Question

A point source is placed at a distance of 30cm from a convex lens (f=15cm). A glass slab of thickness 1cm is kept just in the opposite side of the lens. The refractive index of glass is 1.5. what will be image distance from lens?

Answer 1

`30.33 cm`

Explanation:

Suppose,the glass slab was absent,then the picture would have been like this,

So,applying lens formula,

`1/u - 1/v =1/f`

Given, `u=-30 cm, f=+15 cm`

Putting we get,

`v=30cm`

So,in absence of the glass slab,image would have been formed at `30 cm` from the optical centre of the convex lens.

Now,if we consider a slab only,the ray diagram appears as follows,

so ,we can find a lateral shift of `x` (dotted line represents the pathway of light which would have been traced if slab was not there)

Now,this lateral shift is given by the formula, `x=r- r/(mu)`

Given, `r=1 cm` and `mu=1.5`

so, `x=0.33 cm`

Now,putting the lens and the slab together as wanted in the question,the reconstructed diagram looks like this,

So, now we can say that the image will be formed `v+x=30+0.33=30.33cm` away from the optical centre of the convex lens.