A beam of light converges at a point P. Now a convex lens is placed in the . . .

Question :  A beam of light converges at a point P. Now a convex lens is placed in the path of the convergent beam at 15 cm from P. At what point does a beam converges if the convex lens has a focal length of 10 cm?

Doubt by Saif

Solution : 

Here 

u = +15 cm
f = +10 cm 

Using the Lens Formula 
1/f=1/v-1/u
1/v=1/f+1/u
1/v=1/10+1/15
1/v=(15+10)/150
1/v=25/150
1/v=1/6
v=6 cm

Hence, the beam converges at 6 cm

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