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From the ray diagram shown below, calcul...

From the ray diagram shown below, calculate the focal length of the other convex lens.

Text Solution

Verified by Experts

For first convex lens,
Object distance, `u=-25cm`
Focal length, `f=30cm`
From lens formula, image distnace is given by
`(1)/(v)=(1)/(f)+(1)/(u)=(1)/(30)-(1)/(25)=(-5)/(750)`
`rArr" "v=-150cm`
For second convex lens, image I. will act as object
`therefore" Object distnace"=-(150+20)=-170cm`
`"Image distnace, "v=25cm`
`therefore" "(1)/(f)=(1)/(v)-(1)/(u)=(1)/(25)-((-1)/(170))`
`=(1)/(25)+(1)/(170)=(5+34)/(850)-(39)/(850)`
`rArr" "f=(850)/(39)=+21.8cm`
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Knowledge Check

  • The focal length of the convex lens is

    A
    `+ 10 ` cm
    B
    ` + 20 cm`
    C
    ` - 10 cm`
    D
    ` - 20 cm`

  • Focal length of a convex lens is maximum for

    A
    blue light
    B
    yellow light
    C
    green light
    D
    red light

  • ______ has shorter focal length convex lens

    A
    Objective of a telescope
    B
    Eyepiece of a microscope
    C
    Eyepiece of a telecope
    D
    Both the lenses in a telescope

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