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Solution


Q.

Study the given ray diagrams and select the correct statement from the following:
1 Device X is a concave mirror and device Y is a convex lens, whose focal lengths are 20 cm and 25 cm respectively.  
2 Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 10 cm and 25 cm respectively.  
3 Device X is a concave lens and device Y is a convex mirror, whose focal lengths are 20 cm and 25 cm respectively.  
4 Device X is a convex lens and device Y is a concave mirror, whose focal lengths are 20 cm and 25 cm respectively.  

Ans.

(4) 

Since, for device X the light rays are coming from an object at infinity, passes through the device X and forms a real image on a screen on right side. Since, convex lens forms a real and inverted image at focus when object is at infinity. So, device X is converging lens or convex lens of focal length 20 cm.

For device Y, the beam of parallel rays from infinity is incident on the left side and after reflection a real image is formed on the screen. This device should be a mirror. Also, the light rays are getting converged, so mirror is converging in nature; it is a concave mirror of focal length 25 cm.