Question

Two thin lenses of power+2D and -1.5D are placed in contact .Find the a) power and b) focal length of the combination of lenses c) also state the nature of lens formed by the combination of these lenses. No links pls

Answer 1

1) Combine power of two thin lenses placed in contact has an equivalt to arithmetical sum of the two powers.

Therefore,equivalent power of the combination lens is :+2.0 -1.5 =0.5 D.

b)

Focal length of a lens is reciprocal of its power.

Therefore focal length of the 1st lens is ½ = -0.5 and that of the second is 1/-1.5= -0.66.

c)

Equivalent focal length, F of the combination lens is given by the formula:

1/F = 1/0.5 + 1/1/-0.5= 2-0.5=1.5.

2) Nature of the lens formed is known and defined as equivalent lens.

Answer 2

Concept:

Focal length is the distance between the focal point of a lens and the lens or mirror itself.

Given:

The power of two thin lenses is $+2D$ and $-1.5D$ .

Find:

The power, and focal length of the combination of lenses.

State the nature of the lens formed by the combination of these lenses.

Solution:

(a) The power of the two lenses, $P_1=+2D$ and $P_2=-1.5D$.

The power of the combination is,

$P=P_1+P_2=+2D-1.5D\\P=0.5D$

(b) The focal length of a lens is the reciprocal of its power.

So, $F=\frac{1}{P}$

$F=\frac{1}{0.5}$

$F=2m\\F=20cm$

(c) Since the focal length of the combination of lenses is $+20cm$, the nature of the lens is a convex lens.

The power and focal length of the combination of the lenses are $0.5D$ and $+20cm$. The nature of the lens combined is convex.

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