Reflection on the mirror

Introduction :

Reflection on the mirror is most important topic of NCERT Light chapter in Class 10th . Questions are frequently asked in the CBSE board and ICSE Board exam from plane mirror .

“physics - Light reflection 10th notes “ will be very beneficial for the students who are engaged in the preparation of upcoming board exam .

Contents- Reflection on the mirror

* Light

* Characteristics of light

* Nature of Light

* REFLECTION OF LIGHT

* Incident ray, Reflected ray, Normal , Angle of incidence, Angle of reflection

* Laws of Reflection of Light

* First Law of Reflection

* Second Law of Reflection

* Regular Reflection

* Diffuse Reflection

* Object : Luminous objects , Non-luminous objects

* Image : Real Images , Virtual image

* Lateral Inversion

* Characteristics of an image formed by a plane mirror

* Uses of Plane Mirrors

Reflection of light at plane Surfaces

Light : Light is a form of energy which causes in us the sensation of sight and enables us to see objects .

Characteristics of light :

· Light travels in straight lines.

· The speed of light are different in different medium. Speed of light in vaccume is 3 x 10⁸m/s.

· Light is electromagnetic wave.

· Light show different phenomenon like reflection, refraction, diffusion or scattering, polarization, interference , dispersion etc.

· Light is transversal in nature.

Nature of Light

There are two theories about the nature of light:

wave theory of light and particle theory of light.

Light exhibits the properties of both waves and particles. The modern theory of light called 'Quantum Theory of Light' combines both the wave and particle models of light.

Some of the phenomena of light can be explained only if light is considered as wave whereas others can be explained only if light is considered as particles. Thus, there is evidence for the wave nature of light as well as for particle nature of light.

According to wave theory :

§ Light consists of electromagnetic waves which do not require a material medium for their propagation.

§ The wavelength of visible light waves is very small ( about 4 x 10^-7m to 8 x 10^-7 m).

§ The speed of light waves is very high of 3 x 10⁸ m/s in vacuum.

§ The phenomena of diffraction (bending of light around the corners of tiny objects), interference and polarization of light can only be explained if light is considered to be of wave nature. The particle theory of light cannot explain these phenomena.

According to particle theory :

§ Light is made of particles which travel in a straight line at very high speed.

§ The elementary particle of light is the 'photon'.

§ The phenomena of reflection and refraction of light, and casting of shadows of objects by light, can be explained only if light is considered to be of particle nature.

§ Wave theory of light cannot explain these phenomena.

When light falls on the surface of an object, some of light is returned back in same medium . The process of sending back the light rays from surface of an object, is called reflection of light.

Most of the objects reflect light. Some objects reflect more light whereas other objects reflect less light.

The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces.

Silver metal is one of the best reflectors of light.

A polished block of silver metal reflects almost all the light falling on it and does not transmit any light through it. But the surface of silver metal is easily scratched and soon becomes rough. So, ordinary mirrors are made by depositing a thin layer of silver metal on the back side of a plane glass sheet. The silver layer is then protected by a coat of red paint. The reflection of light in a plane mirror (or any other mirror) takes place at the silver surface in it. Thus, a plane mirror is a thin, flat and smooth sheet of glass having a shining coating of silver metal on one side. The silver coating is protected by a red paint. These days mirrors are being made increasingly by depositing a thin coating of aluminium metal at the back of a glass sheet (instead of silver coating). This is because aluminium is Front much cheaper than silver and it reflects light very well.

Common terms used in PLANE MIRROR

: incident ray, point of incidence, reflected ray, normal (at the point of incidence), angle of incidence, and angle of reflection.

Incident ray :

The ray of light which falls on the mirror surface is called the incident ray.

AO is the incident ray of light.

Point of incidence :

The point at which the incident ray falls on the mirror is called the point of incidence.

Point O on the surface of the mirror is the point of incidence (because the incident ray AO touches the mirror surface at this point).

Reflected ray :

The ray of light which is sent back by the mirror is called the reflected ray.

OB is the reflected ray of light. The reflected ray of light shows the direction in which the light goes after reflection from the mirror.

Normal :

Normal is a line which is perpendicular to the mirror at the point of incidence.

The line ON is the normal to the mirror surface at point O.

The normal is a line at right angles to the mirror surface. In other words, normal makes an angle of 90° with the surface of the plane mirror.

Angle of incidence :

The angle between the incident ray and normal is called the angle of incidence.

The incident ray is AO and the normal is ON, so the angle AON is the angle of incidence.

The angle of incidence is denoted by the letter i.

Angle of reflection :

The angle between the reflected ray and normal is called the angle of reflection.

The reflected ray is OB and the normal is ON, so the angle NOB is the angle of reflection.

The angle of reflection is denoted by the letter r.

(Laws) Rules of Reflection of Light

When a ray of light falls on a plane mirror (or any other plane surface), it is reflected according to some laws, called the laws of reflection of light.

The laws of reflection of light are given below :

1. First Law of Reflection.

According to the first law of reflection of light : The incident ray, the reflected ray, and their corresponding normal , all lie in the same plane.

2. Second Law of Reflection.

According to the second law of reflection of light : The angle of incidence is always equal to the angle of reflection.

If the angle of incidence is ‘ i ‘ and the angle of reflection is r, then according to the second law of reflection : i = r

The laws of reflection of light apply to all kinds of mirrors like plane mirror, spherical mirrors (concave mirrors and convex mirrors), parabolic mirror etc.

What happens when a ray of light falls normally (or the. Perpendicularly) on surface of a mirror.

light is incident normally on a mirror, it means that it is travelling along the normal to the mirror. So, the angle of incidence (i) for such a ray of light is zero (0). And since the angle of incidence is zero, therefore, according to the second law of reflection, its angle of reflection (r) will also be zero (0). This means that a ray of light which is incident normally on a mirror, is reflected back along the same path.

Regular Reflection and Diffuse Reflection of Light

Regular Reflection:

The reflection in which parallel incident rays remain parallel after reflection and go only in one direction is called regular reflection.

§ In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.

§ Regular reflection of light occurs from smooth surfaces like that of a plane mirror (or highly polished metal surfaces).

§ Smooth surface produces a clear image of an object due to regular reflection of light.

§ A highly polished metal surface and a still water surface also produce regular reflection of light and form images. This is why we can see our face in a polished metal object as well as in the still water surface of a pond or lake.

Diffuse Reflection:

The reflection in which parallel incident rays do not remain parallel after reflection is called diffuse reflection.

§ In diffuse reflection, a parallel beam of incident light is reflected in different directions. they are scattered in different directions.

§ The diffuse reflection is also known as irregular reflection or scattering.

§ The diffuse reflection of light takes place from rough surfaces like that of paper, cardboard, chalk, table, chair, walls and unpolished metal objects.

§ A rough surface (like that of paper) does not produce an image of the object due to diffuse reflection of light.

The diffuse reflection of light from a rough surface can be explained as follows :

The particles of a rough surface (like that of paper) are all facing in different directions. Due to this, the angles of incidence for all the parallel rays of light falling on a rough surface are different and hence the angles of reflection for all the rays of light are also different. Since the angles of incidence and the angles of reflection are different, the parallel rays of light falling on a rough surface go in different directions.

The laws of reflection are valid at each point even on the rough surface of an object. The surfaces of most of the objects are rough (or uneven) to some extent. So, most of the objects around us cause diffuse reflection of light and scatter the light falling on them in all directions.

In fact, we can see these objects only because they scatter light rays falling on them in all directions. For example, a book lying on a table can be seen from all parts of the room due to diffuse reflection of light from its surface,

A cinema screen has a rough surface and causes diffuse reflection of light falling on it. The cinema screen receives light from a film projector and scatters it in all directions in the cinema hall so thát people sitting anywhere in the hall can see the picture focused on the screen.

Object :

Anything which gives out light rays (either its own or reflected by it) is called an object.

A bulb, a candle, a pin-head, an arrow, our face, or a tree, are all examples of objects

The objects can be of two types :

Luminous objects :

The objects which emit their own light are called luminous objects.

e.g The objects like the sun, other stars, electric bulb, tube-light, torch, candle and fire, etc., are luminous objects.

Non-luminous objects :

Those objects which do not emit light themselves but only reflect (or scatter) the light which falls on them, are called non-luminous objects .

e.g A flower, chair, table, book, trees, other plants, human beings, fan, bed, mirror, diamond, walls, floor, and road, etc., are all non-luminous objects.

Image :

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

The images are of two types :

Real Images :

The image which can be obtained on a screen is called a real image.

e.g The images formed on a cinema screen .

When light rays coming from an object meet at a point after reflection or refraction then real image is formed.

Real image is always inverted.

Virtual image :

The image which cannot be obtained on a screen is called a virtual image.

· The image of our face in a plane mirror is an example of virtual image

· Virtual images are also called unreal images.

· When light rays coming from an object appear to spread from a point after reflection or refraction then virtual image is formed.

· A virtual image cannot be formed on a screen because light rays do not actually pass through a virtual image.

· Virtual image is always erect.

· A plane mirror always forms virtual images.

· A convex mirror also forms only virtual images.

Lateral Inversion :

When an object is placed in front of a plane mirror, then the right side of object appears to become the left side of image; and the left side of object appears to become the right side of image. This change of sides of an 'object and its 'mirror image' is called lateral inversion.

i.e The effect of reversing the sides of an object and its image is called lateral inversion.

The image formed in a plane mirror is laterally inverted. In other words, the image formed in a plane mirror is 'sideways reversed' with respect to the object.

Characteristics of an image formed by a plane mirror.

1. The image formed in a plane mirror is virtual. It cannot be received on screen.

2. The image formed in a plane mirror is erect. It is the same side up as the object.
3. The image in a plane mirror is of the same size as the object.

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.

5. 'The image formed in a plane mirror is laterally inverted (or sideways reversed).

Uses of Plane Mirrors

(i) Plane mirrors are used to see ourselves. The mirrors on our dressing table and in bathrooms are plane mirrors.

(ii) Plane mirrors are fixed on the inside walls of certain shops (like jewellery. shops) to make them look bigger.

(iii) Plane mirrors are fitted at blind turns of some busy roads so that drivers can see the vehicles coming from the other side and prevent accidents.

(iv) Plane mirrors are used in making periscopes.

Related Article ( Click to read )

If you liked the notes of Reflection on the mirror, then share it with other students.

If you want to ask any question related to topic plane mirror and Light reflection , then write a question in the comment box.

Wednesday, June 9, 2021