Image – Definition, Types and Characteristics

Some of the most important Physics Topics include energy, motion, and force.

What are the Differences between a Real Image and a Virtual Image?

Deviation of a Ray Due To Reflection

When a ray of light changes its original course due to reflection, the angle between the original and the final directions of the ray is a measure of the magnitude of deviation of the ray.

It is obvious from the figure that in the absence of the reflector M₁M₂, the ray AO would have traveled straight in the direction AOC, but has instead been deviated due to reflection. Magnitude of this deviation,
δ = ∠BOC = 180° – ∠AOB = 180° – 2i

Some Phenomena Of Reflection

i) When light falls on a black body, practically no reflection takes place. The black body absorbs almost the whole incident light. Hence, optical instruments like cameras, telescopes etc. are painted black on the insides to avoid unwanted reflection. On the other hand, white objects do not absorb any light, rather reflects it. Hence to stop absorption of light and to increase the brightness, white surfaces are used. This is the reason why white screens are used for projection in a cinema.

ii) Twilight: Twilight is the time between dawn and sunrise and the time between sunset and dusk. The sun itself is not actually visible from this ground level because it is below the horizon. However, the suspended dust particles at upper atmosphere still receive direct sunrays. Due to diffuse reflection, this light spreads in all directions and partially illuminates the ground.

iii) When light falls on a glass slab only a negligible portion is reflected; most of the rays pass through it. Hence the glass slab is coated with aluminium , to make a mirror. As the coating is opaque the incident rays are almost completely reflected from the silvered surface, with only a small portion of incident rays reflected from the front surface.

Image

When light rays from an object come directly into our eyes we see the object at the place where it is actually situated. But if the rays come after reflection or refraction we see the object elsewhere. What we see in the new position is actually its image.

Definition: When rays of light diverging from a point source after reflection or refraction converge to or appear to diverge from a second point, the second point is called the image of the first point.

There are two kinds of images—

1. real image and
2. virtual image.

Real image: When rays of light diverging from a point source after reflection or refraction converge to a second point, the second point is called the real image of the first point.

A real image can be formed on a screen, as evident from Fig. where a convex lens has formed a real image A’ of a point source A.

Some examples of real image are-

1. image formed on the cinema screen,
2. image formed by a camera, etc.

Virtual image: When rays of light diverging from a point source after reflection or refraction appear to diverge from a second point, the second point is called the virtual image of the A’ first point.

A virtual image cannot be formed on a screen as illustrated in Fig. where a plane mirror has formed a virtual image A’ of the point source A.
The image of a tree standing by the side of a pond, on the surface of water is a virtual image. Mirage is also a virtual image.

Differences between real image and virtual image

Real Image	Virtual Image
1. In this case, after reflection or refraction of light, rays converge at a point.	1. In this case, after reflection or refraction of light, rays appear to diverge from a point.
2. Real image can be cast on a screen.	2. Virtual image cannot be cast on a screen.
3. A photograph of an object can be taken directly by placing a photographic plate at the position of its real image.	3. A photograph of an object cannot be taken directly by placing a photographic plate at the position of its virtual image. But using a convex lens a real image of the virtual image can be formed and photo graphed.

Image in a Plane Mirror

Image of a point object: Let A be a point source [Fig.]. A ray of light AO is incident at O normally and retraces its path along OA . Another ray AC follows the path CD after reflection. The reflected rays when produced backwards, meet at A’. It appears that the two reflected rays are coming from A’. So, A’ is the virtual image of A. The line AO A’ joining the object and the image is normal to the mirror.

CN is the normal at the point of incidence. Since OA and CN are parallel,
∴ ∠OAC = ∠ACN = i (alternate angles) and
∠OA’C = ∠NCD = r (corresponding angles)
But ∠ACN = ∠NCD (i = r)
∴ ∠OAC = ∠OA’C
Hence the two right angled triangles AOC and A’OC are congruent.
∴ OA = OA’
Thus, object distance from the mirror = image distance from the mirror
Hence, the image formed by a plane mirror lies on the perpendicular from the object to the mirror as far behind the mirror as the object is in front of it.

Image of an extended object: Let AB be an extended object in front of a plane mirror M₁M₂ [Fig.]. Every point of the extended object may be regarded as a source of light. The complete image of the object will be obtained by locating the position of the images of all the point sources.

Drawing of the image of an extended object: From the topmost point A of the extended object perpendicular AO₁ is drawn and it is extended up to A’ in such a way that A in the previous case, A’ is the image of A. The topmost point of AB. Similarly, B’ is the image of the lowermost point of AB. For every intermediate point of AB, a corresponding image will be formed between A’ and B’. So, A ‘B’ is the image of AB. Obviously AB and A’B’ will be of the same size. So, a plane mirror forms a virtual image of the same size of an extended object.

It is also obvious from the figure that the eye can catch the image within the portion PS of the mirror, as long as the relative positions of the eye and the mirror are not changed.

Lateral inversion: The letter P held in front of a plane mirror will be seen as laterally inverted [Fig.]. The lateral turning is due to the fact that every point image is at the same distance behind the plane mirror gas the point object is in front of it, being the size of the image equal to the size of the object.

If the mirror is held vertical, it does not Fig. invert the image which means turning an image upside down. Only the image is laterally inverted. If we move our right hand facing a mirror, we see the image moving its left hand.

Definition: An image of an object formed in a plane mirror is inverted sideways. This effect of plane mirror is called lateral inversion.

The perpendicular distance of every point of the object from the plane mirror = the perpendicular distance of every image point from the mirror.
i. e., AO₁ = A’O₁ ; BO₂ = B’O₂.
So, the image is laterally inverted.
The images, due to bodies having symmetrical sides such as a sphere or letters like A, H, M, I, O, T, etc. are not affected by lateral inversion, but the images with non-symmetrical bodies like mug or letters like P, B, C etc. are affected.

The characteristics of the image formed by a plane mirror:

1. With respect to mirror, object distance = image distance.
2. The straight line joining the object and its image is perpendicular to the mirror.
3. The image is virtual.
4. The image is formed behind the mirror and has the same size as that of the object.
5. The image is laterally inverted.

Image in a Plane Mirror due to a Convergent Beam

Suppose a converging beam of light is incident on a plane mirror M₁M₂. In absence of the mirror the rays would converge at O [Fig.]. Due to reflection, the rays PQ and RS meet at the point I instead of O. It can be proved that the straight line joining I and O is perpendicular to the mirror and IA = OA . The point O is called the virtual object and the point I is its real image. Thus, a plane mirror can form a real image of a virtual object.

Conversely, if a point source is placed at the point I its virtual image is formed at the point O. This is due to the principle of reversibility of light rays.

1. For a given incident ray if the mirror is rotated through an angle θ, the reflected ray turns through an angle of 2θ.
2. The minimum length of the plane mirror required to have the full length image of a person standing in front of it is equal to half the height h of the person i.e., \(\frac{h}{2}\).
3. If an object moves towards (or away from) a plane mirror with a speed v, the image of the object will move towards (or away from) the mirror with a speed 2v.
4. If two plane mirrors facing each other are inclined at an angle θ with each other, the number of images formed due to multiple reflections, is given by n = \(\frac{360^{\circ}}{\theta}\) – 1. If (\(\frac{360^{\circ}}{\theta}\) – 1) is not an integer, the next integer will indicate the number of images.
5. Due to reflection, the frequency, wavelength and speed of light are not changed.
6. Intensity of light after reflection decreases.
7. For reflection of light from denser medium a phase change of π occurs.