wiz-icon
MyQuestionIcon
MyQuestionIcon
4
You visited us 4 times! Enjoying our articles? Unlock Full Access!
Question

An object is placed first at infinity and then at 20cm from the object-side focal plane of a convex lens. The two images thus formed are 5cm apart. The focal length of the lens is

A
5cm
No worries! We‘ve got your back. Try BYJU‘S free classes today!
B
10cm
Right on! Give the BNAT exam to get a 100% scholarship for BYJUS courses
C
15cm
No worries! We‘ve got your back. Try BYJU‘S free classes today!
D
20cm
No worries! We‘ve got your back. Try BYJU‘S free classes today!
Open in App
Solution

The correct option is B 10cm

The convex mirror formula is,

1f=1v1u

In the first case, the object is placed at infinity, i.e., u=. We put this in the mirror formula and get-

1f=1v1

f=v

In the second case, v=(f+5) cm and u=(f+20) cm. So we put these in the mirror formula and get-

1f=1f+51(f+20)

1f=1f+5+1(f+20)

1f=f+20+f+5(f+5)(f+20)

1f=2f+25(f+5)(f+20)

2f2+25f=f2+25f+100

f2=100

f=10 cm

So, option (B) is correct.


flag
Suggest Corrections
thumbs-up
0
Join BYJU'S Learning Program
similar_icon
Related Videos
thumbnail
lock
Sign Convention for the Measurement of Distances
PHYSICS
Watch in App
Join BYJU'S Learning Program
CrossIcon