What refractive index should be used while calculating the focal length of lens using lensmaker's formula?
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Sun is at a huge distance in comparison with the focal length of the lens.
Hence, the objective distance can be considered at infinity.
Hence the image will be formed at its focus.
Consider the following situation:

Understand the angle subtended by the common tangents with Sun and its image are vertically opposite angles. Hence they are equal to each other and is equal to 0.5 degrees.
Thus the angle BOA = 0.5 degrees.
Also the distance OA=OB=100 cm
Hence the distance AB is
AB=x260(2πr)r=100 cmx=0.5∘AB=0.5360(2×227×100)AB=0.873 cm
Hence, the objective distance can be considered at infinity.
Hence the image will be formed at its focus.
Consider the following situation:

Understand the angle subtended by the common tangents with Sun and its image are vertically opposite angles. Hence they are equal to each other and is equal to 0.5 degrees.
Thus the angle BOA = 0.5 degrees.
Also the distance OA=OB=100 cm
Hence the distance AB is
AB=x260(2πr)r=100 cmx=0.5∘AB=0.5360(2×227×100)AB=0.873 cm
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