Question

The image of a candle flame placed at a distance of 45cm from a spherical lens is
formed on a screen placed at a distance of 90cm from the lens. Identify the type
of lens and calculate its focal length. Identify the type of lens and calculate its
focal length. If the height of the flame is 2cm, find the height of its image​

Answer 1

Explanation:

According to the question;

Object distance, u=−45 cm

Image distance, v=90 cm

By lens formula,

v1−u1=f1

⇒90 cm1−−45 cm1=f1

⇒f1=90 cm1+45 cm1

⇒f1=90 cm1+2=90 cm3

⇒f1=30 cm1

⇒f=30 cm

Since f=30 cm which is positive, hence it is a convex lens of focal length 30cm.

Now,

Height of Flame, h1=2 cm

Magnification, m=h1h2=uv

Putting values of v and u.

Magnification, m=h1h2=−45 cm90 cm

⇒2 cmh2=−2

⇒h2=−2×2 cm=−4 cm

Height of image is 4 cm and the negative sign means image is inverted.

it may help uu.

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