Two identical thin biconvex lenses of focal length 15 cm and refractive index 1.5 are in contact with each other. The space between the lenses is filled with a liquid of refractive index 1.25. The focal length of the combination is ___ cm.
Two identical thin biconvex lenses of focal length 15 cm and refractive index 1.5 are in contact with each other. The space between the lenses is filled with a liquid of refractive index 1.25. The focal length of the combination is ___ cm.
Correct Answer: 10
Solution and Explanation
\(\frac{1}{f_1}\)=(\(\frac{μ_e}{μ_{m-1}}\))(\(\frac{1}{R_1}-\frac{1}{R_2}\))
here |R1|=|R2|=R
⇒\(\frac{1}{fl_1}\)=(1.5-1)(-\(\frac{2}{R}\))=\(\frac{1}{15}\)
⇒\(\frac{1}{R_1}\)=\(\frac{1}{15}\) or R=15 cm
for the concave lens made up of liquid
⇒\(\frac{1}{fl_2}\)=(1.25-1)(-\(\frac{2}{R}\))=-\(\frac{1}{30}\) cm
now for an equivalent lens,
\(\frac{1}{f_e}\)=\(\frac{2}{fl_1}\)+\(\frac{1}{fl_2}\)
=\(\frac{2}{15}\)-\(\frac{1}{30}\)=\(\frac{3}{30}\)=\(\frac{1}{10}\)
or fe = 10 cm
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Concepts Used:
Spherical Lenses
Lenses that are made by combining two spherical transparent surfaces are called spherical lenses. In general, there are two kinds of spherical lenses. Lenses that are made by joining two spherical surfaces that bulge outward are convex lenses, whereas lenses that are made by joining two spherical surfaces that curve inward are concave lenses.
Properties of Convex lens:
- In this, the lenses are thicker in the middle and thinner at the edges.
- They have a positive focal length.
- It intersects the incident rays towards the principal axis
- These lenses are used in the camera, focus sunlight, projector microscope, simple telescope, overhead projector, magnifying glasses, etc.
Properties of Concave lens:
- These lenses are thinner in the middle and thicker at the edges.
- They have a negative focal length.
- It parts the incident rays away from the principal axis.
- These are used in the glasses, spy holes, some telescopes in the doors, etc.