Answer the questions.[Phy-5M]

Question 15 Marks

a. What is a lens? List two main categories of lenses. In which category is a double concave lens placed?
b. A convex lens of focal length 15 cm forms a real image at a distance of 20 cm from its optical centre. Find the position of the object. Is the image formed by the lens magnified or diminished?

Answer

a. A transparent material bounded by two surfaces of which one or both surfaces are spherical / curved.
a. Converging lens
b. Diverging lens
Double concave lens is a diverging lens.
b. To find the position of the object, we can use the lens formula:
$\mathrm{f}=15 \mathrm{~cm}, \mathrm{v}=20 \mathrm{~cm}, \mathrm{u}=$ ?
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\Rightarrow \frac{1}{u}=\frac{1}{v}-\frac{1}{f}$
$=\frac{1}{20 \mathrm{~cm}}-\frac{1}{15 \mathrm{~cm}}$
$=\frac{4-3}{60}$
$\mathrm{u}=-60 \mathrm{~cm}$.
image is diminished.

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Question 25 Marks

i. Draw a ray diagram to show the path of the reflected ray in each of the following cases:
A ray of light incident on a convex mirror
1. parallel to its principal axis, and
2. is directed towards its principal focus
ii. A 1.5 cm tall candle flame is placed perpendicular to the principal axis of a concave mirror of focal length 12 cm . If the distance of the flame from the pole of the mirror is 18 cm , use mirror formula to determine the position and size of the image formed.

Answer

ii. Here $\mathrm{f}=-12 \mathrm{~cm}, \mathrm{u}=-18 \mathrm{~cm}, \mathrm{v}=?, \mathrm{~h}=1.5 \mathrm{~cm}, \mathrm{~h}^{\prime}=$ ?
Mirror formula $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$
$\therefore \frac{1}{v}=\frac{1}{f}-\frac{1}{u}$
$=\frac{1}{-12} \mathrm{~cm}-\frac{1}{-18} \mathrm{~cm}$
$=\frac{-1}{36}$
$\therefore \mathrm{v}=-36 \mathrm{~cm}$
$m=\frac{h^{\prime}}{h}=-\frac{v}{u}$
$\frac{h^{\prime}}{1.5}=-\frac{(-36)}{(-18)}$
$h^{\prime}=-3.0 \mathrm{~cm}$

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Science Answer the questions.[Phy-5M]

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