Light - Part V

Case 5  : Image formed by a concave mirror when the object is placed at a distance beyond the centre of curvature C.

Consider the following figure.

Let the object be a candle AB placed at a distance beyond C.

Applying similar treatments for the rays of light emanating from two extremes of the candle AB. A ray of light AD parallel to CP will be deflected along DFA’ (rule1). Another ray of light ACE will be reflected along ECA (rule 2) Where the two rays DFA’ and ECA meet, we get the image of A as A’. As explained earlier, light from B will go un-deflected along PCB path. The image of AB is A’B’. Clearly the image is reduced and inverted.  The image is formed between C and F.

Thus we can conclude that for an image of an object placed at a distance beyond the centre of curvature C of a concave mirror :

• The image is formed between C and F

• The image is inverted and real

• The image is reduced in size.  
  

Case 6  : Image formed by a concave mirror when the object is placed at a distance far off (or infinite distance) beyond the centre of curvature C.

Consider the following figure.

Let the object be a candle AB placed at infinity.  B is still on line CP. Rays emanating from A are striking the mirror in parallel fashion. They will be reflected by MM' and they will meet at the focal point F (rule 3). The image is thus highly diminished.

Thus we can conclude that for an image of an object placed at an infinite distance beyond the centre of curvature C of a concave mirror :

• The image is formed at F

• The image is inverted and real

• The image is highly reduced in size  
  

Uses of concave mirrors  
From the cases studied above, we can now understand why concave mirrors are widely used as reflectors.

• Case 1 shows why concave mirrors are used as shaving mirrors.

• Case 2 shows why concave reflectors are used in car headlights. The bulb of the head light is
        placed at the focal point. The reflected light is parallel and gives more visibility to the
        driver at night.

• Case 5 shows why concave mirrors are used in solar powered gadgets. The parallel rays of the
       sun meet at the focal point F. The solar energy concentrated at F is used for further
       applications such as heating, etc.
  

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