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Concave Mirror Original post: Sun 11/24/2019 at 11:47 AM

Understanding Concave Mirror

Concave mirror is a curved mirror, where the surface of a concave mirror that reflects light curves backward. In the simplest optics is a mirror. Thousands of years ago the mirror was known to have originated from a shiny stone, the mirror itself is divided into 3 types namely a flat mirror, convex mirror and concave mirror.

Concave mirror is a mirror whose surface is like the inside of a ball. Concave mirror is collecting light (converging) and is also called a positive mirror. From the name we can already imagine the appearance of this mirror. True mirror is concave or curved inward. This mirror has another name, Konkaf.

The characteristics of a mirror is to gather light rays and are of two kinds namely virtual and real depending on the distance and location of the object or object that is copied in the mirror area.

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The nature of concave mirrors

  • Rays come parallel to the main axis reflected through focus (f).

Rays come in line

  • Rays coming through the focus (f) are reflected parallel to the main axis.

Rays come through focus

  • Rays coming through the center of the mirror's curvature (P) will be reflected back through the center of the curvature.

Rays come through the central point

Concave mirror has the following properties:

  1. Concave mirror will reflect rays parallel to the focus point.
  2. Concave mirror is collecting light or is called convergent.

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Concave Mirror Formula

Mirror-concave formulaThe magnification that occurs (M) is:

Zoom in (M)The relationship between focus distance and radius is:

The relationship between focus distance


So: the distance of the object to the mirror
Si: the distance of the shadow to the mirror
F: the distance of focus
R: the radius of the mirror
M: the rotation of the image
ho: the height of the object
hi: the height of the shadow

We can determine the properties of shadows in a concave mirror without having to draw the path of the light if the location of the object is known with the following conditions:

  • If the object in space I (between F and O), then the nature of the image: virtual, enlarged, behind the mirror in R IV.
  • If the object in space II (between F and M), then the nature of the shadow: real, inverted, enlarged, in R III.
  • If the object in space III (between M to infinity), then the nature of the shadow: real, inverted, reduced, in space II.
  • If the object at the focal point (F), then the nature of the shadow: virtual, upright enlarged infinity.
  • If the object is at the center of the mirror's curvature (at point M), then the nature of the image: real, inverted, equal.

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Benefits and Uses of Concave Mirror

Benefits of Mirror Concave

  • As a reflector of car lights, reading lights and others.
  • As a slide projector and cinema reflector.
  • As a solar furnace reflector.
  • To examine the cavities of a diseased tooth.
  • On the bottom of the microscope there is a concave mirror.
  • Flashlight reflector
  • Microwave antenna (satellite dish)
  • Solar cooker
  • Making stage binoculars

Concave mirror is also the result of the reflection of light, this makes the concave mirror called converging or gathering light.

Unlike the convex, concave mirror can reflect light parallel to the focus point like a flat mirror. Concave mirror itself is most easily found in the curve of a tablespoon.

And conversely the convex part of the tablespoon is a convex mirror, hopefully this explanation of concave mirror can increase understanding of the lesson about concave mirror. The benefits of concave mirrors can be concluded depending on the location of objects with respect to the focal point. Most concave mirrors that are parallel to the focus point have the benefit of gathering light and reflecting it.

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Problems example

Problem 01

An object is in front of a concave mirror with a focus of 15 cm. the object is placed 20 cm in front of the concave mirror, then the distance of the image formed to the mirror and zoom in is
Ticket: f = 15 cm
so = 20 cm
Dit: si = ...? M = ...?

Discussion :
si = 60 cm (shadow distance)
M (magnification) = si / s0 = 60cm / 20cm = 3 times

Problem 02

An object 4 cm in height is 8 cm in front of a concave mirror. If the radius of curvature of the concave mirror is 12, determine:
a) the distance of the shadow
b) the magnification of the shadow
c) the height of the shadow
d) the nature of the shadow

Discussion The
distance of the focus point is half of the radius.
Concave Mirror
f = 1/2 × 12 = 6 cm
s = 8 cm
h = 4 cm

a) shadow distance, Enter the data
Example Problem 2

b) magnification of the shadow

Example Problem 2ac) shadow height
c) shadow heightd) the nature of the shadow

  • Real (s' positive sign)
  • reverse
  • Located in front of the mirror
  • Enlarged (due to M> 1)

Problem 03

An object is placed in front of a 20 cm convex mirror whose focus distance is 30 cm. The location and nature of the image formed by the mirror are ...
A. 60 cm in front of the mirror, virtual, upright
B. 60 cm behind the mirror, real, upright
C. 60 cm in front of the mirror, real, upright
D. 12 cm behind the mirror, virtual, upright
E. 12 cm in front mirror, real, upright

Data discussion about:
Convex mirror.
f = - 30 cm
s = 20 cm

Find the distance of the shadow first and the signs:
Example Problem 3s' is negative, so virtual, upright. The distance is 12 cm behind the mirror. Answer D.

Problem 04

An object 15 cm in front of a concave mirror that has a focal point of 12 cm. With the space numbering method, determine the properties of the shadows that are formed!

Discussion The
method of spacing is used for concave mirrors , and convex lenses . The following explanation:

Example Problem 4

For this image , the front is on the left, the back is the right.

Between point O and point F is space I
Between point F and point 2F is space II
After point 2F there again is room III
Room IV is on the opposite side, see the picture above.

The number of space numbers for objects with the shadow space numbers is five. For example, in object II, so in order 5, the shadow is in room III. If the object in room I, the shadow space is IV, the object in room III, of course, the shadow in room II.

To estimate the nature of shadows, consider the following conditions:
Shadows in spaces II and III are real, inverted.
The shadow in room IV is virtual, upright.

Enlarged or reduced?
The object in room II, the shadow in room III, from II to III, will enlarge the number, meaning the shadow will be enlarged.
The object in room III, the shadow in room II, from III to II, will shrink, meaning the shadow will be reduced.
Likewise if the object in space I, the shadow will be enlarged.

Back to the problem above, the focus of the mirror is F = 12 cm, 2F means 24 cm, while the object is 15 cm, means the object in room II. The shadow is in room III, so the sifanya is real, upside down and enlarged.

Problem 05

The virtual image formed by a concave mirror is three times bigger than the object. If the mirror's focal distance is 30 cm, then the object's distance in front of the mirror is ...
A. 5 cm
B. 10 cm
C. 20 cm
D. 30 cm
E. 40 cm
(From the Ebtanas 1996 problem)


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