Unit-5 | LIGHT | Class-09

Overview: Light is a form of energy which gives us the sensation of vision. The bending of light when it passes from one medium to another is called refraction of light. Here we will discuss light, refraction of light, total internal reflection of light, dispersion of light and electromagnetic waves.


Light:

Light is a form of energy that gives us the sensation of vision. It is due to reflection of  light rays we can see clearly through our eyes.

The speed of light in a vacuum is 3x10^8 m/s. Similarly, the speed of light in glass medium is 2x 10^8m/s and in water medium is 2.25x 10^8 m/s.

Refraction of light:

The bedding of light rays when it passes obliquely from one medium to another is called refraction of light. 

Optically Denser and Rarer Medium:

Denser medium is that type of medium in which the day of light travels comparitively slower. Rarer medium is the medium in which the day of light travels faster. 

Water is an optically Denser medium in comparison with air. While, glass is an optically Denser medium in comparison with water and air.

Laws of refraction of light:

The laws of refraction of light are as follows:
  1. The incident ray, normal ray and the refracted ray lie on the same plane at the point of incidence.
  2. The ratio of the sine of angle of incidence to the sine of angle of refraction is always constant for a given pair of media.
i.e. sin i/ sin r = (constant)

This constant term is called refractive index. This is also called as the Snell's law. The drawback of Snell's law is that when the ray passes through the normal, it doesn't bend and the angle of incidence becomes zero.


Refractive Index:

The ratio of the sine of angle of incidence to the sine of angle of refraction for a given pair of media is constant which is called refractive index. It has no unit.

Refractive index (μ) = sin i / sin r
Also,
Refractive index (μ) = speed of light in vacuum (c) / speed of light in medium (v)

Cause of Refraction of Light:

The difference in the speed of light is seen in different media. When light travels from one medium to another, the change in speed of light causes the refraction of light. 


Real Depth and Apparent Depth:

The actual depth of the pond is called real Depth. The virtual depth of an object seen due to refraction is called apparent depth. 

Refractive index in terms of real Depth and Apparent Depth:
Refractive Index (μ) = Real depth / Apparent Depth


Effects of Refraction of Light:

a. Bending of stick in water:

When the stick is immersed in water. The light coming from the stick after it crosses the surface of water gets bend away from the normal because air is a rarer medium and water is a Denser medium. As a result, the stick seems to be bent due to refraction of light.

b. A coin placed at the bottom of a vessel full of water appears to be raised:

The light coming from the coin placed at the bottom of the vessel full of water bends away from the normal after it crosses the water as water is a Denser medium and air is a rarer medium. This is due to refraction of light that the virtual image of the coin is formed which is visible to our eyes. Therefore, the coin appears to be raised.

Critical Angle and Total Internal Reflection:

The angle of incidence whose corresponding angle of refraction is 90° is called critical angle. It is denoted by C or ic. 

When the angle of incidence is further increased beyond the critical angle, the ray of light reflects back to the same medium instead of refraction. This phenomenon is called total internal reflection of light.
Conditions for total internal reflection:
  1. The ray of light must pass from a denser medium to a rarer medium.
  2. The angle of incidence in the denser medium should be greater than the critical angle.
We have a relation between the refractive index of a medium and the critical angle as:
refractive index (μ) = 1 / C

Applications of Total Internal Reflection:

1. Sparkling of a cut diamond:
The structure of a diamond is created in such a way that the faces of diamond allow ray of light to enter the diamond with an angle of incidence greater than the critical angle i.e 24°. So, a ray of light passing through diamond suffers repeated total internal reflection at a larger number of faces. Therefore, the cut diamond sparkles.

2. Light Pipe:
Light pipe is a transparent rod made of glass or plastic. It is used by doctors to view the internal organs of the patients. It is used in telecommunication for transmitting signals.

3. Mirage:
Mirage is an optical illusion of water in which a pond-like structure is seen in a hot desert or a hot coal-tarred roads. 

4. Dispersion of Light:
A band of seven colors obtained on a white screen is called dispersion of light.
The phenomenon of splitting of white light into its constituent colours on passing through a glass prism is called dispersion of light.  

Electromagnetic Waves:

The waves which travel by making electric and magnetic field are called electromagnetic waves. They don't need material medium to propagate. 
Speed of electromagnetic waves are calculated using the equation:
Speed (v) = frequency (f) x wavelength (λ)


Electromagnetic spectrum:

The group of the electromagnetic waves classified according to their wavelength is called electromagnetic spectrum.

Properties of electromagnetic waves:
  1. They don't need any material medium to travel.
  2. They are transverse waves.
  3. They travel at the same speed in vacuum (i.e. 3x10^8 m/s).
  4. They obey laws of refraction and reflection of light.
Uses of electromagnetic waves:
  1. X-rays are used in radiography and radiotherapy.
  2. Gamma rays are used in radiography and treating cancer.
  3. Microwaves are used in microwave ovens to cook/warm food. They are also used in RADAR for communication.
  4. Radio waves are used for communication like in radio, television, telephone transmission.

Numerical Problems:

1. A ray of light is passing through an another medium by making an angle of 45°. If the refractive index of the medium is 1.4, calculate the magnitude of angle of refraction.

Solution:
angle of incidence (i) = 45°
refractive index (μ)= 1.4
angle of refraction (r) = ?
We have,
sin i / sinr = Î¼
or, sin45° / sinr = 1.4
or, 1/√2 ÷ 1.4 = sinr
or, sin^(-1)0.50 = r
So, r = 30.34°


2. The speed of light in air is 3 x 10^5 km/s and the refractive index of paraffin is 1.44. Calculate the speed of light in the paraffin. 

Solution:
speed of light in air (c) = 3 x 10^5 km/s
= 3 x 10^3 x 10^5 m/s
= 3 x 10^8 m/s
refractive index of paraffin (μ) = 1.44
speed of light in paraffin (v) = ?
We have,
μ = c/ v
or, 1.44 = 3 x10^8 / v
or, v = 3x 10^8/ 1.44
So, v = 208333333.3 m/s (2.083333333 x 10^8 m/s)
Therefore, the speed of light in the paraffin is 208333333.3 m/s


#SciPiTutor
#Light
#ScienceNotes
#Notes
#Class9
#Numerical