5.2 Light’s Refraction

What is meant by ‘ Refraction’

 

Refraction of light is a phenomenon where the direction and speed of light  are  changed ( change in velocity) when it crosses the boundary between two materials of different optical densities.

Three ways in which a ray can travel through two  mediums

1  When a ray of light travels from optically less dense medium to optically denser medium , the ray is bent towards  the normal.  After entering the  denser medium the speed of light  decreases.

   

2 When a ray of light travels from optically denser medium to optically less dense medium , the ray is bent  away from the normal. After entering the less dense medium the speed of light  increases.      

      

 

3         When a ray of light crosses the boundry between two different mediums at a right angle or the incident ray parallel to normal, the ray is not bent but the  speed of light is different.

The angles of incidence and refraction are zero.

Common terminology of refraction of light between two medium of different optical densities

Normal                   :     A line at right angles to the  boundary  of the  surface of the

                                      medium.

 

Incident ray           :    A ray of light that is directed onto to the boundry  of the  surface of

                                     the medium

 

Reflected ray        :    A ray that is refracted when passing through  the different

                                    medium.

 

Angle of                                 :               The angle between the incident

incidence                                              ray  and the normal

 

Angle of                                 :               The angle between the refracted

refraction                                               ray  and the normal

                       

The law of refraction

The first law( Snell’s Law) :The ratio of the sine of the angle of incidence to the sine of the angle incidence to the of reflection.

 

The second law  :The incident ray, the normal and the  refracted ray all lie in the same plane

 

Refractive index (n)

 

Refractive index,n  is defined as,

where  n =  Refractive index

             i  =  the angle in medium less dense

              r =  the angle in denser medium                      

·         A material with a higher refractive index has a greater bending effects and higher density.

·         The value of refractive index , n ³ 1

·         The refractive index has no units.

               

The refrective index can be also defined as ,

 

                     

And

 

                     

Example 1

The diagram shows a ray of light passing from air to the block  X.

Calculate the refractive index of the block X.

Solution

Example 2

The diagram shows a ray of light passing from a glass block  to air.

[ Refractive index of glass = 1.54 ]

               

Determine the value of  the angle q .

Solution

Example 3

The diagram shows a light ray entering in a glass prism.

[ Refractive index of glass = 1.51 ]

                            

Draw the ray light after it passes through a point P and determine the angle of refraction when the ray light entering the air again.

Solution

Example  4

The speed of light in vacuum is 3 x 108 ms-1  and the speed  of light in glass is 2 x 108 ms-1 . Determine the refractive index of glass.

Solution

 

Example 5

 

The apparent depth of a fish is 0.50 m. Calculate the real depth of the fish.

[ Refractive index of water = 1.33)

 

Solution

Example 6

The figure shows an man observing the base of a swimming pool . The base of the pool appear to be shallower.The real depth of the swimming pool is 2.0 m.

[ Refractive index of water = 1.33 ]

(a)What is the apparent depth of the swimming pool? 

 

(b) In figure above ,draw  a ray diagram  from point Z to show how the point Z

appear shallower.

 

Experiment to investigate the relationship between the angle of incidence and the angle of refraction.

Hypothesis:

The angle of refraction increases as the angle of incidence increases.

Aim of the experiment :

To investigate the relationship between the angle of incidence and the angle of refraction.

Variables in the experiment:

Manipulated variable: Angle of incidence

Responding variable: Angle of refraction

Fixed variable: Refractive index

List of apparatus and materials:

Glass block, ray box, white paper , protractor,power supply .

 

Arrangement of the apparatus:

    

The procudere of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The glass block is placed on a white paper.

The outline of the sides of the glass block are traced  on the white paper and labelled as ABCD.

The glass block is removed.

The normal  ON is  drawn.

By using a protractor , the angle of incidence is measured = i

The glass block is replaced again on its outline on the paper.

A ray of light  from the ray box is directed along incidence line.

The ray emerging from the side CD is drawn as line PQ.

The glass block is removed again.

The point  O and P is joined and is drawn as line OP.

The experiment is repeated 5 times for the other angles of incidence.

Tabulate the data:

Sinus i
Sinus r

Analysis the data:

 Plot the graph Sin r against Sin i

Experiment to investigate the relationship between real depth and apparent depth.

Hypothesis:

The apparent depth  increases as the real depth increases.

.Aim of the experiment :

To investigate the relationship between apparent depth  and the real depth

Variables in the experiment:

Manipulated variable: real depth

Responding variable: apparent depth

Fixed variable: Refractive index

List of apparatus and materials:

Pin,ruler,water,retort stand ,tall beaker

Arrangement of the apparatus:

 

 

The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

 

A pin is placed at the base of the beaker as object O.

The another pin is clamped  horizontally  onto the retort stand as image position indicator, I

The beaker is filled with water.

By using a ruler ,the real depth of the pin is measured = H

The pin O is seen vertically above the surface of the water.

The position of pin I is adjusted until parallax error between the pin O and the pin I is non- existent.

By using the ruler again ,the position of pin I is measured as the apparent depth = h

The experiment is repeated 5 times for the other value of H

Tabulate the data:

H
h

Analysis the data:

Plot the graph h against H