Notes on Real and Virtual Images and Curved Images | Grade 11 > Physics > Reflection at Plane and Curved Surfaces | KULLABS.COM

Real and Virtual Images and Curved Images

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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.

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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

inclined mirrors
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 90o. 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

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mirrors are inclined at 90o 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 30o, 45o, 60o and 75o.

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.

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  • A mirror which forms a part of a spherical reflecting surface is called the spherical mirror or curved 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 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
  • An image is said to be real when the rays of light producing the image actually pass through the position of the 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
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