## Thursday, January 13, 2011

### Numericals based on concave/convex lens and power

CLASS-X
LIGHT
ASSIGNMENT-3­

Q1. A 4cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20cm. If the distance of the object is 30cm from the lens, find the position, nature and size of the image. Also find its magnification.
Ans: v=60cm, hi= -8cm, inverted, m= -2

Q2. A needle placed at 45cm from the lens forms an image on a screen placed 90cm on the other side of the lens. Identify the type of the lens and determine its focal length.
Ans: f= +30cm, convex lens

Q3. An object 2cm high is placed at a distance of 64cm from a white screen. On placing a convex lens at a distance of 32cm from the object it is found that a distinct image of the object is formed on the screen. What is the focal length of the convex lens and size of the formed on the screen?
Ans: f= 16cm, hi= -2cm (same size, inverted)

Q4. A 10cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 30cm. The distance of the object from the lens is 20cm. Find the position, nature and size of the image formed.
Ans: v= -60cm, hi= +30cm, erect

Q5. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Q6. A concave lens of focal length 20cm forms an image at a distance of 10cm from the lens. What is the distance of the object from the lens?
Ans: u= -20cm

Q7. Find the position and nature of image formed when an object is placed at a distance of 5cm from a concave lens of focal length 15cm. What is its magnification?
Ans: v= -3.75cm, virtual and erect, m= 0.75

Q8. An object located 32cm in front of a lens forms an image on a screen 8cm behind the lens.
a)     Find the focal length of the lens.
b)    Determine its magnification.
c)     Is the lens converging or diverging?
Ans: f= 6.4cm, m= -1/4, convex lens

Q9. An object 60cm from a lens gives a virtual image at a distance of 20cm in front of the lens. What is the focal length of the lens? Is the lens converging or diverging?
Ans: f= -30cm, lens is diverging or concave

Q10. Find the power of a concave lens of focal length 2m.
Ans: P= -0.5D

Q11. A doctor has prescribed a corrective lens of power +1.5D. Find the focal length of the lens. Is the prescribed lens converging or diverging?
Ans: f= +0.67m, converging

Q12. A concave lens of power -2D is placed in contact with a convex lens of power +1.5D. What will be the resultant power?
Ans: P= -0.5D

Q13. A lens has a power of -0.25D. Identify the type of lens and what will be the focal length?
Ans: Concave, f= -4m

Q14. 3lenses of powers 2D, -0.25D, 1.75D are placed in contact. Calculate the net power of the arrangement. Will it produce a converging or a diverging effect? Will the net power change if the order of lens placement is changed?
Ans: P= +3.5D, converging effect, No

Q15. A convex lens forms a real and inverted image of a needle at a distance of 50cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also find the power of the lens.
Ans: 50cm, P= 4D

1. these Q are nice and very helpful but plspost some more.thank u

2. and plss also post the solution....it would help a lot......thank u

3. The first few questions are faulty. Only after using the magnification formula for mirror, you get the answer mentioned here. So either edit the question or the answers. BTW thnx for these sums.

1. no dear....you take magnification +ve those r to be taken -ve

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4. Hi thanks for the questions, will help for my practice.
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