Hướng dẫn giải Bài 10 (Trang 190 SGK Vật lý 11)

10. Một thấu kính hội tụ có tiêu cự f = 20 cm. Vật sáng AB được đặt trước thấu kính và có ảnh A'B'. Tìm vị trí của vật, cho biết khoảng cách vật - ảnh là: a) 125 cm b) 45 cm. Giải  a)  <span id="MathJax-Element-5-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; margin: 0px; padding: 1px 0px; overflow-wrap: normal; word-break: break-word; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 18.08px; letter-spacing: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>d</mi><mo>+</mo><msup><mi>d</mi><mo>&#x2032;</mo></msup><mo>=</mo><mo>&#x00B1;</mo><mn>125</mn></math>">d+d′=±125<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>d</mi><mo>+</mo><msup><mi>d</mi><mo>′</mo></msup><mo>=</mo><mo>±</mo><mn>125</mn></math> ta có: <ul> <li> Trường hợp 1: </li> </ul> <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>1</mn></msub></math> =17,54cm (nhận);  <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>2</mn></msub></math>= -142,5cm(loại) <ul> <li> Trường hợp 2: </li> </ul> <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>3</mn></msub></math>=25cm(nhận); <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>4</mn></msub></math> =100cm (nhận) Vậy, <span id="MathJax-Element-10-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; margin: 0px; padding: 1px 0px; overflow-wrap: normal; word-break: break-word; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 18.08px; letter-spacing: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>1</mn></msub><mo>=</mo><mn>100</mn><mi>c</mi><mi>m</mi><mo>;</mo><msub><mi>d</mi><mn>2</mn></msub><mo>=</mo><mn>25</mn><mi>c</mi><mi>m</mi><mo>;</mo><msub><mi>d</mi><mn>3</mn></msub><mo>&#x2248;</mo><mn>17</mn><mo>,</mo><mn>54</mn><mi>c</mi><mi>m</mi></math>"><math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>1</mn></msub></math>=100cm; <span id="MathJax-Element-10-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; margin: 0px; padding: 1px 0px; overflow-wrap: normal; word-break: break-word; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 18.08px; letter-spacing: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>1</mn></msub><mo>=</mo><mn>100</mn><mi>c</mi><mi>m</mi><mo>;</mo><msub><mi>d</mi><mn>2</mn></msub><mo>=</mo><mn>25</mn><mi>c</mi><mi>m</mi><mo>;</mo><msub><mi>d</mi><mn>3</mn></msub><mo>&#x2248;</mo><mn>17</mn><mo>,</mo><mn>54</mn><mi>c</mi><mi>m</mi></math>"><math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>       d</mi><mn>2</mn></msub></math>=25cm; <span id="MathJax-Element-10-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; margin: 0px; padding: 1px 0px; overflow-wrap: normal; word-break: break-word; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 18.08px; letter-spacing: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi><mn>1</mn></msub><mo>=</mo><mn>100</mn><mi>c</mi><mi>m</mi><mo>;</mo><msub><mi>d</mi><mn>2</mn></msub><mo>=</mo><mn>25</mn><mi>c</mi><mi>m</mi><mo>;</mo><msub><mi>d</mi><mn>3</mn></msub><mo>&#x2248;</mo><mn>17</mn><mo>,</mo><mn>54</mn><mi>c</mi><mi>m</mi></math>"><math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>       d</mi><mn>3</mn></msub></math>≈17,54cm.   b) <span id="MathJax-Element-11-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; margin: 0px; padding: 1px 0px; overflow-wrap: normal; word-break: break-word; display: inline-block; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 18.08px; letter-spacing: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>d</mi><mo>+</mo><msup><mi>d</mi><mo>&#x2032;</mo></msup><mo>=</mo><mo>&#x00B1;</mo><mn>45</mn><mo>;</mo></math>">d+d′=±45;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>d</mi><mo>+</mo><msup><mi>d</mi><mo>′</mo></msup><mo>=</mo><mo>±</mo><mn>45</mn><mo>;</mo></math> ⇒  <span id="MathJax-Element-12-Frame" class="mjx-chtml MathJax_CHTML" style="box-sizing: border-box; margin: 0px; padding: 1px 0px; overflow-wrap: normal; word-break: break-word; display: inline-table; line-height: 0; text-indent: 0px; text-align: left; text-transform: none; font-style: normal; font-weight: normal; font-size: 18.08px; letter-spacing: normal; word-spacing: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>d</mi><mo>=</mo><mn>15</mn><mi>c</mi><mi>m</mi></math>">d=15cm