A student determines the focal length of a device ‘X’ by focussing the image of a distant object on a screen placed 20 cm from the device on the same side as the object. The device ‘X’ is
(a) Concave lens of focal length 10 cm
(b) Convex lens of focal length 20 cm
(c) Concave mirror of focal length 10 cm
(d) Concave mirror of focal length 20 cm
Answer
Since screen is on the same side of object, and image is obtained on the screen
- It means that image is real , and on the same side of object
- That can only be formed in case of concave mirror
Now, let’s find focal length
- Since we focus image of a distant object, it means that object is at infinity.
- Thus, image will be formed at focus
- And, focal length will be distance of screen from device, i.e. Focal length = 20 cm
So, the correct answer is (d) concave mirror of focal length 20 cm
