Ray Optics and Optical Instruments

Reflection of Light by Spherical Mirrors

When you look in the mirror have you noticed something interesting about you and your image in the mirror? Let us carry out a small activity. Stand in front of the mirror and move your right hand. Now lift your left hand. Did you notice that in the mirror the right appears left and vice versa? Let us study in detail about a spherical mirror.

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Spherical Mirror

Suppose you are sitting at the dining table and you don’t like the food, you start playing with the spoon. You look yourself in the spoon and you notice that you look pretty funny. The moment you get the spoon closer you get a magnified image and when taken far, you see an inverted image.

Do you know what’s really happening? To understand what is happening lets us talk about the special class of mirrors known as spherical mirrors. Let us first understand the terms of spherical mirrors.

  • The radius of Curvature (c): It is the distance between Pole and the Center of curvature.
  • Center of Curvature (r):  The Center of Curvature of a spherical mirror is the point in the center of the mirror which passes through the curve of the mirror and has the same tangent and curvature at that point.
  • Aperture: It is a point from which the reflection of light actually happens.
  • Pole (p): Pole is the midpoint of a mirror. It’s twice the focus.
  • Focus: It is any point, where light rays parallel to the principal axis, will converge after reflecting from the mirror.
  • Principal axis: An imaginary line passing through the optical center and the center of curvature of the spherical mirror.
  • Focal Length: It is on the axis of a mirror where rays of light are parallel to the axis converge after reflection or refraction.

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Reflection of Light by Spherical Mirrors


Spherical mirrors are of two types

  • Convex Mirror
  • Concave Mirror

Concave Mirror

A concave mirror is also known as the converging mirror as in these type of mirrors light rays converge at a point after they strike and are getting reflecting back from the reflecting surface of the mirror.

Convex mirror

The convex mirror has a reflective surface that curves outward. These mirrors are “always” form virtual, erect and diminished regardless of the distance between the object and mirror.

Spherical mirror

When parallel rays of light strike the mirror, they are reflected in a way wherein they spread out or diverge. For this reason, a convex mirror is also a diverging mirror too. If these reflected rays are extended behind the mirror by dotted lines, they meet at a point.

This point is the focus of the convex mirror. The concave mirror is used in the vehicle so that the driver is aware of the vehicle coming from behind. They are also used in street light reflectors.

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Reflection of Light

Any polished or shiny surface like that of an water can act as a mirror. When a ray of light falls on such smooth or shiny object light from the object bounces back those rays of light to our eyes and this phenomenon is Reflection of Light.

Laws of Reflection

In the diagram given above, the ray of light that approaches the mirror is the “Incident Ray”. The ray that leaves the mirror is the “Reflected Ray”. At the point of incidence where the incident ray strikes the mirror, a perpendicular line is drawn is the “Normal”. This normal is what divides the incident ray and the reflected ray equally and gives us the “Angle of Incidence” \( \theta_i \) and “Angle of Reflection” \( \theta_r \).

The Laws of Reflection are :

  •  The angle of incidence is equal to the angle of reflection.
  • The incident ray, the normal and the reflected ray, all lie in the same plane.
  • The reflected ray and the incident ray are on the opposite sides of the normal.

Types of Reflection

  •  Regular Reflection
  • Diffused Reflection

Regular Reflection

It is a mirror-like reflection of rays of light. Here the rays of light which are reflected from a smooth and shiny object such as a mirror, are reflected at a definitive angle and each incident ray which is reflected along with the reflected ray has the same angle to the normal as the incident ray.

Diffused Reflection

This is a non-mirror-like reflection of light. In this type of reflection rays of light hit an irregular object with a rough surface, and reflects back in all directions. Here, the incident ray which is reflected along with reflected ray doesn’t have the same angle to the normal as the incident ray.

Reflection of Rays From a Spherical Mirror

Question For You

Q. When the object is the focus of a concave mirror, the image is formed at

  1. focus
  2. the center of curvature
  3. within a focus
  4. infinity

Answer: D. When the object is the focus of a concave mirror, the image is at infinity.

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