Concave mirror

Introduction

A mirror is a polished surface that reflects light to form images. There are mainly two types of mirrors – plane and spherical. A concave mirror and a convex mirror are spherical mirrors. Spherical mirrors form a part of a hypothetical sphere. They are distinguished based on their shapes – a concave mirror has a reflecting surface that bulges inwards, and a convex mirror has a reflecting surface that bulges outwards. But the most crucial difference between the two is the kind of images they form.

Special terms

• Centre of curvature – the centre of the hypothetical sphere of which the concave mirror is a part
• Pole – the point in the middle of the mirror
• Principal axis – an imaginary line passing through the pole and the centre of curvature
• Radius of curvature – the distance between the centre of curvature and the pole
• Focus – the point that lies halfway between the centre of curvature and the pole. Focal length is half the length of the radius of curvature

Concave mirror

A concave mirror is also known as a converging mirror. Its most important physical characteristic is that its reflecting surface is curved inwards. Its reflecting character is to converge light rays towards one particular point of focus. They are different from convex mirrors because they form images of different sizes and types depending on the distance of the object. The name converging mirror is used for a concave mirror because they focus light at one point. So they are often used in contraptions that are aimed at concentrating light. They do this because the normal on the mirror surface is at a different angle at different points. So an incoming parallel ray of light is reflected to focus on a particular point.

Characteristics of a concave mirror

A concave mirror has the following characteristics:

• A concave mirror converges or focuses light at one point
• The image is virtual and magnified if the object is placed very close to the mirror
• The image formed is real, and diminished in size when the object is very far from the concave mirror

Image in a concave mirror

For surfaces to be reflective, they do not have to be flat. A concave mirror is an example of a non-flat reflective surface. The basic principle at play in image formation by a concave mirror is displayed when the object being reflected is very far from the mirror so that the light rays coming from it are in a parallel beam. Each ray will strike the surface of the concave mirror and be reflected so that the angle of incidence (the angle at which the ray strikes the surface) is equal to the angle of reflection (the angle at which the ray is reflected). If the mirror section is small compared to the radius of curvature, then the image formed will be focused at one point because the rays will pass through the focus. But if the mirror is huge, the image formed will be blurry because some of the rays will not pass through the focus. When this happens, it is called spherical aberration. This is a flaw that is corrected either by using small mirrors or by using parabolic mirrors.

Characteristics of the images formed in a concave mirror

• Object at infinity

1. 1. The image is real
   2. It is formed at the focus
   3. It is inverted
   4. Image is highly diminished

• The object is beyond the centre of curvature

1. 1. Image is formed between the focus and the centre of curvature
   2. Image is real
   3. It is inverted
   4. The image is diminished

• The object is at centre of the curvature

1. 1. Image is real
   2. Inverted
   3. Formed at the centre of curvature
   4. The image is of the same size as the object

• When the object is between centre of curvature and focus

1. 1. The image is formed beyond the centre of curvature
   2. The image is real
   3. The image is inverted
   4. It is enlarged

• The object is at the focus

1. 1. The image is formed at infinity
   2. The image is inverted
   3. The image is real
   4. The image is enlarged

• When the object is between the pole and the focus

1. 1. The image is upright or erect
   2. The image is virtual
   3. The image is formed behind the mirror
   4. The image is enlarged

Uses of a concave mirror

A concave mirror is used for several purposes. Some are as follows:

• Reflecting telescopes use concave mirrors
• Used in magnifying mirrors designed for applying make-up or shaving
• In torches, headlights, and spotlights, a concave mirror is used to gather light from a small source of light and magnify it
• A concave mirror is also used for gathering light and concentrating it onto a spot, as in the case of solar power concentrators
• In laser constructions, optical cavities use concave mirrors
• A concave mirror is sometimes used in dental mirrors for magnifying images
• Modern mirror landing aid uses concave mirrors

Conclusion

A concave mirror is an integral part of optics. They are spherical mirrors whose reflective surface is curved inwards. Their edges are nearer to the viewer than the sphere’s centre. They are used for focusing light or for magnifying images. The rays will be concentrated at the focus as long as the section of the mirror is small. If the mirror is too big, the image will be blurry and distorted. It is essential to understand the principles behind image formation in a concave mirror because of their varied uses in various applications. From telescopes to lasers to dental mirrors, concave mirrors are used widely for several purposes.