Question

Can anyone solve 18th question pls fast

Answer 1

hlo mate wish you happy new year

given here:

m = -1 it means that the image is real and formed on the same side as that of the object from the mirror so as to be obtained on a screen as it is real,  which also suggests that the mirror must be concave as only concave mirrors form real image
thus,

v = -40 cm 
now, 

m = -v/u
-1 = - (-40/u)
u = -40 cm 


Now,
the above is possible only if the object is kept at the centre of curvature of the mirror, thus,
R = -40 cm
now, 
f = R/20
f = -20 cm 
now, 


If the object is moved 20 cm towards the mirror then the object will be at the focus of the mirror and the image will be formed at infinite.


Hope this information will clear your doubts