Question 6 - Case Based Questions (MCQ) - Chapter 10 Class 10 - Light - Reflection and Refraction

A very thin narrow beam of white light is made incident on three glass objects shown below. 

Study the nature and behaviour of the emergent  beam in all the three cases.

(a) Following are the possibility of two emergent  beams being similar. Choose the correct answer:   

(i) (i) and (ii)  

(ii) (i) and (iii)  

(iii) (ii) and (iii)  

(iv) No similar emergent beams  

Answer 

In all 3 cases

• In (i) the emergent beam is white and laterally displaced .  
• In (ii) emergent beam is a spectrum of seven colours bent in different angles.  
• In (iii) the emergent beam from the second prism is  white only .  

Similarity between (i) and (iii) as both emergent  rays are white in colour

So, the correct answer is (ii) (i) and (iii) 

(b) When light enters from air to glass, the angles of  incidence and refraction in air and glass are 45°  and 30°, respectively. Find the refractive index of  glass. (Given that sin 45°= 1/√2  ; sin 30°= 1/2)  

(i) √2 

(ii) 2 √2  

(iii) 1/(√2) 

(iv) 1  

Ans.  

So, the correct answer is (i) √2  

(c) The light changes its path as its medium changes.  Which of the following is an incorrect statement.  

(i) Speed of light is different in different media.  

(ii) Light changes its path because light only travels in a straight line.  

(iii) Speed of light is dependent on the medium  through which it is passing.  

(iv) The light chooses the path with minimum time, as it changes its medium.  

Ans.

• Speed of light is different in different media - Speed of light is less in glass as compared to in air. Hence, we can say that the speed of light is different in different media.
  Option (i) is correct
  
  
• Light changes its path because light only  travels in a straight line - Light doesn't change it’s path , it just bends to an angle while traveling in different media.
  Option (ii) is incorrect.   
  
  

• Speed of light is dependent on the medium  through which it is passing - This is true .
  Option (iii) is correct
  
  
• The light chooses the path with minimum time, as it changes its medium - By definition, light always chooses the path which will take the least time.
  Option (iv) is also correct

So, the correct answer is (ii) 

(d) What is the unit of refractive index?  

(i) Pascal 

(ii) Joule  

(iii) No unit 

(iv) μ m  

Ans. 

Refractive index is the ratio of sine of angle of incidence to the sine of angle of refraction which is a constant , hence no unit .

So, the correct answer is (iii) No unit  

(e) Light travel fastest in:  

(i) Air 

(ii) Vacuum  

(iii) Glass 

(iv) Diamond 

Ans. 

• Light travel fastest where there no obstructions
• In vacuum, there is no obstructions
• Hence, light travel fastest in vacuum

So, the correct answer is (ii) vacuum.