CBSE Class 10 Science 2018 · Paper 31(B) · Question 26(a)

Any four of the following: (i) Mark the boundary (outline) of the glass slab with a sharp pencil, and do not move the slab until all the pins are fixed. (ii) Keep the angle of incidence between $30^{\circ }$ and $55^{\circ }$. (iii) Fix the two pins on each side at least $5$ cm apart. (iv) Draw the rays and lines with a sharp pencil. (v) Fix the pins vertically, and look at the feet of the pins, not their heads, while lining them up. (Other correct precautions, such as using a slab with clean, smooth and parallel faces, also earn credit.)