Concave mirrors form both real and virtual images of objects, while convex mirrors form a virtual and erect image.
We find mirrors at our home, in our cars, beauty salons, etc. The list is endless! However, mirrors are called mirrors in a common language, while scientifically, each one of these is categorized into concave and convex mirrors.
Concave and convex mirrors are spherical mirrors. Now, if I ask you what a concave mirror is and how you differentiate it from a convex mirror, you say that it is possible by looking at the images of these two drawn below:
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Concave and Convex Mirror Images
In the above introduction, you understood the basic diagram of concave and convex mirrors. A ray diagram for a concave mirror varies with the object placed at varying positions.
Before starting with the ray diagram of each case, we need to know the following terms:
Principal Axis
The line passing through the centre of the sphere is called the principal axis.
Pole
The centre of the reflecting surface is called the pole (P).
Centre of Curvature
A concave mirror is carved out of a sphere, and its centre is called the centre of curvature (C).
The Radius of Curvature
Radius of the sphere (R)
Focus
The midpoint between C and P.
Sign Conventions:
When the object is placed in front of the mirror, the object is taken as negative.
Signs of the radius of curvature and focal length are also taken negatively.
Ray Diagram of Concave
Now, to understand it in detail, we will first look at the ray diagram concave mirror:
The Image Formed by a Concave Mirror
A concave mirror forms different images for the objects lying at different positions; let’s look at various cases one-by-one:
1. An Object Placed at Infinity
When an object is placed at infinity, the images coming from the distant object parallel to the principal axis converge at the focus ‘F’, as shown in the ray diagram below:
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2. An Object is Placed at C
When an object is placed at C, the real and inverted image is formed at C itself, as you can see in the ray diagram below:
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3. An Object Placed Beyond C
When an object is placed beyond C, the real and inverted image is formed between C and F, as you can see in the image below:
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4. An Object Placed Between C and F
When the object is placed between C and F, the real and inverted image is formed beyond C, as you can see in the ray diagram below:
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5. An Object is Placed at F
When an object is placed at F, the image coming from the distant object pass through C, strike the surface of the mirror, and hence, the reflected ray comes out parallel to the incident ray, as shown below:
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In all the above cases, the image is formed in front of the mirror. Now, we will look at the case of virtual images formed by the concave mirror.
6. An Object Placed Between P and F
This is a special case for the ray diagram of the concave mirror. When an object is placed between P and F, an image is formed behind the mirror. The rays appear to meet each other, so we represent these rays by a dotted line, as shown in the ray diagram below:
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So, the ray diagram for the image formed for the objects placed at different positions is:
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Image Formation by a Convex Mirror
We know that the convex mirrors form a virtual and erect image, and now we will look at the ray diagrams of the convex mirror:
1. An Object Placed at Infinity
When an object is placed at infinity, the incident rays passing parallel to the principal axis converge at F. The image formed is virtual and the zero-sized image is formed as you can see in the ray diagram below:
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2. An Object Placed Between Infinity and P
When an object is placed between infinity and P, the virtual and diminished image is formed between F and P, as you can see in the ray diagram below:
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3. An Object Placed at P
When an object is placed at P, the image will form at the pole itself.
The nature of the image is virtual, upright, and of the same size as that of the object.
Now, let’s look at the convex mirror image formation table:
FAQs on Concave Mirrors and Convex Mirrors
1. What is the principal difference between image formation by concave mirrors and convex mirrors, as explained in the CBSE syllabus?
The principal difference is that a concave mirror can form real or virtual images depending on the object's position, while a convex mirror always forms a virtual, diminished, and erect image regardless of object placement. This distinction is key in understanding their applications in daily life and exam questions.
2. Explain with an example the real-world use of concave mirrors in daily life.
Concave mirrors are used in applications needing concentrated beams of light, such as
- headlights of vehicles
- searchlights
- shaving and makeup mirrors (for a magnified image)
- solar cookers
- medical instruments like ophthalmoscopes
3. Why are convex mirrors preferred as rear-view mirrors in vehicles?
Convex mirrors are preferred as rear-view mirrors because they provide a wider field of view compared to concave or plane mirrors. They always form small, virtual, and upright images, making it easier and safer for drivers to observe more area behind the vehicle without image inversion.
4. How does the position of an object in front of a concave mirror influence the nature and position of the image formed?
The nature and position of the image depend on where the object is placed relative to the mirror's principal focus (F) and center of curvature (C):
- At infinity: Image at focus, highly diminished, real & inverted
- Beyond C: Between F and C, diminished, real & inverted
- At C: At C, same size, real & inverted
- Between C and F: Beyond C, magnified, real & inverted
- At F: Image at infinity, highly enlarged, real & inverted
- Between F and pole (P): Behind the mirror, magnified, virtual & erect
5. Can the image formed by a convex mirror ever be projected onto a screen? Why or why not?
No, images formed by a convex mirror are always virtual and cannot be projected onto a screen because the reflected rays appear to diverge from a point behind the mirror, never actually meeting at a real point where a screen could be placed.
6. Describe the sign conventions used for spherical mirrors in image formation as per CBSE 2025–26 guidelines.
The sign conventions include:
- Distances measured from the mirror along the principal axis
- Distances toward the mirror are negative (object distance, focal length for concave mirror)
- Distances away from the mirror are positive (focal length for convex mirror)
- Height above the principal axis is positive; below, negative
7. What happens to the image quality if the lower half of a concave mirror is covered with an opaque material?
If the lower half of a concave mirror is covered, the image size and location remain the same, but the brightness and clarity reduce. This is because fewer reflected rays contribute to forming the image, leading to lower intensity.
8. How do ray diagrams help in understanding image formation by spherical mirrors for CBSE board exams?
Ray diagrams provide a visual method to locate and predict the characteristics (size, location, and nature) of images formed by mirrors, which is crucial for answering 3-mark and 5-mark exam questions. They help clarify the logic of reflection and the path of rays, an essential CBSE skill.
9. Compare and contrast the uses of concave and convex mirrors in science and technology.
- Concave mirrors focus light and produce real/magnified images, so they're used in headlights, telescopes, and solar cookers.
- Convex mirrors diverge light rays, always creating small, virtual images, making them ideal for surveillance and rear-view mirrors.
10. Why do concave mirrors form both real and virtual images, but convex mirrors cannot?
Concave mirrors have a reflecting surface that curves inward, allowing reflected rays to converge at real points or appear to diverge from virtual ones, depending on object position. Convex mirrors curve outward, causing all reflected rays to diverge as if from a virtual point behind the mirror. As a result, concave mirrors can form both real and virtual images, but convex mirrors only produce virtual images.
