Question

What should be the position of the object when a concave mirror is used as a doctor’s mirror ?

Answer 1

Hint: To answer this question we will need a basic idea about what a concave mirror is. Later we will learn about the different forms of images formed by a concave mirror when an object is placed in front of it. and we will then decide which is the best position at which the object should be placed in front of a concave mirror while visiting a doctor.

Complete answer:
Concave mirror: A concave mirror has a curled inward curve in the middle. Real and virtual pictures are created by concave mirrors. When the concave mirror is put very close to the item, it produces a virtual and enlarged picture; however, as the distance between the object and the mirror increases, the size of the image decreases and real images are produced.

There are various types of images formed by this mirror when objects are placed at different positions;
1. When the object is placed at infinity: image formed is point size , much diminished, both real and inverted.
2. When the object is placed beyond $C$ (center of curvature): at this position the image formed is at $F$(foci). The image thus formed is diminished, real and inverted.
3. When the object is placed at $C$: at this position the image is formed between $F$ and $C$. The image formed has the same size as that of the object and it is real and inverted.
4. When the object is placed between $C$ and $F$ (focus): The image is larger compared to the size of the object and it is real and inverted.
5. When the object is placed at $F$: This results in the formation of a highly enlarged image which is real and inverted, at infinity.
6. When the object is placed between $F$ and $P$ (Pole): The image formed here is virtual and erect and it is larger than the object.

Hence clearly when we visit a doctor we need to see an image which is real , inverted and can be formed till infinity. Hence the correct answer would be foci.

Note:Do not think that doctors can use convex mirrors. This is not possible, doctors need a magnified and real image hence they use concave mirrors only. Do not confuse the image generated when an object is placed between $F$ and $C$ with the image formed when an object is placed at F. Although the image formed when an object is placed between $F$ and $C$ is magnified, the image formed when an object is placed at $F$ is bigger.