What happens when you heat an iron rod on a flame?

• Do you find any change in colour on heating an iron rod?

When you heat an iron rod, some of the heat energy is emitted as light. First it turns red (lower energy corresponding to higher wavelength) and as the temperature rises it glows orange, yellow, blue(higher energy and of lower wavelength) or even white (all visible wavelengths) if the temperature is high enough.

• Do you observe any other colour at the same time when one colour is emitted?

When the temperature is high enough, other colours will also be emitted, but due to higher intensity of one particular emitted colour (e.g., red), others cannot be observed.

Max Planck broke with the ‘continuous energy’ tradition of electromagnetic energy by assuming that the energy is always emitted in multiples of hυ;

For example: hυ, 2 hυ, 3 hυ... nhυ That is, the energy for a certain frequency E can be represented by the equation E = hυ, where ‘h’ is Planck’s constant which has the value 6.626 X 10^-34Js and ‘υ’ is the frequency of the radiation absorbed or emitted.

The energy (E) for the red colour (higher wavelength or lower frequency) is lower compared to the energy of blue colour (lower wavelength or higher frequency). The energy emitted from a material body increases with increase in heat energy.

The significance of Planck’s proposal is that, electromagnetic energy can be gained or lost in discrete values and not in a continuous manner.

Hence, emission or absorption of light spectrum is a collection of a group of wavelengths.

• Do you enjoy Deepavali fireworks?

Variety of colours is seen from fireworks.?

• How do these colours come from fireworks?

Take a pinch of cupric chloride in a watch glass and make a paste with concentrated hydrochloric acid. Take this paste on a platinum loop and introduce it into a non-luminous flame.

• What colour do you observe?

Carry out similar activity with strontium chloride.