Question

Two thin lenses of power, +3.5D and -2.5D are placed in contact, then the power and focal length of the lens combination is
1)+1D,+100cm
2)+2D,+150cm
3)+1D,+200cm
4)+2D,+100cm​

Answer 1

Answer:

• 1) +1 D , + 100 cm is the required answer.

Explanation:

Given:-

• Power of lens ,P1 =+3.5 D

• Power of Lens ,P2 = -2.5 D

To Find:-

• Power of Combination & Focal length

Solution:-

As we know that Power of Combination is defined as the sum of individuals power of given lens .

• P = P1 + P2 +...

Where,

• P power of combination
• P1 power of lens
• P2 power lens

Substitute the value we get

→ P = +3.5 + (-2.5)

→ P = +3.5 - 2.5

→ P = +1 D

• Hence, the power of combination of two lens is 1 D.

Now, we have to calculate the combination focal length.

As we know that 1 D = 100 cm & also relationship between Power of lens & Focal length

• P = 1/f

Therefore, the combination focal length is +100cm as power of combination is + 1 D.

Answer 2

Answer:

Given :-

• Power of Lens 1 = +3.5 D
• Power of Lens 2 = -2.5 D

To Find :-

• Power and focal length

SoluTion :-

As we know that

$\huge \tt \: P = R1 + R2$

$\tt \: P = + 3.5 + ( - 2.5)$

$\tt \: P = 3.5 - 2.5$

$\tt \: P = +1D$

Power of Lens = +1 D.

Now,

As we know that

$\huge \tt \: P = \frac{1}{f}$

As we know that

1D = 100 cm

$\tt \: 100 = \dfrac{1}{f}$

$\tt \: f = 1 \times 100$

$\tt\: f = 100$

Hence :-

$\huge \bf \bigodot \: Option \: A$