
a convex lens, an inverted image, prism?
which is twice as large as the object, 14. Form Two students went to the
is formed. How far from the lens picnic. The weather was pleasant.
must the object be placed to obtain They played various games and then
an image four times the size of the had snacks. Suddenly, Gebu, one of
object? them, noticed seven colours extended
11. A beam of light consisting of red, like a bow in the sky. He said, “Wow,
green, and blue is incident on a right- what is rainbowᴉ?” The Ramso,
angled prism as shown in Figure one of them, asked him, “What is
5.47. The refractive indices of the a rainbow?” He then explained its
material of the prism for red, green, formation to all students. After that,
and blue colours are 1.39, 1.44, and
1.47, respectively. Which colour everyone in the group thanked him
suffers from total internal reflection? for the knowledge he had given to
Explain. them.
(a) If you were in the place of Gebu,
how would you have explained
such a natural phenomenon?
Blue
(b) Which device can be used to
Green obtain such a phenomenon?
Red (c) If Gebu facing the rainbow, then
where was the sun?
90° 45°
(d) What does Gebu show the moral
Figure 5.47 values?
210
Student’s Book Form Two
Physics Form 2 Final.indd 210 25/10/2025 10:28
