2. Converging/conves Lenses 会聚透镜/凸镜
When parallel rays of light (travelling parallel to the principal axis) pass through a
lens, they are brought to a focus at a point known as the principal focus
focal length
depends on
how curved the
lens is
3. Forming a Real Image
1. 首先画一条从物体顶部穿过透镜中心的光线。这条射线将继续沿直线传播
2. 接下来画一条从物体顶部出发的光线，平行于到透镜的轴。当光线从透镜中出来时，它会直接射向主焦点
3. 图像是在上述两条射线相交的点上找到的
f<x<2f
In this case, the image is:
Real 真实的
Enlarged 扩大的
Inverted 倒过来的
4. Forming a Real Image x>2f
In this case the image is:
Real
Diminished (smaller)
Inverted
5. Forming a Real Image x=2f
In this case the image is:
Real
Same size as the object
Inverted
6. Magnifying glass 放大镜
If the object is placed closer to the lens
than the focal length, the emerging rays
diverge and a real image is no longer
formed
如果物体离透镜的距离比焦距更近，
则产生的光线发散，不再形成真实的
像
In this case the image is:
Virtual
Enlarged
Upright
x<f
When using a magnifying glass, the lens
should always be held close to the
object
7. Concaving Lenses 凹镜 A concave lens is a diverging lens
which makes the rays of light
disperse and spread further apart.
It does the opposite of a convex
lens.
凹透镜是发散透镜
使光线分散，分散得更远。
它的作用与凸透镜相反。
