Real and virtual image

Light of Class 7

Real Image

In study of light , Let an object O is placed then a real imageexist which the perceived location is actually a point of convergence of the rays of light that make up the image. If we will fix a screen in front of mirror or lens then a real image appear and the image will generally become visible on the screen.

In different types of mirror or lens the images appears are different some of the examples of real images includes the image seen on a cinema screen in this process the source or picture being the projector on screen , the image produced on a detector in the rear of a camera, and the image produced on an eyeball retina.

real image

In the ray diagrams (such as the images on the right), real rays of light are always represented by full, solid lines that give the indication that it is real.

A real image is formed when light rays coming from an object actually meet at a point after reflection from a mirror or refraction through a lens . In this processes the image formed on the screen is real image.

Virtual Image

In the same way, we can define virtual image as an image in which the outgoing rays from a point on the object always diverge from it . Then it will appear to converge in a point or behind the mirror or lens. Because the rays never really converge, one cannot project a virtual image.

virtual image

Virtual image formed by concave lens and convex mirror

Because the image will not obtained on a the real screen then it is called a virtual image. A virtual image is seen only by looking into a the mirror or a lens. In daily life when we see the image of our face formed by a plane mirror cannot be obtained on a screen; it can be seen only by looking into the mirror. This is an example of virtual image.

Characteristics of the image formed by a plane mirror

The image formed by a plane mirror is virtual.
The image formed by a plane mirror is erect.
The size of the image formed by a plane mirror is same as that of the size of the object. If object is 10 cm high, then the image of the object will also be 10 cm high.
The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of it. Suppose, an object is placed at 5 cm in front of a plane mirror then its image will be at 5 cm behind he plane mirror.
The image formed by a plane mirror is laterally inverted i.e., the left hand appears as right hand and vice-versa. As shown in fig.

Use of plane mirror

Plane mirror is used as looking glass.
Plane mirror is used in a solar cooker to reflect the sun light.
Plane mirrors are used in a solar cooker to reflect the sun light.
Plane mirrors are used in periscopes usually used in submarines.

Spherical Mirror

Spherical mirrors are the reflecting part of spherical surface.

Concave mirror : Concave mirror is the part of the hollow sphere whose outer surface is silvered and the inner surface acts as reflecting surface (fig.)

concave mirror

Convex mirror :Convex mirror is the part of a hollow sphere whose outer surface acts as reflecting surface and the inner surface is silvered as shown in fig.

convex mirror

Definitions in connection with Spherical Lens:

Centre of curvature (C): The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part, because a lens has two surfaces, so it has two centres of curvature. In figure (a) and (b) points C₁ and C₂ are the centres of curvature.

Radius of curvature (R): The radius of curvature of the surface of a lens is the radius of the sphere of which the surface forms a part. R₁ and R₂ in the figure (a) and (b) represents radius of curvature.
Principal axis (C₁ C₂): It is the line passing through the two centres of curvature (C₁ and C₂) of the lens.

characteristics of lenses

Optical centre: In the given figure if a ray of light is incident on a lens after refraction through the lens the emergent ray is parallel to the incident ray, then the point at which the refracted ray intersects, the principal axis is called the optical centre of the lens. In the given figure the optical centre of the lens. It divides the thickness of the lens in the ratio of the radii of curvature of its two surfaces. Thus:

optical centre

For a ray passing through the optical centre, the incident and emergent rays are parallel. However, the emergent ray suffers some lateral displacement relative to the incident ray. The lateral displacement decreases with the decrease in thickness of the lens. Hence a ray passing through the optical centre of a thin lens does not suffer any lateral deviation, as shown in the figure (b) and (c) above.

Pole : The mid point of spherical mirror is called its pole. It is denoted by P.
Aperture: The part of the spherical mirror exposed to the incident light is called the aperture of the mirror. In other words, diameter of a spherical mirror is known as its aperture.
Principal axis: A line joining the center of curvature (C) and pole (P) of spherical mirror and extended on either side is called principal axis.
Principal focus: A point on the principal axis of a spherical mirror where the rays of light parallel to the principal axis meet or appear to meet after reflection from the mirror is called principal focus. It is denoted by F.
Focal plane : A plane normal or perpendicular to the principal axis and passing though the principal focus (F) of the spherical mirror is called focal plane of the mirror.
Focal length : The distance between the pole (P) and principal focus (F) of a spherical mirror is called the focal length of the mirror. It is denoted by f. Focal length of a mirror is given by f = PF