Chapter 12 : Geometrical Optics

              Chp12 : Geometrical Optics

Answer the following questions.

1.  What do you understand by Reflection of light? Draw a diagram to illustrate reflection at a plane surface.

Ans.  “When light travelling in a certain medium falls on the surface of another medium, a

part of it turns back in the same medium, this is called Reflection of light”. 

2.  Describe the following terms used in Reflection:

Ans. a) Normal:

The imaginary line perpendicular to the reflecting surface of mirror at the point of incidence is called Normal.

  b) Angle of incidence:

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

  c) Angle of reflection :

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

3.  State Laws of Reflection? 

Ans. Following are the laws of reflection :

• The incident ray, reflected ray and normal at the point of incidence lie in the same pane.
• The angle of incidence is equal to the angle of reflection.

4.  Define Refraction of Light?

Ans. “Bending effect of light as it passes from one transparent material into another is

called Refraction of light”.

4. Define the following terms used in refraction:

Ans.  Angle of incidence:

The angle that the incident ray makes with the normal is referred as the angle of incidence. 

        Angle of refraction:

The angle that the refracted ray makes with the normal line is referred to as the angle of refraction.

5.  What is meant by Refractive index of a material?How can it be determined ?

Ans.  When a ray of light passes from one particular medium to another, the ratio of the

sin of the angle of incidence to the sin of the angle of refraction is constant. This constant

ratio is called the refractive index. 

     n = Sin i/Sin r

6.  What is meant by the term Total Internal reflection? 

Ans.  The angle of incidence for which the angle of refraction becomes 90 is called

critical angle. When the angle of incidence becomes larger than the critical angle, no

refraction occurs. The entire light is reflected back into the denser medium. This is known

as Total Internal reflection.

7.  State the Laws of Refraction of Light?

Ans.  Following are the laws of refraction given below :

•  The incident ray, the normal, and the reflected ray at the point of incidence all lie in the same plane.
• For two particular media, the ratio of the sin of the angle of incidence to sin of the angle of refraction is a constant.

8.  State the conditions for Total Internal reflection?

Ans. Conditions for total internal reflection are:

• The ray of light passes from denser to a less dense medium.
• The angle of incidence in the denser medium is greater than the critical angle

9. What is Critical Angle? 

Ans.  “The critical angle is defined as the angle of incidence in the optically denser

medium for which the angle of refraction in the less dense medium is 90 °”

10. What are Optical Fibers? Describe how total internal reflection is used in propagating of light through optical fibre ?

Ans.   An optical fibre cable is a bundle of glass fibres with thickness of a human hair.                                         Working of Optical Fibre:

pg 45 (from Total internal reflection is used in optic fibre..............................  with small loss of energy).

10.  Define the following terms applied to a lens:

Ans.  (i) Principal axis:

Each of the two surfaces of a spherical lens is a section of a sphere. The line passing through the two centres of curvatures of the lens is called Principal axis.

          

         (ii) Optical centre:

A point on the principal axis at the centre of lens is called Optical centre.

         (iii) Focal length:

The focal length is the distance between the optical centre and the principal focus

11.  What is meant by the Principal focus of a:

Ans.  Principal focus of Convex lens:

The light rays travelling parallel to the principal axis of a concave lens after refraction meet at a point on the principal axis called principal focus or focal point. Hence convex lens is called converging lens.

       Principal focus of Concave lens:

The parallel rays appear to come from a point behind the lens called principal focus. Hence concave lens is also called diverging lens.

12. Describe how light is Refracted through Convex lens?

Ans.   Refraction of light through convex lens can be described with the help of three

principal rays as:

• The ray parallel to the principal axis passes through the focal point after refraction by the lens.
• The ray passing through the optical center passes straight through the lens and passes deviated.
• The ray passing through the focal point becomes parallel to the principal axis after refraction by the lens.

13. A coin is placed at a focal point of a converging lens. Is an image formed? What is its nature?

Ans.  A coin is placed at a focal point of a converging lens. No image is formed because

the refracted rays are parallel and never meet. 

14.What are the Differences between Real and Virtual Images?

Ans.   Real Image

• Real image usually appear inverted.
• On mirror, Real images lie in front of the reflecting surface.
• On lens, Real images lie on the other side of the object.
• Light rays meet at a focal point in front of the mirror.

          Virtual Image

• Virtual image usually appears erect.
• Virtual image cannot be obtained on screen.
• On lens, Virtual images lies on the same side of the object.
• Light rays meet at a focal point behind the mirror.

15. How does a converging lens form a virtual image of a Real object? How does a diverging lens can form a real image of a Real object?

Ans.   At pg 50 and 51

16. Define Power of a lens and its Units?

Ans.  “Power of a lens is defined as the reciprocal of its focal length in metres”

 Unit :    SI unit of power of a lens is dioptre denoted by a symbol D.

16.   Define the terms Resolving Power and Magnifying Power?

Ans,   Resolving Power:

Resolving power of an instrument is its ability to reveal the minor details of the object under examination.

      Magnifying Power:

The ratio of the angles subtended by the image as seen through the optical device to that subtended by the object at the unaided eye.

17.  What is meant by the terms Nearsightedness and Farsightedness? How can

these defects be corrected?

Ans. Nearsightedness (myopia): “Some people cannot see distant objects clearly

without the aid of spectacles. This defect of vision is known as Short sight or

Nearsightedness”.

Correction of Nearsightedness: The nearsighted eye can be corrected with glass or

contact lenses that use diverging lenses.

Farsightedness (hypermetropia): “The disability of the eye to form distinct images

of nearby objects on its retina is known as Farsightedness”.

Correction of Farsightedness: Farsightedness can be corrected with the aid of a

suitable converging lens.

18. Draw Ray diagrams.

Ans.   pg 50,51

19. Mention magnification for telescope, microscope and  simple microscope?

Ans.  pg 58,56,57