(i) "In refraction of light through a rectangular glass slab, the emergent ray is always parallel to the…
(i) "In refraction of light through a rectangular glass slab, the emergent ray is always parallel to the direction of the incident ray." Why ? Explain with the help of a ray diagram. What happens when a ray of light falls normally on one of the faces of a rectangular glass prism ? Draw diagram.
(ii) An object is placed at a distance of 30 cm from the optical centre of a concave lens of focal length 20 cm. Use Lens formula to determine the position of the image formed in this case.
Solution
(a) (i) The extent of bending of the ray of light at the opposite parallel faces of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray.
[diagram: rectangular glass slab showing incident ray, refraction inside slab, and emergent ray parallel to incident ray, points K,N,M,Q,R labelled]
NORMAL INCIDENCE
[diagram: ray of light passing straight through a slab at normal incidence, i and r marked]
[diagram: triangular prism showing a ray refracting at two faces with angles i1 and i2 marked]
(ii) u = -30 cm, f = -20 cm
$\dfrac{1}{v}$ - $\dfrac{1}{u}$ = $\dfrac{1}{f}$ or $\dfrac{1}{v}$ = $\dfrac{1}{u}$ + $\dfrac{1}{f} \dfrac{1}{v}$ = 1/(-30 cm) + 1/(-20 cm)
= 1/(-12 cm) or v = -12 cm
The image is at -12 cm on the same side of the lens.