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Thursday, 5 November 2015

Light - Reflection and Refraction (Questions)


Light - Reflection and Refraction (Questions)

Q. For the same angle of incidence 45 degree, the angle of refraction in two transparent media 1 and 2 is 20 degree and 30 degree respectively. out of 1and 2 which is optically denser medium and why ?

Ans: As the angle of refraction is less for medium 1, So medium 1 will be optically denser medium than medium 2.

Q. For the same angle of incidence in media p , q , r the angles of refraction are 45 , 35 and 15 degrees respectively. In which medium will the velocity of light be minimum?

Ans: Taking the refractive index of the medium as sin i / sin r, the refractive index of medium is the maximum and the refractive index of medium 'p ' is the least. In other words, out of the three media,,the rarest medium. Therefore, as light travels fastest in rarer media, the velocity of light will be aximum in and minimum in 'r '.

Q.For what angle of incidence, the lateral shift produced by parallel sided glass plate is zero?
Ans: For <i = 0.

Q. What are the factors on which the lateral shift depends?
Ans: Thickness of the refracting medium, angle of incidence and its refractive index


Q. A coin in a glass beaker appears to rise as the beaker is slowly filled with water. Why? 
Ans: It happens on account of refraction of light. A ray of light starting from the coin goes from water  to air and bends away from normal. Therefore, bottom of the beaker on which the coin lies appears to  be raised.

Q. An object under water appears to be at lesser depth than in reality. Explain why?
Ans: This is due to refraction of light. We know 
Real depth / Apparent depth = n         Or Apparent depth = Real depth / n                 

Since n > 1, so apparent depth < real depth

Q.Why does bending of light takes place ?
Ans: The bending of light takes place because it travels with different velocity in different media

Q.Does the refractive index of substance change with the colour of light ?
Ans: Yes, refractive index depends upon the colour of light (wavelength)

Q.What is the advantage of "total internal reflection" over reflection ?
Ans: The loss of intensity of light is less in total internal reflection than reflection

Q.In refraction of light through a rectangular glass slab, the emergent ray is parallel to the  direction of the incident ray. Why 
Ans: This is because glass slab has parallel reflecting surfaces
The incident ray bends towards the normal inside the glass slab and the refracted ray bends away from the normal when it emerges from the glass slab. The extent of bending of the light ray at the two opposite parallel surfaces is equal and opposite. Hence the incident ray and the emergent ray are parallel to each other.

Q.Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be angle of incidence. Show the same with the help of diagram.
Ans. Incident ray and reflected ray will always be parallel to each other if two plane mirrors are placed perpendicular to each other


Q. What is refraction of light? What are the laws of refraction? 
Ans: Deviation of ray of light from its original path when it travels from one transparent homogeneous medium to another transparent homogenous medium is called the refraction of light. There are two laws of refraction: (i) Incident ray, reflected ray and normal lie in the same plane. (ii) The ratio of sine of angle of incidence and sine of angle of refraction is constant i.e. Sin i / Sin r =n21. This is also called as Snell’s law.

Q. Define Refractive Index? 
Ans: It is the ratio of speed of light in vacuum to the speed of light in medium is called Refractive index.  

Q. Give the ratio of velocities of two light waves travelling is vacuum and having wave lengths 4000A and 8000A .
 Ans: In Vacuum, light of all the wave lengths travel with the same velocity i.e. 3 X 108 m/sec

Q. What is the value of Relative refractive index of air? 
Ans: One.

Q. A coin in a glass beaker appears to rise as the beaker is slowly filled with water. Why? 
Ans: It happens on account of refraction of light. A ray of light starting from the coin goes from water to air and bends away from normal. Therefore, bottom of the beaker on which the coin lies appears to be raised.


Q. What is lateral shift? Explain with the help of a diagram. 
Ans: When a ray of light travels through a glass slab from air, it bends towards the normal and when it comes out of the other side of the glass slab it bends away from the normal. It is found that the incident ray and the emergent ray are not along the same straight line, but the emergent ray seems to be displaced with respect to the incident ray. This shift in the emergent ray with respect to the incident ray is called lateral shift or lateral displacement. The incident and the emergent rays, however, remain parallel. 

 Q. A student has three concave mirrors A, B, C of focal lengths 20 cm, 15 cm and 10 cm respectively. For each concave mirror he performs the experiment of image formation for three values of object distance of 30 cm, 10 cm and 20 cm. 
Giving reason answer the following: 
(a) For the three object distances identify the mirror which will form an image equal in size to that of object. Find at least one value of object distance. 
(b) Out of the three mirror identify the mirror which would be preferred to be used for shaving purpose. 
Ans: (a) Mirror B of focal length 15cm this is because an image equal in size to that of object is formed if object placed at C 
(b) For shaving purpose object is placed between f and O so that magnified virtual image can be produce . Thus the mirror which would be preferred to be used for shaving purpose is B 

 Q. When sunlight is concentrated at a paper at the principal focus of a convex lens, what happen to the paper? 

Ans: Paper stat burning because Sun light ray, after refracting through the lens, converges at a very sharp point.

Q. "A concave mirror of focal length ‘f ’ can form a magnified, erect as well as an inverted image of an object placed in front of it." Justify this statement stating the position of the object with respect to the mirror in each case for obtaining these images. 

Ans: When object is placed between P and F, an erect, magnified, virtual image is formed. When the object is placed between F and C as well as F, an inverted, real magnified image is formed

 Q. Which kind of mirrors are used in the headlights of a motor- car and why? 
Answer: Concave mirror are used to get powerful parallel beam of light in the forward direction

Q. When sunlight is concentrated at a paper at the principal focus of a convex lens, what happen to the paper?
 Ans: Paper stat burning because Sun light ray, after refracting through the lens, converges at a very sharp point

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