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Every day we come across plenty of spherical surfaces unknowingly. But, we hardly realise how amazingly it works. For example, the spectacles you are wearing, the vehicle mirror you have, everything comes with a spherical surface.
A change in the light direction or just a bending in the light wave moving from transparent mediums is caused due to wave speed. This phenomenon is known as refraction. You will come across this phenomenon in nature. Lenses tend to converge or diverge light rays passing through it only due to refraction. Let us check out more about refraction.
A mirror or surface that resembles the shape of a sphere is termed a spherical mirror or surface. In other words, a spherical mirror looks like it is a part that has been cut from a sphere. Spherical mirrors are of two types – Concave mirrors and Convex mirrors.
Reflection of Light on Spherical Surfaces
A mirror or surface that resembles the shape of a sphere is termed a spherical mirror or surface. In other words, a spherical mirror looks like it is a part that has been cut from a sphere. Spherical mirrors are of two types, namely, concave mirrors and convex mirrors.
Concave mirror
Also referred to as converging mirrors, this type of mirror is known to converge the rays that fall on it. When a beam of light strikes at the concave mirror, the light rays converge at a single point. And, this type of mirror curves inward. Such mirrors are used as vehicle mirrors as well as in street lights.
Concave Mirror Characteristics
• Light converges at a spot when it reaches the concave mirror reflecting surface and reflects back.
• A magnified and simulated picture is generated when the concave mirror is put very close to the object, so it is also known as a converging mirror.
• However, as the distance between the item and the mirror is increased, the size of the image shrinks and a true image emerges.
• The concave mirror can produce a diminished or magnified, which can be real or virtual.
Terms related to concave mirrors
It is necessary to familiarise yourself with a few terms related to concave mirrors:
Centre of Curvature: In the case of a curved mirror, the centre of curvature can be described as the centre of a hollow glass sphere, from which the curved mirror is taken as a part.
Radius of curvature: The radius of the hollow glass sphere of a spherical mirror is known as the radius of curvature of a curved mirror.
Principal axis : The imaginary line that runs through the pole and the spherical mirror’s centre of curvature is known as the principal axis.
Principal focus: For a spherical mirror, the principal focus is described as the point where the reflected rays meet or seem to meet. The primary focus of a concave mirror is taken on the front, while the primary focus of a convex mirror is taken at the back.
Convex mirror
Unlike concave mirrors, this type of mirror has an outward curve. The images formed by the Convex mirror are erect, virtual, and diminished. The size of the image is independent of the distance between the mirror and the object. When the beam of light strikes the surface, the lines get reflected by either diverging or spreading out. For this reason, these mirrors are known as diverging mirrors.
Rules for image formation due to convex mirror
Rule 1
The incident ray of light that is travelling parallel to the principal axis. Creates a reflected ray that appears to originate from the principal focus on the other side of the mirror.
Rule 2
If the incident ray of light appears to head towards the principal focus on the other side of a convex mirror, then the reflected ray is parallel to the principal axis.
Rule 3
If the incident ray of light is not parallel or perpendicular to the principal axis and is heading straight for the pole of the convex mirror, then the rays reflected such that the reflected ray has an angle of reflection equal to the angle of incidence measured between the principal axis and each ray.
Rule 4
A ray of light heading towards the centre of curvature on the other side of the mirror gets reflected in such a way that it retraces its path.
Image formation by a convex mirror
The reflecting surface in convex mirrors is on the outside of the sphere, making the centre of curvature and the focal point on the other side of the mirror from the object. The focal length being on the opposite side of the mirror from the object is assigned a negative number.
Case 1: When object is placed at Infinity
The incident right rays of the object coming from Infinity to the mirror aimed at the focal point ( that is behind the mirror ) will be reflected such that the reflected rays are parallel to the principal axis.
Similarly, the light rays approaching the mirror parallel to the principal axis are reflected as if they came from the focal point. In the case of a convex mirror. If the object is at infinity, the image will be a dot on the focal point.
Case 2: When object is placed anywhere on the principal axis
When an object is placed anywhere on the principal axis, the image formed appears somewhere between the mirror and the focal point.
The ray tracing for convex mirrors follows this general sketch. Two rays leave the tip of the object, one approaches the mirror parallel and reflects as if it came from the focal point.
Refraction of Light at Spherical Surfaces
When two spherical lenses come in contact with each other (face to face), it forms a spherical lens shape. Spherical lenses are of two types – Concave lenses and Convex lenses. When the ray of light passes through the Concave lens, it forms a virtual, erect and diminished image.
Laws of Refraction
There are two laws of refraction.
All the three rays – incident ray, normal and refracted ray lie in the same plane.
The ratio of the sine angle of incidence to the sine angle of refraction is always constant, for a given pair of mediums. This law is also known as snell’s law.
Conclusion
Both convex and concave mirrors are used in our daily lives, and both of them act differently. In some instances, the size of the image formed by the mirrors depends on the distance between the mirror and the object. Here we’ve explained all the facts and factors related to the reflection and refraction of light at plane and spherical surfaces.
