Help me with physics

Question

An object is placed 375 mm in front of a concave mirror with focus 250 mm.if the object is moved 25 mm away from the mirror,then it's shadow will shift to ...mm

 

a.80

b.82

c.83

d.84

e.85

Answer 1

The lens equation can be used to analyze this type of problem (the lens equation works for mirrors also).

 

   1/f =  1/p + 1/q     (some books have d_o  in place of p and d_i in place of q)

 

For the original object distance,  p =  375  and  f = 250.   Solving gives q = 750.

 

If  the object is moved 25 mm further from the lens,  p becomes 400 and the new q is 667.    

 

Thus the image is 750 - 667 = 83 closer to the mirror.