Q28.A convex lens of focal length 20 cm is placed in front of convex mirror with principal axis coinciding each other. The distance between the lens and mirror is 10 cm. A point object is placed on principal axis at a distance of 60 cm from the convex lens. The image formed by combination coincides the object itself. The focal length of the convex mirror is _____ cm.
What This Question Tests
This multi-step problem requires applying lens formula first, then understanding that for the final image to coincide with the object, the light rays must strike the convex mirror normally after passing through the lens.
Concepts Tested
Formulas Used
1/f = 1/v - 1/u (Lens)
1/f = 1/v + 1/u (Mirror)
📚 NCERT Sections This Tests
9.8 — A Beam Of Light Converges At A Point P. Now A Lens Is Placed In The
Physics Class 12 · Chapter 9
9.8 A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12cm from P. At what point does the beam converge if the lens is (a) a convex lens of focal length 20cm, and (b) a concave lens of focal length 16cm?
9.19 — A Screen Is Placed 90Cm From An Object. The Image Of The Object On
Physics Class 12 · Chapter 9
9.19 A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20cm. Determine the focal length of the lens.
9.20 — (A) Determine The ‘Effective Focal Length’ Of The Combination Of
Physics Class 12 · Chapter 9
9.20 (a) Determine the ‘effective focal length’ of the combination of the two lenses in Exercise 9.10, if they are placed 8.0cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notion of effective focal length of this system useful at all? (b) An object 1.5 cm in size is placed on the side of the convex lens in the arrangement (a) above. The distance between the object and the convex lens is 40 cm. Determine the magnification produced by the two-lens system, and the size of the image.
📋 Question Details
- Chapter
- Ray Optics
- Topic
- Lens-mirror combination
- Year
- 2022
- Shift
- 25 Jul Shift 2
- Q Number
- Q28
- Type
- Numerical
- NCERT Ref
- Class 12 Physics Ch 9: Ray Optics and Optical Instruments
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