What is visible spectrum? Why is red used as the stopping light at traffic signals? (iii)Two triangular glass prisms are kept together connected through their rectangular side. A light beam is passed through one side of the combination. Will there be any dispersion? Justify your answer.

(i) Visible Spectrum:
The visible spectrum is the range of electromagnetic wavelengths that can be detected by the human eye. It spans the colors of the rainbow and includes wavelengths roughly between 380 nanometers (violet) and 750 nanometers (red). The colors of the visible spectrum, in order, are violet, indigo, blue, green, yellow, orange, and red. When white light passes through a prism or a raindrop, it disperses into its component colors, creating the spectrum of visible light.

(ii) Use of Red in Traffic Signals: Red is used as the stopping light at traffic signals for several reasons:
• Visibility: Red light has a longer wavelength compared to other colors in the visible spectrum. It is more easily visible from a distance and under different lighting conditions.
• Symbolic Meaning: Red has a universally understood association with stopping or caution. It is a strong and attention-grabbing color that signals drivers to come to a halt.
• Human Perception: The human eye is more sensitive to longer wavelengths (like red) during low-light conditions, making red signals more noticeable in the dark.

(iii) Dispersion of Light through Two Prisms:
When a light beam passes through a single prism, it undergoes dispersion, separating into its component colors due to the different refractive indices for different wavelengths. However, when two identical prisms are placed together in the manner described (connected through their rectangular sides), there is no net dispersion.

This is because the first prism disperses the light into its component colours, and the second prism recombines the colours, effectively reversing the dispersion. The overall effect is that the emergent light from the second prism is nearly the same as the incident light. There may be some small deviations due to the angles of the prisms and imperfections in the glass, but the net result is that there is no significant dispersion. The given setup will behave like a glass slab, resulting in recombination of the seven colours to produce white light. The prisms, in this configuration, act as a dispersive system followed by a converging system, cancelling out the dispersion.