Object at F or inside F
No image at F, and a large upright image just inside F.
A dentist's mirror
A dentist holds a small concave mirror close to a tooth and sees a large, upright picture of the tooth in it.
Where must the tooth be, compared with the focus F of the mirror?
What this lesson covers
The idea
For a concave mirror, an object at F forms its image at infinity, while an object between P and F gives a virtual, erect and enlarged image behind the mirror.
A dentist's mirror
A dentist holds a small concave mirror close to a tooth and sees a large, upright picture of the tooth in it.
Where must the tooth be, compared with the focus F of the mirror?
- Between the mirror and F
- Exactly at F
- Beyond C
Slide the object
Red ray: parallel to the axis. Purple ray: towards the pole P. Slide the object to F, and then to a place between F and the mirror.
Two special places
At F the reflected rays came out parallel, so they never met. Between F and the mirror they spread out, and their dashed extensions met behind the mirror.
For a concave mirror, an object at F forms its image at infinity (no image is formed), while an object between P and F gives a virtual, erect and enlarged image behind the mirror.
Notes
A concave mirror gives no image for an object at F (the image would be at infinity) and a virtual, erect, enlarged image behind the mirror for an object between P and F.
Check yourself
An object is placed at the focus F of a concave mirror. What happens to the reflected rays?
An object is between P and F of a concave mirror. What is its image like?
A dentist wants a large, upright image of a tooth using a concave mirror. Where should the tooth be?
For which position of the object does a concave mirror give a virtual image?
- They meet at C and form a same-size image. Rays meeting at C would give an image at C. That is for an object at C, not at F.
- They go out parallel to each other, so no image is formed — correct. Yes!
- They meet at F and form a point-sized image. A point-sized image at F is formed by an object at infinity, not by an object at F.
- In front of the mirror, real, inverted and enlarged. That image is formed for an object between C and F. Between P and F the image is behind the mirror.
- Behind the mirror, virtual, erect and diminished. That is the image of a convex mirror. A concave mirror enlarges an object that is between P and F.
- Behind the mirror, virtual, erect and enlarged — correct. Yes!
- Between the pole and the focus — correct. Yes!
- At C. An object at C gives a real, inverted image of the same size.
- Beyond C. An object beyond C gives a real, inverted, diminished image.
- Between C and F. An object between C and F gives a real image beyond C.
- Between P and F — correct. Yes!
- At F. An object at F gives no image: it would be at infinity.