Question 14 [OD, Set-1, 2017] - MCQ from Past Year Papers - Chapter 10 Class 10 - Light - Reflection and Refraction

Study the given ray diagrams and select the correct  statement from the following: 

(a) Device X is a concave mirror and device Y is a  convex lens, whose focal lengths are 20 cm and 25  cm respectively  

(b) Device X is a convex lens and device Y is a concave  mirror, whose focal lengths are 10 cm and 25 cm  respectively  

(c) Device X is a concave lens and device Y is a convex  mirror, whose focal lengths are 20 cm and 25 cm  respectively  

(d) Device X is a convex lens and device Y is a concave  mirror, whose focal lengths are 20 cm and 25 cm  respectively 

Answer

For X

• Because the screen is on the other  side of the object, thus it is a lens
• Since the image is obtained on a screen, it is a real image. Hence, it is a convex lens.
• Since he focuses the sun’s image , the object placed at infinity.
• And, for a convex lens, object at infinity forms an image at 2F .
• Hence, the 2 × Focal length = 20 cm, and focal length is 10 cm  
• Thus, X is a convex lens of focal length 10 cm

For Y

• 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 25 cm . 
• Thus, Y is a concave mirror of focal length 25 cm

So, the correct answer is (b)