點解有煙會到個條鐳射線(英)

You could never see infrared (紅內線, NOT紅外線 as said in your question, there is noting in this world known as 紅外線) even there is smoke present. It because infrared radiation is of wavelength outside the visible range of human eyes.

Laser rays (鐳射線) is different from infrared. The laser ray that you can see is of wavelength within the visible range of human eyes. Because light (including laser light) travels in a straight line, you could only see the light when it is directed right onto your eyes, or after reflection on some objects/surfaces.

However, if there is smoke particles present in the air when a beam of laser is passing through, these smoke particles would scatter the laser light in all directions. As such, the scattered light enters your eyes and the beam appears visible to you.

In fact, it is a kind of electromagnetic wave. So long as has the energy of the object can emit electromagnetic waves, in other words, the temperature object can emit electromagnetic waves. So the higher the temperature of the object, the electromagnetic wave emitted is stronger. Then the sun was in high temperature can emit visible light (seven kinds of color composite), the burning of coal can emit red and yellow light, can now think of light levels infrared is the lowest, which is as long as the temperature of the object can emit infrared. The burning of coal but also can emit infrared.