10th Physics | Light | Plane Mirror

Plane Mirror

Any smooth, highly polished reflecting surface is called a mirror. A plane mirror is a highly polished plane surface.

CHARACTERISTICS OF IMAGE FORMED BY PLANE MIRRORS

Following are the important characteristics of image formed by plane mirrors.

Image formation by a plane mirror

Figure 1

1.    The image formed in a plane mirror is always virtual. Such an image cannot be taken on a screen. (figure 1)

2.    The image formed in a plane mirror is always erect. It is the same side up as the object. (figure 1)

3.    The image in a plane mirror is of the same size as the object. (figure 1)

4.    The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of the mirror i.e. image distance is always equal to the object distance in a plane mirror. (figure 1)

5.    The image formed in a plane mirror is laterally inverted i.e. the left side of the object becomes the right side of the image and vice-versa as shown in the figure 2.

Figure 2: Diagram to show lateral inversion

Question:     If an object is placed at a distance of 10 cm in front of a plane mirror, how far would it be from its image?

Solution:

The object would be at a distance of 10 cm + 10 cm = 20 cm from its image.

Question:     State four characteristics of the image formed in a plane mirror.

Solution:

The image formed in a plane mirror is :

(a) always virtual and erect.

(b) of the same size as the object.

(c) at the same distance behind the mirror as the object is in front of it.

(d) laterally inverted (sideways reversed).

Objects and Images

OBJECTS:

Object is a point of intersection of the incident rays.

Objects are of two types :

REAL OBJECT

If the incident rays are diverging from the point of intersection of incident rays then it is called a real object.

VIRTUAL OBJECT

If the incident rays are converging from the point of intersection of incident rays then it is called a virtual object.

IMAGES

Image is a point of intersection of the reflected or refracted rays.

An image is formed when the light rays coming from an object actually meet (or appear to meet) at a point after reflection or refraction.

Images are of two types

REAL IMAGE

The real image formed due to real intersection of reflected or refracted rays. Real image can be obtained on screen.

or

The image formed when two or more reflected or refracted rays intersect each other at a point in front of the reflecting surface (mirror) is known as a real image. It can be obtained on a screen.

e.g. the images formed on a cinema screen are real images.

VIRTUAL IMAGE

The virtual image formed due to apparent intersection of reflected or refracted light rays. Virtual image can’t be obtained on screen.

or

The image formed when two or more reflected or refracted rays appear to intersect at a point behind the normal. It can’t be obtained on a screen.

e.g. The image of our face in a plane mirror is a virtual image.

Note: Human eye can’t distinguish between real and virtual image because in both case rays are diverging.