### Bohrs Atomic Model

Bohrs Atomic Model Scientist Neils Bohr gave some corrections to Rutherford's atomic model in 1913. This corrected model is called the Bohr's atomic model. The main postulates of this model are - a) The electron moves around the nucleus in a circular motion, about an axis. This circular motion is called an energy level or orbit. They are also known as the shell or permanent orbit or principal energy level. Electrons do not absorb or emit any energy when they revolve these fixed orbits. If we express the permanent orbit as n, then n = 1, 2, 3, 4, etc.  In other words, if n = 1, the principal energy level is K,  when n = 2, L is the principal energy level,  when n = 3, M is the principal energy level,  when n = 4, N is the principal energy level, etc. b) According to Bohr's model, the angular momentum of an electron in an energy level is mvr =  nh / 2𝜋 . Here, m = the mass of the electron (9.11×10 -31  kg) r = the radius of the orbit that the electron follows v = the veloc

## Nature of Light

What type of wave of light?

Light is an electromagnetic wave.

What is the wavelength of light?

Wavelength of light is from 4⨯10-7 m.

## The three major Properties of light

The three major properties of light are given below:

a) Light is a form of energy.

b) Light is a kind of electromagnetic wave.

c) Reflection, refraction, interference, diffraction, dispersion and polarization of light take place.

When can we see an object?

When light comes to our eyes from an object then we can see the object. The image of the object is formed in the retina and develops a sense of vision similar to the object in our brain through a complex process.

## Reflection

Definition of reflection

When light travelling through the first medium falls on the surface of a second medium, then a portion of the light returns back to the first medium from the surface of separation of two medium. This phenomenon is called the reflection of light.

What do you mean by reflection of light?

In a homogenous transparent medium (e.g: glass) light travels in straight line with the same velocity. But when light travelling through the first medium falls on the surface of a second medium, then a portion of the light returns back to the first medium from the surface of separation of two medium. This phenomenon is called reflection of light.

## What do you mean by regular and irregular reflection?

Regular reflection: If a parallel beam of light falls on a smooth surface and after reflection remains parallel, on is converted into a convergent or a divergent beam, then such type of reflection is called regular reflection.

For example: when a parallel beam of light incident on a plane mirror or on a well polished metal surface, the beam remains parallel even after reflection. In this case, the angle of incidence for each incident ray is the same and the angle of reflection for each of the ray is also the same due to regular reflection.

Irregular or diffused reflection: If a parallel beam of light is incident on a surface and after reflection it is neither parallel nor converted into a convergent or a divergent beam, then such reflection is called diffused or irregular reflection. In this case, the rays incident at different angles at different point of incidence on the rough surface, as a result the corresponding angle of reflection of these rays become different. Due to this, the reflected rays are no more parallel. Objects which we see around us most of their surfaces are not smooth. As a result the reflected rays which enter to our eyes are diffused in nature. Due to this the objects appear dim instead of bright most of the surfaces which appear smooth to valued eyes actually are not smooth. When these objects are viewed with a microscope then they appear rough.

## Laws of Reflection

Why rays incident normally on the mirror returns back along the same path?

According to the law of reflection we know, the angle of incidence is equal to the angle of reflection. That is, the angle of incidence for incident ray and the angle of reflection for reflection ray is same. For this reason rays incident normally on the mirror returns back long the same path.

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## Mirror

What is a mirror?

A mirror is a smooth surface on which regular reflection takes place. A clear image of an object placed in front of a mirror is formed due to reflection of light mirrors are mainly of two types. Namely -

a) Plane mirror

b) Spherical mirror

What is plane mirror?

If the reflecting surface is plane and smooth and regular reflection of light takes place on it, then this surface is called plane mirror.

Why metal coating is given behind a mirror?

Generally, a mirror is prepared by giving metal coating on one surface of glass. This process of coating with mercury or silver on glass is called silvering. Without metal coating a mirror might be a piece of glass. When light incidents on a glass then only 4.5% of light is reflected. But on a metal coated mirror the amount of reflection of light is 45%. Thus, for increasing the rate of reflection of light, metal coating is given behind a mirror.

## Image

What is called an image?

The reflection of an object seen at the new position due to the presence of the mirror is called the image of that object. Images are of two types. They are -

a) Real image

b) Virtual image

What is real image?

If light rays emitted from a point after being reflected or refracted on a surface actually converge at a second point, then this second point is called the real image of the first object.

What is virtual image?

If light rays emitted from a point after being reflected or refracted on a surface seem to diverge from a second point then this second point is called the virtual image of first point.

How is image formed?

If a beam of rays after being reflected from or refracted through any surface meets at a second point or appears to diverse from the second point, then the second is called the image of the first point. This image is formed in many kinds of mirror or lens.

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## Question Bank

Reflection

• What is reflection?