An object of height 3 cm is placed at 10 cm from a concave lens of focal length 15 cm. find the size of the image.

Explanation

(i)
`1/f=1/v-1/u`
`f=-15` cm [left side of lens]
`u=-10` cm [left side of lens]
`1/v=1/f+1/u`
`" "=1/(-15)+1/(-10)`
`" "=((1/-15) times 30+(1/-10) times 30)/30`
`" "=(-2-3)/30=(-5)/30=(-1)/6`
`v=-6cm`
`m=v/u=h_(2)/h_(1)`
`" "=(-6)/(-10)=h_(2)/(3)`
`" "=(-18)/(-10)=h_(2)`
Size of the image `h_(2)=1.8cm`
(ii) Given, time, t = 9.8 s
Speed of sound, v = 330 `ms^(-1)`
Height of cloud, `d=?`
`v=d//t, " "d=vt`
`" "=330 times 9.8`
`" "d=3234 m`