An optical device has been given to a student and  he determines its focal length by focusing the  image of the sun on a screen placed 24 cm from the device on the same side as the sun. Select the  correct statement about the device  

(a) Convex mirror of focal length 12 cm  

(b) Convex lens of focal length 24 cm  

(c) Concave mirror of focal length 24 cm  

(d) Convex lens of focal length 12 cm  



  • Because the screen is on the same  side of the object which means it cannot be a lens. Thus, it is a mirror  
  • Since the image is obtained on a screen, it is a real image. Hence, it is a concave mirror.
  • Since he focuses the sun’s image , the object placed at infinity.
  • And, for a concave mirror, object at infinity forms an image at the focus .
  • Hence, the focal length of the concave mirror is 24 cm

Image of the sun on the screen placed at 24 cm from device.jpg

So, the correct answer is (c)

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CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo