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A concave mirror is a mirror with a curved reflecting surface which is bulging inwards or curved inwards. It is also known as a converging mirror as this reflecting surface collects light incident on its surface, and reflects the parallel rays of light towards a common focus. It can be thought to have been made from the inside of a sphere which has been silvered on the inside. In this knowledge article, we shall get acquainted with some of the uses of concave mirror.

Image Credits: Byjus

There are two rules of reflection of light when it comes to concave mirrors. They are:

  • Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection.
  • Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection.
  • Any ray passing through the centre of curvature of the mirror will reflect back along its own path since the radius of a circle always hits the edge of the circle at 90 degrees – it hits it normally so the angle of incidence and reflection will both be zero.

Image Credits: Physics Classroom

Here, five incident rays along with their corresponding reflected rays, are drawn. Each ray intersects at the image location and then diverges to the eye of an observer.Every light ray would follow the law of reflection and every observer would observe the same image location. However, two of these rays are sufficient to determine the image location since only two rays are required to find the intersection point.

  • When the object is at infinity, a real image is formed at the focus. The size of the image is smaller than the object.
  • If the object is behind the center of curvature, a real image forms between the center of curvature and focus. The size of the image is smaller than the object.
  • If the object is placed at the center of curvature and focus, at the center of curvature a real image is formed. The size of the image is same as that of the object.
  • If the object is placed in between the center of curvature and focus, behind the centre of curvature, the real image is formed. The size of the image is smaller than that of the object.
  • If the object is placed at the focus, the real image is formed at infinity. The size of the image is much larger when compared to the object.
  • If the object is placed in between focus and pole, there is a formation of a virtual image. The size of the image is larger as compared to that of the object.
Position of the Object Position of the Image Image size Nature of Image
At infinity At the focus Highly diminished, point sized Real and inverted
Beyond C Between F and C Diminished Real and inverted
At C At C Same size Real and inverted
Between C and F Beyond C Enlarged Real and inverted
At F At infinity Highly enlarged Real and inverted
Between P and F Behind the mirror Enlarged Virtual and erect

Chart Source: Key Difference

Uses of Concave Mirrors

Depending on the distance between the object reflected, and the mirror, different types of images are formed by concave mirrors. Thus, they are put in a wide variety of uses, some of which are as follows:

  • Shaving mirrors: Objects which are reflected by concave mirrors have a magnified image produced on the mirror. The precision and uniformity required for shaving are helped by using a concave mirror which produces a magnified image of the skin and the hair.
  • Makeup mirrors: Since the enlarged image of the skin can be seen, it helps in applying makeup properly and evenly. Blending can be better with the help of concave mirrors.
  • Headlights: A source of light is placed in the focal point of a mirror. The mirror reflects the light in a parallel beam to the axis of the mirror on to the road.
  • Torch: A torch operates on the same principle as headlights, making use of a concave mirror and a light source.
  • Satellite dishes: Satellite antennas receive weak signals from the communication satellites via space. These signals or waves are almost parallel, and when they strike the concave surface of the antenna, they are reflected and converge on the focus of the antenna. Amplification of the signal thus occurs here, and a receptor placed at the focal point receives this amplified signal and transmits it further. The mirror thus works as a receptor and an amplifier.
  • Telescopes: Used for astronomy, telescopes have a concave mirror placed at the bottom of the telescope. On the other end is a plain mirror. The plain mirror is used to form the image by the mirror. The concave mirror focuses the light and creates a clearer image of the object. Thee concave mirror helps in gathering more light and produces a clear image of a distant object.
  • Medical use: Dentists and ENT specialist doctors use concave mirrors to get a magnified clearer view of the teeth, skin and ear cavities.
  • Solar powered gadgets: The solar-powered gadgets operate on the same principle. Parallel sunrays are converged on a focal spot and then used to generate energy for activities like cooking, recharging batteries, boiling water etc.
  • Microscopy: These are used to magnify objects for studying in electron microscopes and magnifying glasses.
  • Visual bomb detectors make use of these concave mirrors as well.
  • The mirror used in the flashlight of cameras is also a concave mirror.

To know more about the reflection and refraction of light, visit here.

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