an object 50 cm tall is place on the principle axis of a convex lens. its 20cm tall image is formed on the screen placed at a distance of 10cm from the lens. find the focal length of the lens

Lexy is correct because angle r is between the incident wave,  n  = 0, and a diffracted wave.

I'm having some difficulty seeing the diagram clearly.but the question says there's a net inward force on the object.so we know right away,  from newton's 2nd law of motion, thatthat baby is accelerating.  it doesn't even matter which way thenet force is pointing.  if there's any net force, then either theobject's speed or else its direction must be changing.

Chromium  +  hydrogen sulfate  =  chromium(iii) sulfate  +  hydrogenso then it will be  a. cr + h2so4? cr2(so4)3 + h2