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ICSE Class 9 Physics Question 5 of 19

Reflection of Light — Question 19

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19
Question

Question 1(xix)

We use a concave polished metallic surface as a reflector in a torch to obtain a parallel beam of light. The position of the bulb is :

  1. at the centre of curvature of the reflector.
  2. at the focus of the reflector
  3. between the focus and centre of curvature of the reflector
  4. can be any of the above options
Answer

at the focus of the reflector

Reason — In a torch, we want the light rays to come out as a parallel beam as this helps the light travel a long distance in a specific direction and a concave mirror reflects light rays parallel to the principal axis if the source is placed at its focus.
So, if we want a parallel beam of light (which is the case in a torch), the bulb should be placed at the focus of the concave mirror.