Question

a convex lens of power 2d is placed at the distance of 40m from a wall. at what distance from the lens should be placed so that image is formed on the wall.??

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Answer 1

Answer:

• 200 cm is the required answer .

Explanation:

Given:-

• Power of lens ,P = 2 D

• Object distance ,u = -40 cm

To Find:-

• Image distance ,v

Solution:-

Firstly we calculate the focal length of the lens. As we know the relationship between Focal length and Power .

• P = 1/f

Substitute the value we get

→ 2 = 1/f

→ 2f = 1

→ f = 1/2

→ f = 0.5 m

→ f = + 50 cm

Now,

Using lens formula

• 1/v - 1/u = 1/f

Substitute the value we calculate

→ 1/50 = 1/v - 1/(-40)

→ 1/50 = 1/v + 1/40

→ 1/v = 1/50 - 1/40

→ 1/v = 4-5/200

→ 1/v = -1/200

→ v = -200 cm

• Hence, the image distance is 200 cm .

Answer 2

Answer:

Required Answer :-

As we know that

P = 1/f

2 = 1/f

2f = 1

f = 1/2

f = 0.5 m

f = + 50 cm

Now,

1/u + 1/v = 1/f

1/50 = 1/v - 1/(-40)

1/50 = 1/v + 1/40

1/v = 1/50 - 1/40

1/v = 4 - 5/200

1/v = -1/200

v = -200 × 1

v = -200 cm