(a) Name the electromagnetic radiation used to detect fake currency.

(b) Redraw the diagram given below and complete the path of the light ray AВ through the glass prism till it emerges out of the prism. Critical angle of the glass is 42°.

(a) Ultraviolet (UV) radiation

(b) The completed diagram is shown below :

Explanation:

• The ray AB enters the prism normally at point B, so it passes straight into the prism without bending.
• It then travels vertically upward and strikes the slant face of the prism.
• The angle between this face and the vertical side is 30°, so the angle of incidence at the slant face becomes 60°.
• Since this angle is greater than the critical angle (42°), the ray undergoes total internal reflection at the slant surface.
• After reflection, the ray travels at an angle of 60° with the normal of slant face and move towards opposite face.
• The ray incident on this face at an angle of 30° which is smaller than the critical angle (42°) so the ray refract at this face.