Image formation by a concave mirror for Class 10 involves understanding how light rays reflect off the curved surface to create either a real or a virtual image, primarily depending on the object's position relative to the mirror.
The formation of an image that occurs in a concave mirror mainly depends on the distance between the object and the mirror. This is a crucial concept as different object distances result in varying image characteristics. Both real and virtual images are formed by the concave mirror.
Types of Images Formed by Concave Mirrors
Concave mirrors can form two types of images:
- Real Image: An image formed by the actual intersection of reflected light rays. Real images are inverted and can be projected onto a screen.
- Virtual Image: An image formed when reflected light rays appear to diverge from a point behind the mirror. Virtual images are erect and cannot be projected onto a screen.
How Object Position Affects Image Formation
The characteristics of the image (its position, nature, and size) change significantly as the object is moved closer to or farther away from the concave mirror. Let's look at the different cases:
Key Positions for Image Formation:
- Principal Axis: The straight line passing through the pole and the centre of curvature.
- Pole (P): The centre of the reflecting surface of the mirror.
- Principal Focus (F): The point on the principal axis where parallel rays of light converge after reflection (for a concave mirror).
- Centre of Curvature (C): The centre of the sphere from which the mirror is a part.
Image Formation Cases:
| Object Position | Image Position | Nature of Image | Size of Image |
|---|---|---|---|
| At Infinity | At F | Real & Inverted | Highly Diminished (Point) |
| Beyond C | Between F and C | Real & Inverted | Diminished |
| At C | At C | Real & Inverted | Same Size |
| Between C and F | Beyond C | Real & Inverted | Enlarged |
| At F | At Infinity | Real & Inverted | Highly Enlarged |
| Between P and F | Behind the mirror | Virtual & Erect | Enlarged |
This table summarizes the outcomes based on where the object is placed. Notice the transition from real, inverted, and diminishing images to real, inverted, and enlarging images as the object moves from infinity towards F.
The Special Case: Object Very Close
As stated in the reference, When the object is placed very close to the mirror, a virtual and magnified image is formed. This corresponds to the case where the object is placed between the Pole (P) and the Principal Focus (F). In this scenario, the reflected rays diverge but appear to originate from a point behind the mirror, forming a virtual, erect, and significantly magnified image.
Understanding Ray Diagrams
To understand how these images are formed, ray diagrams are used. These diagrams trace the path of two or three specific light rays originating from the object and reflecting off the mirror. Common rays used are:
- A ray parallel to the principal axis passes through the principal focus (F) after reflection.
- A ray passing through the principal focus (F) becomes parallel to the principal axis after reflection.
- A ray passing through the centre of curvature (C) is reflected back along the same path.
- A ray incident at the Pole (P) is reflected back symmetrically to the principal axis.
The point where the reflected rays (or their extensions) intersect is where the image is formed.
Practical Applications
The ability of concave mirrors to form different types of images makes them useful in various applications, such as:
- Shaving mirrors: Used to form a magnified, virtual image when held close to the face (object between P and F).
- Headlights of cars: A bulb placed at the focus creates a powerful, parallel beam of light.
- Reflecting telescopes: Used as the primary mirror to collect and focus light from distant objects.
- Solar furnaces: Large concave mirrors concentrate sunlight at the focus to generate heat.
In summary, image formation by a concave mirror is a versatile process governed by the object's distance from the mirror, resulting in a range of real and virtual images with varying sizes and positions.
