Lens formula

Lens formula is an concept from the MPSC syllabus, tested in 2 past MPSC questions (2018). Below are those real questions with their correct answers and explanations, plus the concepts worth being comfortable with first.

Real questions

Years tested

1 · 2018–2018

In context

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Asked in

2018

What you need to know first

Spherical lensesNear point of eye

What mastering this unlocks

Convex lens propertiesImage formation

Real questions that test this

एका वस्तूची उंची 3 सेमी असून 24 सेमी वक्रता त्रिज्या असणाऱ्या अंतर्वक्र आरशापासून 20 सेमी अंतरावर ठेवली आहे. सुस्पष्ट प्रतिमा मिळवण्यासाठी आरशापासून पडदा किती अंतरावर असावा ? तसेच प्रतिमेचा आकार किती ? / An object 3 cm in size is placed 20 cm in front of a concave mirror of radius of curvature 24 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image ? Also, what is the size of the image ?

1)-30 सेमी, +4.5 सेमी / -30 cm, +4.5 cm

2)-30 सेमी, -4.5 सेमी / -30 cm, -4.5 cm✓ Correct

3)+4.5 सेमी, -30 सेमी / +4.5 cm, -30 cm

4)-4.5 सेमी, +30 सेमी / -4.5 cm, +30 cm

Using mirror formula 1/f = 1/v + 1/u with R=24 cm (f=-12 cm) and u=-20 cm, we find v=-30 cm. Magnification gives the image size as -4.5 cm.

ज्या व्यक्तीचे सुस्पष्ट दृष्टीचे लघुत्तम अंतर 50 cm आहे, अशा व्यक्तीच्या चष्माच्या भिंगांचे नाभीय अंतर किती असेल ? / What focal length should the spectacles have for a person for whom the least distance of distinct vision is 50 cm ?

1)50 cm, अंतरवक / 50 cm, concave

2)50 cm, बहिर्वर्क / 50 cm, convex✓ Correct

3)25 cm, अंतरवक / 25 cm, concave

4)25 cm, बहिर्वर्क / 25 cm, convex

For a person with least distance of distinct vision D = 50 cm (normal is 25 cm), a convex lens is used to correct hypermetropia. Using the lens formula with object distance u = -25 cm and image distance v = -50 cm gives a focal length of 50 cm.