In this topic, we study how images are formed by concave and convex mirrors, by using ray diagrams.
From the earlier topic on Reflections, we know that there are two types of mirrors: Plane and Curved. Curved (or spherical) mirrors are of two types, Concave and Convex.
Ray diagrams and image formation in curved mirrors
When an object is placed in front of a mirror, every point on the object reflects or emits light rays in different directions. For simplicity, ray diagrams usually consider only two rays originating from a point on the object, typically the top of the object. The point where at least two reflected rays intersect, or appear to intersect, gives the position of the image formed by the mirror.
When drawing ray diagrams for concave and convex mirrors, we use the following rules:
Incident ray
Reflected ray
- Parallel to the principal axis
b. Appears to diverge out of F in convex.
2. Passing through the focal point (F) in concave, or directed towards it in convex
Parallel to the principal axis
3. Passing through the centre of curvature (C) in concave, or directed towards it in convex
Reflected back in the same direction as the incident ray
4. Incident on the pole (P) obliquely
Reflected with the same angle as that of incidence
Having set the rules for drawing ray diagrams, we can use them to form images in curved mirrors.
Image formation in a concave mirror
A concave mirror converges light rays onto a single point. The type of image formed depends on the position of the object it is reflecting.
- Real image: It can be focused on a screen and captured on a film or sensor. The object and the image will be on the same sides of the mirror. There is no lateral inversion of the image.
- Virtual image: We can only see this but cannot focus it on a screen. It will appear behind the mirror. The image is laterally inverted.
Image: Formed on the focal point. It is point size, real and inverted
Image: Between C and F. Diminished, real and inverted
Image: At C. Same size as that of the object, real and inverted
Image: Beyond C. Enlarged, real and inverted
Image: At infinity. Highly enlarged, real and inverted
Image: Behind the mirror. Enlarged, virtual and erect
Image formation in a convex mirror
Unlike a concave mirror, a convex mirror always forms a virtual, diminished, and erect image, regardless of the object’s position.
Object: At infinity
Image: Behind the mirror, on focal point. Point sized, virtual and erect
Image: Behind the mirror. Diminished, virtual and erect
Summary of image formation
1. As object moves from infinity towards C:
a. Image is real and inverted
b. Image position moves from focus, F towards C
c. Image size increases from point size to object size
2. As object moves from C towards F:
a. Image is real and inverted
b. Image position moves from C towards infinity
c. Image size increases from object size to highly enlarged
3. As object moves from F towards P:
a. Image is virtual and erect
b. Image position is behind the mirror
c. Image size is enlarged
Image is always virtual, erect, diminished irrespective of the object’s position. Its position is always behind the mirror.
1. Object at infinity:
a. Image position at F
b. Image size is point size
2. As object moves from infinity towards mirror
a. Image position is behind the mirror, between F and P
b. Image size is diminished
In the next section, we see how to calculate the size and position of image for both concave and convex mirrors using mirror formula.
Required article
Images are good to understand