Concave and Convex Mirrors

A mirror can be termed as a highly polished and smooth reflecting surface. The most common types of mirrors that are used are plane mirrors. The spherical mirror is part of the spherical reflecting surface. There are two categories of spherical mirrors - concave mirrors and convex mirrors.

Concave mirror: 

Concave mirror has a reflecting surface that caves inwards. The concave mirrors essentially converge light to only one prime focus point. This is why they are also known as converging mirrors. These mirrors are used for focusing light and the image that is formed by the concave mirror differs in size based on object position with respect to the mirror. The image exists either in virtual or real form. Erect or magnified and inverted, of the same size as that of the object or diminished, and it all depends on the position of the object. 

Convex mirror: 

The convex mirror is the curved mirror where the reflective surface bulges out towards the position of the light source. The convex mirrors tend to reflect light in the outwards direction, diverging light rays, and hence they are not used for focusing light. The image is erect, virtual, and smaller than the object, however gets larger as the object comes closer to the mirror. These mirrors are also referred to as diverging mirrors. 

Mirrors and the reflection laws

When we have a look at ourselves in the flat mirror, we appear to be of the same size as we actually are and our distance appears to be as behind the mirror as we actually are standing in front of the mirror. This is due to how reflection happens and it is known as the laws of reflection. According to the 1st law of reflection, the light, which hits a mirror would essentially bounce back at the same angle. If the specific mirror is flat, then it would lead to the image appearing as life-size. The image type that is created by the flat mirror, is known as the virtual image since it appears behind the mirror. The reflection laws are valid and hold true for any type of surface or mirror. 

The different terms that are used in spherical mirrors are pole, a centre of curvature, a radius of curvature, principal axis, aperture, focus, focal length. The pole is the midpoint of the mirror while the centre of curvature is the centre of the sphere, which is part of the image formed by the mirror. The radius of curvature refers to the distance between the centre of curvature and the pole. Its focal length is twice the length of the mirror. 

The principal axis is essentially the imaginary line that passes via the pole and centre of curvature of a spherical mirror. Aperture is used for denoting the mirror size. The focus is the point on the principal axis, where light rays that are parallel to the principal axis will appear to diverge from (for convex mirror) or converge ( for concave mirror) after reflecting from the mirror. The focal length is the distance between the focus of the mirror and the pole.

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A mirror is a part of a smooth and highly polished reflecting surface. Most commonly used mirrors are plane mirrors. A spherical mirror is a part of a spherical reflecting surface. There are two types of spherical mirrors - convex mirror and concave mirror.

Convex Mirror:

Convex mirror is a curved mirror for which the reflective surface bulges out towards the light source. Convex mirrors reflect light outwards (diverging light rays) and therefore they are not used to focus light. The image is virtual, erect and smaller in size than the object, but gets larger (maximum upto the size of the object) as the object comes towards the mirror. Such mirrors are also called diverging mirrors.

Concave Mirror:

A concave mirror has the reflecting surface that caves inwards. Concave mirrors converge light to one prime focus point. Therefore, they are also called converging mirrors. They are used to focus light. The image formed by a concave mirror varies in size depending on the position of the object with respect to the mirror. The image can be real or virtual, erect or inverted and magnified, diminished or of the same size as that of the object, all depending on the position of the object.

Real images can be brought onto a screen and they are always inverted.

Mirrors and reflection Laws of Reflection:

When we look at ourselves in a flat mirror, we will appear in the same size as we are and will appear to be just as far behind the mirror as we will be actually standing in front of the mirror.

This is because of how reflection takes place. This is concluded as the laws of reflection. The first law of reflection says that the light that hits a mirror would bounce back at the same angle. If the mirror is flat that would cause the image to appear life-size. 

The type of image created by a flat mirror is called the virtual image as it will appear behind the mirror. 

Laws of reflection are valid for any type or mirror or surface. 

Terms Used in Spherical Mirrors:

• Pole (P): It is the midpoint of a mirror.

• Centre of Curvature (C): It is the centre of the sphere of which the mirror forms a part.

• The Radius of Curvature (R): It is the distance between the pole and the centre of the curvature. It is twice the focal length of the mirror.

• Principal Axis: An imaginary line that is passing through the pole and the centre of curvature of the spherical mirror.

• Aperture: It is used to denote the size of the mirror.

• Focus: It is the point on the principal axis, where the light rays parallel to the principal axis will converge (in the case of a concave mirror) or appear to diverge from (in the case of a convex mirror) after reflection from the mirror.

• Focal Length: The distance between the pole and the focus of the mirror.

Properties of the Images Formed by a Convex Mirror:

Whatever be the position of the object in front of the convex mirror, the images is always smaller than the object, erect, virtual and also formed within the focus. 

Why is that a Convex Mirror Never Forms a Real Image?

A real image occurs where rays converge, whereas virtual image occurs when rays diverge and only appear to come from a point. The real images cannot be produced by a convex mirror as it diverges the rays. 

Real Image:

The real image is formed as a result of the actual convergence of the reflected light rays. It can be received on a screen and it is always inverted.

Do Concave Mirrors Always Form Real Images?

The concave mirror forms an image and that depends on the two parameters: the object distance and the focal length of the mirror.

If the object is placed between the pole and the focus of a concave mirror, a magnified and erect virtual is found to be formed.

Difference Between Convex and Concave Mirrors:

Image Formation by Concave and Convex Mirrors:

Convex Mirror Ray Diagram:

• When an object is placed at infinity a virtual image will be formed at the focus point. The image will be highly diminished as compared to the object.

• When an object is placed at a finite distance from the mirror, the virtual image will be formed between the pole and focus. The size of the image is smaller as compared to the object.

Concave Mirror Ray Diagram:

• When an object is at infinity, a real image is formed at the focus point. The size of the image is much smaller as compared to the object.

• A real image will be formed between the focus and centre of curvature, when the object is placed beyond the centre of curvature. The size will be small as compared to the object.

• When an object or thing is set at the centre of curvature, the real image gets formed at the centre of the curvature. The size of the image is the same as that of the object.

• When an object/thing is set between the curvature and focus, the real image will be formed beyond the centre of curvature. The size will be larger as compared to the object.

• When an object is placed at the focus, the real image will be formed at infinity. The size of the image is much larger as compared to the object that is placed at the focus.

• When the object is placed between the pole and focus, a virtual image is formed behind the mirror. The size is larger as compared to the object.

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Uses of Convex Mirrors:

The convex mirror is used as a side-view/rear – view mirror of a vehicle because it forms an erect and smaller image. Convex mirror gives a wide rear view.

The convex mirror is suitable for convenient shops and big supermarkets and any other corner for a wide observation.

They can be used as street light reflectors because they can spread the light over a bigger area.

They are put on the corners of roads so that the drivers can see any vehicles and avoid collisions by taking due measures.

Uses of Concave Mirrors:

The concave mirror is a converging mirror, so it is used for many purposes. 

• It is used in a torch, automobile headlamps, lighthouses etc to reflect the light and make a fine beam.

• It is used in the aircraft landing at the airports to guide the airplane.

• It is used in the shaving process where you can get an enlarged and erect image of the face.

• It is used in solar ovens also. It collects a large amount of solar energy and focuses to a point where the vessel containing water or item to be cooked is placed.

• Concave mirrors are used in satellite dishes, they are also used by dentists and ENT doctors use them to obtain a larger image.

• Concave mirrors are used in electronic microscope, astronomical telescopes, visual bomb detectors etc.