Sam kk857jj In this set-up, we need to have parallel rays so that we can form the image on focal plane by the refraction of the convex lens. The plane mirror is used to reflect the parallel rays and reverse its direction. So no matter how far the mirror is placed, it can still reflect the parallel rays as parallel rays.
![[DSE Physics] Formation of image by concave lens 凹透鏡的成像](http://i.ytimg.com/vi/j7Jh7fyQtV0/mqdefault.jpg)
![[DSE Physics] Formation of image by concave lens 凹透鏡的成像](/img/tr.png)
![[DSE Physics] Nature of images formed by convex lens 凸透鏡成像的性質](/img/n.gif)
thank you
its very useful for me
Why the mirror's position isn't important for determining focal length of convex lens
Sam kk857jj In this set-up, we need to have parallel rays so that we can form the image on focal plane by the refraction of the convex lens. The plane mirror is used to reflect the parallel rays and reverse its direction. So no matter how far the mirror is placed, it can still reflect the parallel rays as parallel rays.
Thx
阿sir 請問2:25嗰個實驗個image點解會去咗左邊，正常唔係應該同個object 同一個位咩？
正常應該係同一個位，但係image同object同一個位就會睇唔到，所以我地將個object放側少少，咁image同objec就可以啱啱子喺隔離。
點解要中文講一次英文又要講一次
back focal length 後焦距 我是對focal 很困惑
我找不到我家的放大鏡 那凹透鏡的焦距要怎麼樣測量?
都仲有其他方法，不過就無咁準，你可以將一件實物放喺image大概位置，當你喺凹透鏡邊邊見到實物同影像重疊時，就即係實物處於影像位置，咁你就可以量度到像距，再計返focal。(用文字就咁講好難明)