Hướng dẫn giải Bài 9 (Trang 203 SGK Vật lý 11)

Mắt của một người có điểm cực viễn Cv cách mắt 50cm. a) Mắt người này bị tật gì? b) Muốn nhìn thấy vật ở vô cực không điều tiết người đó phải đeo kính có độ tụ bao nhiêu? (kính đeo sát mắt). c) Điểm Cc cách mắt 10cm, khi đeo kính mắt nhìn thấy điểm gần nhất cách mắt bao nhiêu? (kính sát mắt). Giải: a) <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>O</mi><msub><mi>C</mi><mi>v</mi></msub><mo>=</mo><mn>50</mn><mi>c</mi><mi>m</mi><mo><</mo><mo>∞</mo></math>, nên người này bị cận thị b) Tiêu cự kính đeo: <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>f</mi><mi>K</mi></msub><mo>=</mo><mo>-</mo><mi>O</mi><msub><mi>C</mi><mi>v</mi></msub><mo>=</mo><mo>-</mo><mn>50</mn><mi>c</mi><mi>m</mi><mo>=</mo><mo>-</mo><mn>0</mn><mo>,</mo><mn>5</mn><mo>(</mo><mi>m</mi><mo>)</mo></math> Độ tụ kính đeo: <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>D</mi><mi>k</mi></msub><mo>=</mo><mfrac><mn>1</mn><msub><mi>f</mi><mi>k</mi></msub></mfrac><mo>=</mo><mfrac><mn>1</mn><mrow><mo>-</mo><mn>0</mn><mo>,</mo><mn>5</mn></mrow></mfrac><mo>=</mo><mo>-</mo><mn>2</mn><mfenced><mrow><mi>d</mi><mi>p</mi></mrow></mfenced></math> c) <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>O</mi><msub><mi>C</mi><mi>c</mi></msub><mo>=</mo><mn>10</mn><mfenced><mrow><mi>c</mi><mi>m</mi></mrow></mfenced></math> <img class="wscnph" 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" width="390" height="59" /> <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>d</mi><msub><mo>'</mo><mi>c</mi></msub><mo>=</mo><mo>-</mo><mfenced><mrow><mi>O</mi><msub><mi>C</mi><mi>c</mi></msub></mrow></mfenced><mo>=</mo><mo>-</mo><mn>10</mn><mi>c</mi><mi>m</mi></math> <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mi>c</mi></msub><mo>=</mo><mfrac><mrow><mi>d</mi><msub><mo>'</mo><mi>c</mi></msub><mo>.</mo><mo> </mo><mi>f</mi></mrow><mrow><mi>d</mi><msub><mo>'</mo><mi>c</mi></msub><mo>-</mo><mi>f</mi></mrow></mfrac><mfrac><mrow><mo>-</mo><mn>10</mn><mo>.</mo><mfenced><mrow><mo>-</mo><mn>50</mn></mrow></mfenced></mrow><mrow><mo>-</mo><mn>10</mn><mo>+</mo><mn>50</mn></mrow></mfrac><mo>=</mo><mn>12</mn><mo>,</mo><mn>5</mn><mi>c</mi><mi>m</mi><mo>=</mo><mi>O</mi><mi>C</mi><msub><mo>'</mo><mi>c</mi></msub></math> Khi đeo kính, mắt nhìn thấy điểm gần nhất cách mắt 12,5cm.