चित्र में \(\ell=2 \,m\) लम्बे तथा \(d =20\, \mu m\) व्यास के एक प्रकाश तन्तु को दिखाया है। यदि प्रकाश की किरण इस तन्तु के एक सिरे पर \(\theta_{1}=40^{\circ}\) कोण पर आपतित होती है तो दूसरे सिरे से निकलने से पूर्व इसके परावर्तनों की लगभग संख्या होगी? (फाइबर का अपवर्तनांक \(1.31\) है और \(\sin 40^{\circ}=0.64\) )

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.