Two rays are enough
Show on a big screenYou want to mark where a bulb's image forms. You do not need every ray.
Step 4 gives v and m. Here v is the image distance. And m is the image height divided by the object height.
- An object stands 30 cm from a concave mirror, f = 10 cm.
- One ray leaves parallel to the axis. It reflects through F.
- A second ray leaves through C. It comes straight back.
- Where they cross is the image: v = −15 cm, m = −0.5.
Loading lab…
The rule
Draw any two standard rays. Where they cross, or seem to cross, is the image. A third ray only checks the drawing. A standard ray has a known path after the mirror. Draw them from the top of the object. They then cross at the top of the image.
| Ray from the top of the object | After it hits the mirror |
|---|---|
| Parallel to the axis | Goes through F |
| Through F | Goes parallel to the axis |
| Through C | Returns along its own path |
| Hits the pole | Leaves at an equal angle on the other side of the axis |
Worked example
In step 4, v is minus. The image is in front of the mirror. The minus on m means the image is inverted. The 0.5 means half the size.
A ray through C travels along the radius, which is the normal. So its angle of incidence is 0°. It reflects straight back.
In the picture, the parallel ray and the ray through C cross. They cross 15 cm in front of the mirror. The image is real, inverted and smaller.
Now put the object inside F. The reflected rays spread out and never cross in front. Extend them backwards and they meet behind the mirror. That is a virtual, upright image.
Try the lab to finish this lesson.