An object is placed 21 cm in front of a concave mirror of radius of curvature 10cm. A glass slabe of thickness 3cm and refractive index 1.5 is then placed close to the irror in the space between the object and the mirror. The distance of the near surface of the slabe from the mirror is 1cm. The final image from the mirror will be formed at

Explanation

Because of the two refractions at the surfaces of the glass slab, the image of the object O is formed at O..

Shift `OO.=t(1-1/mu)=3cm(1-1/1.5)=1cm`
`therefore` PO. = 21 cm - 1 cm = 20 cm = radius of curvature.
`therefore` O. lies on the centre of curvature of the concave mirror. Therefore, the ray falling on the concave mirror is normal to its surface. Hence after reflection, rays will retrace their path and the final image will be formed at O itself.