Section 47.6 Image Formation by Thin Lens
A converging lens can form a real as well as a virtual image, but a diverging lens can form only a virtual image. To find the image of an object we select representative points on the object and find the image of each point by tracing rays from that point. We will study objects that have the shape of a simple arrow so that we can also study the lateral magnification of the object. In this section, we will study image formation by the ray tracing method, i.e., by drawing rays rather than using lens equation.
Subsection 47.6.1 Real Image by a Convex Lens
Real image of a point light source is formed if physical rays actually meet after passing through the lens. For a convex lens this happens if object is placed at a distance further than the first focal point of the lens. Figure 47.6.1 shows the setup we will use to illustrate the method of finding location, size, and orientation of the real image. The object is the arrow OP.
As usual we start with drawing the lens, the center plane of the lens, and the symmetry axis. Then we position the foci \(\text{F}_1\) and \(\text{F}_2\) symmetrically about the lens, and place the object at the object OP distance from the center plane of the lens. It is a good idea to keep the height of the object smaller then the height of the lens. It is also best to draw the object from the axis rather than straddling the axis. Label appropriate points of the object.
In this example only the ends points O and P need labeling. We will work out the image of point P and by symmetry the image point of O will be on the axis in the plane containing the image point of P. To aid the drawing of rays, we draw a vertical line at the center of lens and all changing in ray directions are drawn fron this central line of the len.
Steps in the Drawing: Any two rays from P and striking the lens will work, but since we know the behavior of three special rays discussed in Subsection 47.5.1, we will choose two of them. The simplest one is the chief ray, labeled \(11'\) from P that passes through the lens undeviated. Next is a ray, labeled \(22'\) that will be incident on the lens parallel to the axis and pass through the second focal point \(\text{F}_2\text{.}\) These rays cross at point Q, which is the image of point P.
Now, if you draw the third special ray, e.g., \(33'\text{,}\) which passes through the first focal point before being incident on the lens, this ray will also pass through point Q. As a matter of fact, any arbitrary ray starting at P that goes through the lens will end up passing through Q.
Note that, even though we did not need to draw ray \(33'\) to locate the image point Q, it is wise to draw this ray just to make sure other rays were drawn correctly. So, you should develop the habit of drawing all three special rays!
Subsection 47.6.2 Virtual Image by a Convex Lens
If an object is withing a focal length of a convex lens, no image will form on any screen on the other side of the lens like we saw in Figure 47.6.1. When rays from the object emergy from the lens, they diverge as shown in Figure 47.6.2. When you are looking through the lens, the rays that started from P will appear to to come from point Q. This is virtual image of point P.
Steps in the Drawing: As before, any two rays from P and striking the lens will work. The simplest one is the chief ray, labeled \(11'\) from P that passes through the lens undeviated. Next is a ray, labeled \(22'\) that will be incident on the lens parallel to the axis and pass through the second focal point \(\text{F}_2\text{.}\) The emerging rays \(1'\) and \(2'\) are diverging away from each other. So, we draw their extensions backwards as shown by dashed lines. They meet at Q, which is the virtualimage of point P of the object.
Now, if you draw the third special ray, e.g., \(33'\text{,}\) which passes through the first focal point before being incident on the lens, this ray will emerge in a direction that will seem like it is cming from Q. As a matter of fact, any arbitrary ray, such as \(44'\) from P will appear to be coming from Q.
Since transmitted physical rays are not meeting at Q, this image cannot be placed on a screen. If you placed at screen at location of IQ, there will be no image. The screen will just have direct illumination from object OP. When you look through the lens, what you see is the image IQ and not the object OP.
Subsection 47.6.3 Image by a Concave Lens
Image by a concave lens is always virtual. That means you can look at the image when you view and object through the lens, but you cannot place the object on any screen. Ray tracing works same way here as it does for the virtual image in a cnvex lens shown in Figure 47.6.2. As shown in Figure 47.6.3, we draw two rays from a point on the object to locate its image by extension of the transmitted rays.
