Light reflection and refraction Worksheet-14

Light reflection and refraction Worksheet-14

 

  1. For what position of the object will a convex lens form a virtual and erect image?

 

  1. For what position of an object, a real, inverted and diminished image is formed by a convex lens?

 

  1. An object is placed between F and 2 F of a convex lens. What is the nature of the image formed  ?

 

  1. Where should an object be placed in order to use a convex lens as a magnifying glass ?

 

  1. What is the difference between lens formula and mirror formula ?

 

  1. When the image formed by a lens is always virtual, erect and diminished, what is the nature of the lens?

 

  1. What kind of lens can form a

(i) Virtual, erect, diminished image ?

(ii)  virtual, erect, magnified image ?

 

  1. What is one dioptre ?

 

  1. How is power of a lens related to its focal length ?

 

  1. Which lens has greater power, a convex lens of focal length 10 cm or a convex lens of focal length 20 cm?

 

  1. What is meant by number of lens ?

 

  1. What is a spherical mirror ?

 

  1. What is the basic difference between a concave mirror and a convex mirror?

 

  1. In a concave mirror, is principal focus a real point? If yes, why?

 

  1. The principal focus of a convex mirror lies at the back of the mirror. Explain.

 

  1. How do you measure quickly the approximate focal length of a concave mirror ?

 

  1. What is meant by linear magnification of a concave mirror?

 

  1. State the expression for linear magnification of a concave mirror in terms of object distance and image distance.

 

  1. What is the nature of image formed when an object is held at a distance of 10 cm from the pole of a concave mirror of focal length 15 cm ?

 

  1. A concave mirror of focal length 20 cm is to be used as a shaving mirror. Which of the following is the suitable object distance from the mirror ?

(i) 10 cm (ii) 20 cm (iii) 30 cm.

 

Answer:

  1. When the object is situated between optical centre and principal focus of the lens.

 

  1. A convex lens forms a real, inverted and diminished image when the object is placed at a distance more than twice the focal length of the lens.

 

  1. Real, inverted and magnified.

 

  1. A convex lens acts as a magnifying glass, when it forms a virtual, erect and magnified image of an object. For this, the object is to be placed between principal focus and optical centre of the lens.

 

  1. The lens formula is and the mirror formula is

where the symbols have their usual meaning.

 

  1. The lens must be concave.

 

  1. (i) concave lens

(ii) convex lens.

 

  1. One dioptre is the power of a lens of focal length one meter.

 

  1. Power of a lens is reciprocal of the focal length of a lens.

 

  1. A convex lens of smaller focal length has greater power.

 

  1. Number of a lens represents power of the lens (in dioptres). It may be positive or negative.

 

  1. A spherical mirror is that mirror whose reflecting surface is a part of a hollow sphere of glass. One side of the mirror is reflecting and the other side is opaque.

 

  1. In a concave mirror, the reflecting surface is towards the centre of the sphere of which the mirror is a part. In a convex mirror, the reflecting surface is away from the centre of the sphere, of which the mirror is a part.

 

  1. Yes, because the parallel rays reflected from the mirror actually pass through the principal focus of concave mirror.

 

  1. Principal focus of a convex mirror is a virtual point. The parallel rays reflected from convex mirror only appear to come from the principal focus.

 

  1. Hold the concave mirror facing some distant object like a tree.

Hold a cardboard screen in front of the mirror.

Try to adjust the position of the screen so that a sharp image of distant object is formed on the screen.

The distance of screen from the mirror gives the approximate focal length of the concave mirror.

 

  1. Linear magnification (m) of a concave mirror is the ratio of the size of the image (h2) to the size of the object  (h1) i.e.

 

  1.   

 

  1. The object lies between pole and principal focus of concave mirror. Therefore, the image formed is virtual and erect.

 

  1. For using a concave mirror as a shaving mirror, u < f. Therefore, object distance can only be 10 cm.