Question

While a moving picture is being screened, a boy introduced a glass slab thickness $3cm$ and refractive index $1.5$ between the projector and the screen. In order to have a clear picture on the screen, the screen show to be moved through a distance of :

• A. $1cm$ away
• B. $1cm$ nearer
• C. $2cm$ away
• D. $3cm$ away

Answer 1

The correct option is A $1cm$ away

Refractive index of slab, $n_{slab}=1.5$

Refractive index of air , $n_{air}=1$

Thickness of slab, $t=3cm$

Shift due to slab $=t(1-\frac{n_{air}}{n_{slab}})=3(1-\frac{1}{1.5})=1cm$

So, the image produced by the projector gets shifted by $1cm$ away from the slab due to refraction. Thus the screen is now has to be shifted by $1cm$ away.