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Home > Articles > Mirror Formula - Definition, Explanation, Sign Convention, Solved Examples, Applications
Updated on 03rd June, 2024 , 6 min read
The law of reflection states that when a ray of light is made to fall on the reflecting surface, the incident ray, the reflected ray, and the normal to the surface of the mirror all lie in the same plane and the angle of incidence is equal to the angle of reflection.
The following are the most common types of mirrors:
The reflected images in their normal proportions but reversed from left to right are formed by a plane mirror.
These are curved outward spherical mirrors, and the image obtained is virtual, diminished, and erect for a real object.
These are curved inward spherical mirrors, and the image obtained from these mirrors is dependent on the placement of the object.
The mirror formula is the relationship between the focal length of the mirror, the object's distance u from the pole of the mirror, and the image's distance v from the pole.
The mirror equation is
1/v +1/u = 1/f.
The Mirror formula represents the relationship between the object distance (u), image distance (v), and focal length (f) of a Spherical Mirror. The mirror equation is written as follows:
1/f = 1/v+1/u
Here, u and v represent the distances of the object and image from the mirror's pole, respectively. And Focal Length (f) is the distance between the primary focus and the pole.
Using the equation below, we can calculate the magnification (m) of the object using u and v.
m=−v/u
The radius of curvature (R) is twice its focal length.
R=2f
and f=R/2
Hence, the Mirror equation can be written as:
1/u+1/v = 1/f= 2/R
The new Cartesian Sign Convention is used to avoid confusion in understanding the ray directions. Please take a look at the diagram for a better understanding.
Read More About: Mirror Formula Derivation
The Mirror Equation is used in the following ways:
Check out:
Q1. Calculate the magnification of a concave mirror if the image of an object formed in front of a concave mirror with a focal length of 12 cm is formed at a point 10 cm away from the mirror.:
Solution:
To calculate the magnification (M) of a concave mirror, we can use the mirror formula and the magnification formula. The mirror formula is:
1/f=1/do + 1/di
where:
- f is the focal length of the mirror.
- do is the object distance (the distance from the object to the mirror).
- di is the image distance (the distance from the image to the mirror).
The magnification of (M) is given by:
M= hi/ho = -di/do
where:
- hi is the height of the image.
- ho is the height of the object.
- di is the image distance.
- do is the object distance.
Given:
- The focal length, f= -12 cm (the focal length is negative for concave mirrors).
- The image distance, di = -10 cm (the image distance is negative since the image is formed in front of the mirror).
1/f=1/do + 1/di
1/-12 = 1/do + 1/-10
1/do= 1/-12 + 1/10
= -5/60 + 6/60 = 1/60
Therefore, do = 60 cm
= -(-10)/60 = 10/60 = 1/6
Therefore, the magnification of the concave mirror is 1/6. This means the image is 1/6th the size of the object and is inverted (as indicated by the negative sign in the magnification formula).
Q2. The radius of curvature of a convex mirror used for rear view on a bus is 4m. Determine the position of the image if a car is 2m away from the mirror.
Solution:
To determine the position of the image formed by a convex mirror, we use the mirror formula:
1/f = 1/v + 1/u
where:
- f is the focal length of the mirror,
- v is the image distance,
- u is the object distance.
For a convex mirror, the focal length (f) is positive and is given by:
f = R/2
where (R) is the radius of curvature of the mirror.
Given:
- The radius of curvature R = 4,
- The object distance u = -2 cm (since the object distance is always taken as negative for real objects in mirror formulas).
f = 4m/2 = 2m
1/f = 1/v + 1/u
½ = 1/v + 1/-2
1/v = ½ + ½
1/v = ½ - ½
=½ + (-½)
=½ - ½ = 0
Therefore, 1/v = 0; and v= ∞
This result indicates that the position of the image is at infinity, which implies that for an object placed at 2 meters from a convex mirror with a radius of curvature of 4 meters, the image appears to be very far behind the mirror, almost at an infinite distance.
In reality, due to the nature of a convex mirror, the image formed is virtual, erect, and diminished, located behind the mirror but virtually at a far distance, giving the appearance that it is at infinity.
Also Check:
A mirror typically has a flat or curved surface made of glass, is covered in reflective material, and is made of glass. Mirrors are used in many scientific and technological components and are not just for aesthetic purposes. Before mirrors were created, people frequently used water pools to see their reflections.
Justus von Liebig invented modern mirrors in Germany in 1835
A flat (planar) reflective surface is referred to as a plane mirror. For light rays striking a plane mirror, the angle of reflection coincides with the angle of incidence.
A two-way mirror, also referred to as two-way glass, is a type of glass that is clear on one side and reflective on the other, giving the impression of a mirror to those who see the reflection while allowing those on the clear side to see through as through a window.
There are four different types of mirrors: spherical, inclined, rotating, and plane mirrors. Furthermore, spherical mirrors are divided into two types: concave spherical mirrors and convex spherical mirrors.
The mirror formula is represented by the following equation: 1 f = 1 v + 1 u. The object distance is the separation between the object and the mirror, the image distance is the separation between the mirror and the image, and the focal length of the mirror is where.