Light reflection and refraction Worksheet-17

Light reflection and refraction Worksheet-17

 

  1. The refractive index of glass is 3/2 and refractive index of water is 4/3. What would be the refractive index of water with respect to glass?

 

  1. What are the conditions for no refraction of light ?

 

  1. A ray of light passing through centre of curvature of a spherical mirror retraces its path on reflection from the mirror.

 

  1. A concave mirror is used as a head mirror by ENT specialists. The same mirror can also be used as a shaving mirror. Why?

 

  1. State mirror formula. How does f change when object distance u from the mirror is changed ?

 

  1. The formula for linear magnification of a spherical mirror is What determine the sign of m ? What is the significance of this sign?

 

  1. The image formed by a convex mirror of focal length 20 cm is a quarter of the object What is the distance of the object from the mirror?

 

  1. How much time will light take to cross 2 mm thick glass pane if refractive index of glass is 3/2 ?

 

  1. How do you find the rough focal length of a convex lens? Is the same method applicable to a concave lens ?

 

  1. Which factors determine the focal length of a lens ?

 

Answer:

  1. Refractive index of water w.r.t. glass

∴ light travels slower in glass than in water.

 

  1. Refraction of light shall not occur when

(i) Light is incident normally on a boundary of two media.

(h) Boundary that separates the two media which have equal refractive indices.

 

  1. This is because normal to spherical mirror at any point is the line joining that point to the centre of curvature of the mirror. Therefore, any ray passing through C falls normally on the mirror. It would retrace its path on reflection from the mirror.

 

  1. A concave mirror converges the rays of light falling on it in a direction parallel to the principal axis, on to its focus. That is why ENT specialists use it as a head mirror.

When an object is held between the pole and principal focus of a concave mirror, the mirror forms a virtual, erect and magnified image of the object. That is why concave mirror is used as a shaving mirror also.

 

  1. The mirror formula is , where the symbols have standard meaning.

When u is changed, v changes, but f remains constant. This is because focal length of mirror depends only on radius of curvature of the mirror.

 

  1. In the formula, the sign of m is determined by the signs of h1 and h2.  When m is positive, the image is virtual and erect and when m is negative, the image is real and inverted.

 

  1. Given f  = 20 cm,

∴ v = –u/4

From

         

or      ;  u = –60 cm

 

  1. Here, t = ?,  x = 2  mm = 2 × 10–3 m, 

From         m/s

         

 

  1. The rough focal length of a convex lens is obtained by forming sharp image of a very distant object on a screen. The distance of the screen from the lens gives  the rough focal length of the lens. This method will not be applicable to a concave lens, as image formed by a concave lens is virtual so it can not be taken on a screen.

 

  1. The focal length of a lens depends on (i) radii of curvature of the surfaces of the lens, (ii) nature of material of the lens, (iii) nature of medium in which lens is placed.