# Change in position of image due to insertion of glass slab

1. Jun 9, 2006

### Amith2006

# A convex lens placed between an object and screen forms a sharp image of the object on screen. A glass slab of refractive index n is inserted between object and lens. In order to obtain the sharp image on the screen again the lens has to be displaced by a distance x. What is the thickness of the slab?
I solved it in the following way:
Let t be the thickness of the slab.
Shift in position of image due to the insertion of glass slab = [t(n-1)]n
i.e. x = [t(n-1)]n
t = (nx)/(n – 1)
But the answer given in my book is n/x.

2. Jun 9, 2006

### Andrew Mason

I would suggest you check this carefully. Where did you get this from?

AM

3. Jun 9, 2006

### Amith2006

Hootenanny Sir,

4. Jun 9, 2006

### Hootenanny

Staff Emeritus
My first instinct would be to draw a diagram. However, I would think that the refraction of the light would cause the object to 'appear' closer to the lense if you understand my meaning. If you could calculate the change in horizontal distance due to this refraction, I think you would be there.

5. Jun 9, 2006

### Andrew Mason

Where did you get ?

The source of your problem is your statement x = t(n-1)n. This is not correct.

AM