Light 5

Q.1. What is meant by Refraction? Why does this take place ?
A. The bending of light rays, as it travels from one medium to another having different refractive indices, is called refraction of light.
The phenomenon of refraction takes place because of difference in the speed of light in different media.
For eg, Light travels with 3x10^8 m/s in vacuum or air , 2x10^8 m/s in glass and 2.25x10^8 m/s in water.

Q.2. Is optical density and density same ? Explain.
A. No. Optical density and density are not the same physical quantities.
Optical density is measured by the speed of light in that medium, whereas, density is the mass contained per unit volume.
For eg, Kerosene floats above water suggesting that it is lighter than water, i.e. its density is lower than that of water. But the refractive index of kerosene is 1.44 whereas that of water is 1.33, suggesting that kerosene is optically denser than water.

Q.3. What are the factors on which refractive index of a medium depends ?
A. The factors on which refractive index of a medium depends are :
i. Nature of the medium, in other words its optical density. More is the optical density, more is its refractive index, and hence, less is the speed of light in it.
ii. Colour of the light used or the wavelength. Red light, having largest wavelength in the visible region, shows lowest refractive index and hence, bends less. Violet, having shortest wavelength, shows highest refractive index, and hence, bends more.
iii. Temperature. Due to increase in temperature the density of the medium decreases, so light moves fast through it. As a result decreases.

Light 4

Characteristics of images formed in a plane mirror :
1. Size of the image is always equal to the size of the object. Hence, its magnification is unity.
2. The image formed is laterally inverted. When you stand in front of the mirror, your right hand appears left and your left hand appears right.
3. The image formed is as far behind the mirror as the object is in front of it. The image distance is equal to the object distance.

If the mirrors are not plane surfaces then they are called spherical mirrors. There can be two types of spherical mirrors : Concave and Convex.
Few definitions related to spherical mirrors are :
1. Centre of curvature: The centre of an imaginary sphere of which the mirror is a part.
2. Radius of curvature : The radius of the hollow imaginary sphere of which the mirror is a part. It is denoted by R.
3.Pole : The central point of the spherical mirror.
4. Focus : When parallel beam of light is incident on a spherical mirror, then after reflection the rays meet at this point in case of concave mirror, and appear to diverge out from the point in case of a convex mirror, then that point is called the Focus.
5. Focal Length : The distance between the pole and the focus is called focal length. Generally, it is denoted by f , and the relation f = R/2 always holds true.
6. Magnification : The ratio of the size of the image to the size of the object is called magnification. It is denoted by m.

Light 3 - Reflection

The phenomenon of rebouncing of light rays into the same mediumon striking a surface is called Reflection of light.

Laws of Reflection
Following are the two laws of reflection of light:
1. The incident ray, the reflected ray and the normal at the point of incidence all lies in the same plane.
2. The angle of incidence is always equal to the angle of reflection at the point of incidence.

Angle of incidence : It is the angle which the incident ray makes with the normal at the point of incidence.
Angle of Reflection : It is the angle which the reflected ray makes with the normal at the point of incidence.

Types of Reflection :
1. Regular Reflection : When a parallel beam of reflected light rays is obtained when parallel light rays are incident on a smooth reflecting surface, then that is called Regular Reflection.
Example: Light rays falling on a mirror gets reflected back.
2. Irregular or Diffused Reflection : When a non-parallel beam of reflected rays are obtained when parallel incident light rays fall on an uneven surface , then that is called Irregular or Diffused Reflection.
Example: Light rays falling on a wall.

Note: The Laws of Reflection holds true for both regular and irregular reflection. Incase of irregular reflection, however, the laws hold true for each point of the incident beam of light.

Light 2

Objects that emit light of their own are called Luminous objects. For example - Sun, stars etc. Whereas objects that do not emit light but are visible because light from other sources fall on them are called Non-luminous objects. For example - moon, chair, table, wall etc.
If light rays coming from an object meet or appear to meet at a point after reflection or refraction, then that point is called image of the object. If the rays actually meet then that is called a real image, and if the rays appear to meet then that is a virtual image.

Light 1 : Introduction

Light is an electromagnetic radiation that helps in the sensation of sight.The different properties of light can be understood only if we consider its dual character - wave and particle.
Many optical phenomena can be understood if we consider light as waves. Some characteristics of light waves are -
1. They are electromagnetic in nature. They consist of oscillating electric and magnetic fields at mutually perpendicular planes in same frequency.
2. They do not require any material medium for their propagation. Light from Sun reaches Earth travelling vast expanse of space where nothing exists.
3. Light waves travel with a speed of 30,00,00,000 m/s in vacuum or free space.
But, the speed of light decreases when it travels through a medium of higher density.
A light wave propagates from source to destination in one single straight line, which we generally refer to as a 'Ray'. A collection of such rays make a beam of light