Physics Class 12 NCERT Solutions: Chapter 9 Ray Optics and Optical Instruments Part 4

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Lens equation and magnification

Lens Equation and Magnification

Q: 9. An object of size is placed in front of a concave lens of focal length. Describe the image produced by the lens. What happens if the object is moved further away from the lens?

Answer:

Size of the object,

Object distance,

Focal length of the convex lens,

Image distance

According to the lens formula, we have the relation:

Hence, the image is formed on the other side of the lens, away from it. The negative sign shows that the image is erect and virtual.

The magnification of the image is given as:

Hence, the height of the image is .

If the object is moved further away from the lens, then the virtual image will move toward the focus of the lens, but not beyond it. The size of the image will decrease with the increase in the object distance.

Q: 10. What is the focal length of a convex lens of focal length in contact with a concave lens of focal length? Is the system a converging or a diverging lens? Ignore thickness of the lenses.

Answer:

Focal length of the convex lens,

Focal length of the concave lens,

Focal length of the system of lenses

The equivalent focal length of a system of two lenses in contact is given as:

Hence, the focal length of the combination of lenses is. The negative sign indicates that the system of lenses acts as a diverging lens.

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