Real and Virtual Images and Curved Images

Real and Virtual Images

Real Image
An image is said to be real when the rays of light producing the image actually pass through the position of the image as shown in the figure. The real image is produced when a converging beam is an incident on the plane mirror. Such type of image can be projected or obtained on the screen.

Virtual Image

An image is said to be virtual when the rays of light producing the image do not actually pass through the position of the image but only appear to come from that point as shown in the figure. Such an image cannot be projected on the screen.

Location of Image by No Parallax Method

A virtual image of an object can be located by the method of no parallax, which is described below.

Consider a plane mirror m and an object say a pin is placed at point O in front of the mirror so that its image is formed at a point I behind the mirror as shown in the figure. A pin A behind the mirror is moved towards and away from the mirror and for each position of A, the relative motion between I and A is noted by moving the eyes to left and right sides across the mirror. If they move together, A is at the place of I and they are called in coincident position. But, if A and I do not coincide, then they appear to move together, A ia at the place of I and they are called in coincident position. But, if A and I do not coincide, then they appear to move relative to each other. The relative movement between A and I are called parallax.

Images in Inclined Mirrors

When two plane mirrors are inclined to each other, a large number of images of a point object, placed in front of them, are formed due to multiple reflections between them.

For this, consider two plane mirrors OX₁ and OX₂ which are inclined at 90^o. When a point object is placed at point A, its image due to mirror OX₁ is formed at point I₁ behind the mirror. I₁ is in front of the image mirror OX₂ so that its image is obtained at point I₁₂ behind the mirror, no other images will be formed by the two mirrors. From the principle, we can write

$$ AB = BI_1, \: CI_1 = CI_{12} $$

Again, the mirror OX₂ forms an image of the object behind the mirror at point I₂. This image lies in front of the mirror OX₁ so that its image is obtained at point I₂₁ behind the mirror; no further images will be forms by the two plane

mirrors are inclined at 90^o to each other three images I₁, I₂ and I₁₂ or I₂₁ of a point object are formed. So enlarging this for an inclination at an angle Ï´, we can write

The number of images produced, \(N = \frac {360^o}{\theta } -1 \).

Fro the validity of the above formula, draw the ray diagram for the location images when the angle inclination is 30^o, 45^o, 60^o and 75^o.

For example, estimate the minimum size of a mirror to see the full image of a man standing in front of the mirror as shown in the figure.

Consider a plane mirror XY and a man HF is standing in front of the mirror. Suppose F, E, G and H are the foot, eye and head of the man respectively. To see head, light coming from the head must be reflected by the mirror to reach eye. So from the laws of reflection of light, we have

$$ HL = LE = AC \dots (i) $$

Again, to see foot F, light coming from the foot must be reflected by the mirror to reach the eye. So we can write

\begin{align*} EM = MF = CB \dots (ii) \\ \text {Thus the minimum size of mirror} = AB = AC + CB = LE + EM \\ &= \frac 12 HE + \frac 12 EP \\ &= \frac 12 (HE + EP) \\ &=\frac 12 HF \\\end{align*}

That is, the required size of the mirror is half the size of the man.

Curved mirror

A mirror which forms a part of a spherical reflecting surface is called the spherical mirror or curved mirror. There are two types of spherical mirrors:

1. Concave mirror
2. Convex mirror

Concave Mirror

A spherical mirror in which reflection takes place on the inner side of the sphere of which the mirror forms a part is called the concave mirror. A concave mirror is depressed at the middle on the reflecting surface.

Convex Mirror

A spherical mirror in which reflection takes place on the outer side of the sphere of which the mirror is a part is called the convex mirror. A convex mirror bugles at the middle on the reflecting surface.