Question

i) A lens produces a magnification of +1/3. Is this a converging or diverging lens? If the focal length of the lens is 30cm , draw a ray diagram showing the image formation in this case.
ii) Under what condition will a glass lens placed in a transparent liquid become invisible.

Answer 1

Answer:

Part a)

It is a diverging lens because it produce diminished virtual image

Part b)

When refractive index of lens and refractive index of the medium is same then the lens will become invisible in the medium

Explanation:

Here we know that the magnification due to lens is +1/3

so here we know that

$\frac{v}{u} = \frac{1}{3}$

since magnification is less than 1 and it is virtual image

so it is diminished image with magnification less than 1

this shows that the nature of the lens must be diverging lens

so we have

$u = 3v$

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$\frac{1}{\frac{u}{3}} - \frac{1}{u} = \frac{1}{30}$

$\frac{2}{u} = \frac{1}{30}$

$u = 60 cm$

$v = 20 cm$

so the ray diagram is given as following

Part b)

When refractive index of lens and refractive index of the medium is same then the lens will become invisible in the medium

#Learn

Topic : ray optics

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