1
You visited us 1 times! Enjoying our articles? Unlock Full Access!
Question

# A double convex lens is made of glass which has refractive index 1.55 for violet rays and 1.50 for red rays. If the focal length of violet rays is 20 cm, the focal length for red rays is :

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

## The correct option is C 22 cmGiven–––––––:∙ fV=20cm∙ μV=1.55∙ μR=1.50∙ Refractive index doesn't have a unit.To find: fR= ?Solution––––––––––: 1f=(μ−1)×(1R1−1R2) Since lens is the same, R1 and R2 are common for both.∴1fV=(μV−1)×(1R1−1R2)∴1fR=(μR−1)×(1R1−1R2)∴(1fV)(1fR)=(μV−1)×(1R1−1R2)(μR−1)×(1R1−1R2)=(μV−1)(μR−1)∴ 1fV÷1fR= fRfV=μV−1μR−1∴ fR=fV×(μV−1)(μR−1)=20×(1.55−1)(1.50−1)=20×0.550.5=20×5550=2×11=22Answer–––––––––: Focal length for red rays is 22cm.

Suggest Corrections
0
Join BYJU'S Learning Program
Related Videos
Playing with Refractive Index
PHYSICS
Watch in App
Explore more
Join BYJU'S Learning Program