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Mirror Formula Derivation: Definition, Equation, Applications and Proof

Kasturi Talukdar

Updated on 22nd May, 2023 , 8 min read

Mirror Formula Derivation Overview

The mirror formula, also known as the mirror equation, is obtained through a derivation that establishes a relationship between the object distance, image distance, and focal length. It is applicable to both plane mirrors, which produce straight images that reverse the front and back of real objects, and spherical mirrors, such as concave and convex mirrors. Spherical mirrors have a shape resembling a piece cut from a spherical surface or material. The derivation of the mirror formula is a commonly encountered topic in board and competitive examinations. In this article, we will explore the derivation process and provide sample questions related to the mirror formula.

Understanding Spherical Mirrors and Their Properties

Spherical mirrors are curved glass surfaces that reflect light back to the surface. By understanding the properties of these mirrors, we can derive the formulas for calculating their magnification and other properties.

Let's start with basic anatomy: Spherical mirrors have two parts—the radius of curvature (R) and the focal point (F). Radius of curvature (R) is the distance from the mirror's center to its edge, while focal point (F) is the point where all incoming light converges when it reflects off the mirror.

The shape of a spherical mirror can vary depending on its curvature. Mirrors with short radii of curvature (R1) will produce a convex mirror, while mirrors with long radii of curvature (R2) will produce a concave mirror. In either case, the reflected images will be distorted depending on how far away they are from the focal point (F).

By understanding these basic properties of spherical mirrors, we can begin to derive the formulas for calculating their magnification and other characteristics. This is essential for professionals in all fields who depend on accurate readings from mirrors, from optics engineers to physicists.

What Is the Mirror Formula?

The mirror formula is an equation that relates the object distance (u), image distance (v), and focal length (f) of a spherical mirror. It is commonly used in optics to determine the position and characteristics of an image formed by a mirror.

The mirror formula is expressed as:

1/f = 1/v + 1/u

Where:

  • f represents the focal length of the mirror.
  • v represents the distance of the image from the mirror (positive for real images and negative for virtual images).
  • u represents the distance of the object from the mirror (positive for objects in front of the mirror and negative for objects behind the mirror).

This formula is derived from the lens/mirror maker's formula, which is based on the principles of refraction and reflection. By using the mirror formula, one can calculate the focal length of a mirror if the object and image distances are known, or vice versa. It is a fundamental tool in understanding the behavior of spherical mirrors in optics.

Read more about the Enantiomers.

Sign Convention for Mirror Formula Derivation

In the derivation of the mirror formula, a sign convention is followed to determine the sign (+/-) of the object distance (u), image distance (v), and focal length (f). This convention helps in representing the direction and nature of the distances and focal length involved in the formula. The sign convention for mirror formula derivation is as follows:

Object Distance (u):

  • The object distance (u) is positive (+) when the object is located in front of the mirror (on the same side as the incident light).
  • The object distance (u) is negative (-) when the object is located behind the mirror (on the opposite side of the incident light).

Image Distance (v):

  • The image distance (v) is positive (+) when the image is formed on the opposite side of the mirror (on the side of the reflected light).
  • The image distance (v) is negative (-) when the image is formed on the same side as the object (virtual image).

Focal Length (f):

  • The focal length (f) is positive (+) for a concave mirror.
  • The focal length (f) is negative (-) for a convex mirror.

By adhering to this sign convention, the mirror formula equation (1/f = 1/v + 1/u) can be used to correctly determine the characteristics and position of the image formed by the mirror.

Assumptions of Mirror Formula Derivation

The mirror formula is derived based on a few assumptions. These assumptions include:

  1. Thin Mirror: The mirror is assumed to be thin, meaning its thickness is negligible compared to its radius of curvature. This assumption allows us to treat the mirror as a two-dimensional surface.
  2. Spherical Mirror: The mirror is assumed to have a spherical shape. While this assumption may not hold exactly for all mirrors, it is a reasonable approximation for most practical purposes.
  3. Small Angles: The angles involved in the derivation are assumed to be small. This assumption allows us to use the small-angle approximation, where the tangent of a small angle is approximately equal to the angle itself. It simplifies the calculations and makes the derivation more manageable.
  4. Paraxial Rays: The mirror formula is derived using paraxial rays, which are rays that are close to the principal axis and make small angles with it. This assumption ensures that the rays are close enough to the principal axis, making the calculations more accurate.
  5. Reflection and Refraction Laws: The derivation assumes that the laws of reflection and refraction hold. These laws state that the angle of incidence is equal to the angle of reflection for reflection, and the incident and refracted rays lie in the same plane and obey Snell's law for refraction.

 Mirror Formula Derivation

Consider the ray diagram where the object AB is positioned on the principal axis of a concave mirror. The object is placed beyond the center of curvature (C), resulting in the formation of an image A'B' between the center of curvature (C) and the principal focus (F) of the concave mirror. In this scenario, we are specifically considering a concave mirror that forms a real image.

 

mirror formula derivation

 

By examining the ray diagram, we can observe that triangles ΔABC and ΔA′B′C are similar. Therefore, the ratios of their corresponding sides are equal:

AB / A′B′ = CB / CB′

Similarly, triangles ΔABP and ΔA′B′P are also similar, leading to the relationship:

AB / A′B′ = PB / PB′

Combining these two relations, we find:

AB / A′B′ = CB / CB′ = PB / PB′

Next, measuring distances from the pole (P), we have:

CB = PB – PC

CB′ = PC – PB′

Substituting the values of CB and CB' into the equation above, we get:

(PB – PC) / (PC – PB′) = PB / PB′

Applying the Cartesian sign convention, we assign the following values:

PB = –u

PC = –R

PB′ = –v

Replacing PB, PB', and PC in the equation, we have:

(-u - (-R)) / (-R - (-v)) = -u / -v

Simplifying further, we obtain:

(-u + R) / (-R + v) = u / v

Further simplification leads to:

v(-u + R) = u(-R + v) - vu + vR

= -uR + uv

Alternatively, we can express it as:

uR + vR = 2uv

Dividing both sides by uvR, we get:

1/v + 1/u = 2/R

Since we know that f = R/2, we can substitute it into the equation:

1/f = 1/v + 1/u

This equation holds true even when a virtual image is formed by a concave mirror.

Hence, the mirror equation for a spherical mirror is:

1/f = 1/v + 1/u

For a convex lens, the mirror formula is:

1/f = 1/v - 1/u

The Variables in the Mirror Formula Derivation: Object and Image Distances

The "Mirror Formula" is a mathematical expression of how light rays are reflected off of a curved mirror. It is composed of two variables: object distance (do) and image distance (di).

Object Distance

Object distance is the distance between the object and the center of curvature of the reflecting surface. This distance is measured in meters, with a negative sign indicating that the object is located on the same side as the mirror.

Image Distance

Image distance is determined by measuring how far away from the center of curvature the reflected image appears. This distance is also measured in meters, with a positive sign indicating that it appears on the opposite side from the mirror.

Together, these two variables provide an accurate reflection of light rays - that is why they are used to determine image distances for various applications such as lens design and beam divergence calculations. By taking into account both object and image distances, you can ensure accurate reflections from your mirrors.

Mirror Formula Derivation: Examples

Reflection of Light

The mirror formula describes how light reflects off of a curved surface, such as a convex or concave mirror. For instance, if you were to draw a line from the center of curvature (C) to the focal point (F) and then another line from the center of curvature to any point on the mirror's surface (Q), then the angle between these two lines will be equal to twice the angle between F and Q. Once this angle is known, calculating the reflected light follows from basic trigonometric principles.

Reflection of Acoustic Waves

The same principle applies for acoustic waves. If one were to place a speaker at C and a microphone at Q, then measuring the angle between these two points will tell you exactly how sound will reflect off of the curved surface. From this data, it's possible to determine how loud certain frequencies would be in different parts of a room - allowing us to better tune our acoustics for optimal sound quality.

Reflection of Electromagnetic Waves

Finally, optical techniques such as spectroscopy depend heavily on understanding how electromagnetic radiation interacts with different materials. By using our mirror formula, we can calculate exactly where radiation is most likely to reflect off of an object - allowing us to make precise measurements about its composition.

Mirror Formula Derivation: Application

The Mirror Equation can be utilized in the following manners:

  1. Given the object distance and the focal length of the mirror, the mirror equation enables us to determine the image distance.
  2. When provided with the image distance and the focal length of the mirror, the equation allows us to calculate the object distance.
  3. By knowing both the object distance and the image distance, the mirror equation facilitates the calculation of the focal length of the mirror.
  4. In conjunction with the magnification equation, the mirror equation can be employed to derive the value of either the image height or the object height when one of them is known.

Mirror Formula Derivation: Things to Remember

  1. Optics is the scientific discipline that focuses on the study of light, including its characteristics and behaviors.
  2. Light, an electromagnetic radiation, enables human vision and renders objects visible, observable without the aid of specialized equipment.
  3. The focal length of a lens dictates its capacity to either converge or diverge light rays.
  4. By employing mirror formulas and equations, we can ascertain the location where an image will form if we possess knowledge of the object's position and the mirror's focal length.
  5. The Mirror Equation represents a mathematical formulation that establishes a connection between the object distance, image distance, and focal length of a mirror.
  6. The mirror formula can be expressed as 1/f = 1/v + 1/u, where f represents the focal length, v signifies the image distance, and u denotes the object distance.

Conclusion

In conclusion, the process of mirror formula derivation is a crucial mathematical tool that can be used to solve problems related to ray optics and to find the characteristics of images produced in mirrors. Understanding the basic steps and formulas associated with the mirror formula derivation is a fundamental requirement for anyone dealing with optics, allowing quick and accurate calculations that provide useful results. Through these calculations, the properties of images formed in mirrors can be accurately determined. So, the next time you're looking for the image characteristics in a mirror, remember the process of mirror formula derivation and use it to maximize your accuracy and efficiency.

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