Refraction of Light // Physics// Snell's law// Radiation// Lens// Prism// Laws of Refraction

 

Physics

     Refraction of Light

1. What is called refraction of light?

Ans: When the ray of light travels obliquely from one transparent medium to another, the bending of a ray of light at the surface of separation of the two media is called the refraction of light. 

2. Write Snell's law.

Ans: For a fixed pair of media and a fixed colour of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction always remains constant. This law is known as Snell's law.

3. What is radiation?

Ans: Radiation is the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles that cause ionization.

         

4. What is lens?

Ans: A transparent refracting medium confined by two spherical surfaces is called lens.

5. What is prism?

Ans: A prism is a transparent optical instrument or object which disperses the light rays into its various constituent light rays. For example - white light is dispersed into seven colours of light that are called spectrum of white light.

6. What is called critical angle?

Ans: When a ray of light of a fixed colour is refracted from a denser medium to a rarer medium, the value of angle of incidence for which angle of refraction becomes one right angle is called the critical angle of the denser medium with respect to the rarer medium.

7. What is called concave and convex lens?

Ans: The lens whose mid-portion is thin but the edges are thick is called concave lens. The lens whose mid-portion is thick but the edges are thin is called convex lens.

8. What is called focal plane?

Ans: The imaginary plane at the principal focus perpendicular to the principal axis is called focal plane of the lens or focal plane of the sperical mirror.

9. What is called the power of lens? 

Ans: The capacity of a lens to converge or diverge a beam of light is called the power of lens.

The power of a lens is expressed as the inverse quantity of its focal length. After refraction, the disatnce from the lens at which a parallel beam of light converges or appears to diverge after refraction is its focal length. So, the power of a lens indicates how fast or how close it converges or diverges the light rays after refraction.

 

 

10. Write down the laws of refraction.

Ans: First Law: The refracted ray will lie on the same plane as the incident ray and the normal.

Second law: If the refractive index of the first medium is n1, the refractive index of the second medium is n2, the angle of incidence is θ1, the angle of refraction is θ2, then                                              n1 Sinθ1    =   n2 Sin θ2

          If the first medium is air then taking n1 = 1

          We can say 

As the value od n2 is greater than 1, θ1    =   n2 < θ1 i.e after refraction, the ray of light will bend towards the normal. If the volume of n, is greater, when the medium is denser.

According to the second law of refraction, when a ray of light enters from a rarer medium to a denser  medium, the refracted ray bends towards the normal. When it enters from a denser medium to a rarer medium, it goes away from the nomal.

11. Why does refraction of light occur during the change of medium?

Ans: Since the velocity differs from medium to medium, the refraction of light takes place during the change of medium.

        Q. A ray of light is incident on a medium of n = 1.6 with an angle of incidence 45 degree.                     With what angle will it enter into the second medium?

Solution:

We know,

 n1 Sinθ1    =   n2 Sin θ2

 

         Q. A ray is incident from air to another medium at an angle of 60 degree and refracted at the angle of 45 degree. What is the refractive index of the second medium?

                   Solution:

We know that 

 n1 Sinθ1    =   n2 Sin θ2

1 x Sin 60० = n2 Sin 45०

n2  = Sin 60०/ Sin 45०

= 1.22

Text Book Q/A:

1. The lens of eye makes inverted image on retina Why do we not see everything inverted?

Ans: The eyelens forms inverted image of an object on the retina. Even then we see the object upright. This is because the sensation of vision does not come from eyes directly. It comes from the brain. When light falls on retina, the light sensitive cells connected with the nerve converts it into electrical signal. The signal is then sent to the brain through optic nerve. The brain turns upright the inverted image formed on the retina. Thats why we always see upright images of objects.

 

2. What is an important difference between eye and camera?

Ans: An important difference between eye and camera is the accomodation power of eyes. To form a distinct image on retina, the eyelens always increases its focal length. In case of camera it can't be done.

3. The velocity of light in a denser medium is less. Is there anything which can go with the velocity greater than the velocity of light?

Ans: No, nothing can go with the velocity greater than the velocity of light.

4. At noon why is rainbow not seen?

Ans: At noon, sunlight falls on water drops at right angle causing no refraction of light. That's why rainbow is not seen at noon.

5. Drops of water can work like a lens. What is focal length of this lens?

Ans: The focal length of water depends on the radius of water drop. Let the radius of water drop is r and the refractive index of water is n and the focal length is f.

1/f  = (n-1) (1/r  + 1/r)

      = (n-1) x 2/r

f     = r/2 (n-1)