Tag Archives: light

Tritium Illumination

Radioluminesence is the emission of light due to bombardment by ionising radiation, the most common example of which is tritium illumination. Tritium is an isotope of hydrogen, made up of one proton and two neutrons (i.e. it is hydrogen-3). It has a half-life of 12.3 years and decays by emitting beta particles (high speed electrons) to form helium-3.

In a tritium illumination light source the tritium gas is trapped in a glass tube that has been coated on the inside with a phosphor. When the gas decays, the electrons produced strike the phosphor and their kinetic energy is transferred into light energy. By choosing different phosphors, different colours of light are produced.


Novelty keychains containing tritium light sources.

Because tritium illumination requires no power source and lasts for a long time, it is commonly used in situations where long-term but low-power lighting is required. For example, tritium illumination is used on watchfaces, compasses, instrument dials and gunsights. The encapsulation of the tritium source prevents any radiation risk, and if a tritium light source is broken open then simply leaving the area and allowing the gas to disperse will mitigate any health risks.

On the emission of light

There are many circumstances in which a system will emit light.

During incandescence objects emit light because of their temperature. Everything above absolute zero emits electromagnetic radiation due to its temperature as the electrons in the object vibrate back and forth due to the motion of the atoms that make up the object. The type and amount of EM radiation emitted depends on the temperature – this is how infrared thermometers work, by measuring the intensity and wavelength of the IR radiation emitted.


A red-hot piece of metal demonstrating incandescence.

Luminescence is the emission of light not due to temperature, and can be broken up into many sub-processes, listed below.

Fluorescence is one of the most familiar of these processes. An object fluoresces when it absorbs electromagnetic energy of one sort and subsequently emits another, usually longer-wavelength energy. This is how hidden “ultraviolet ink” works and why clothes sometimes “glow” under UV illumination. Fluorescence is a subset of photoluminescence, in which light emission is the result of absorption of photons, with the other photoluminescent process being phosphorescence, a much slower process than fluorescence in which the emission of photons is highly delayed. It is phosphorescence which is responsible for the light produced by glow-in-the-dark materials that are “charged” by light.

fluorescent-mineralsA selection of fluorescent minerals.

PhosphorescenceA phosphorescent statue.

Chemiluminescence is the process by which light is emitted during a chemical reaction, such as the reaction which occurs in glow sticks. Bioluminescence is a subset of this, when the process occurs in living organisms like fireflies. The other subset of chemiluminescence, electrochemiluminescence occurs when an voltage is applied to a solution; this is how LEDs operate. Cathodoluminescence is itself a subset of electroluminescence, occurring when electrons strike a material such as a phosphor, causing the electron’s energy to be converted to light. This is how old-fashioned cathode ray tube (CRT) televisions operate.

chemiluminescenceA solution of luminol demonstrating chemiluminescence.

bioluminescence-squid-fireflyL-R: A squid and a firefly demonstrating bioluminescence.

ledsA selection of electroluminescent blue LEDs.

Crystalloluminescence is the process by which light is emitted during crystallisation and fractoluminescence when the bonds in crystals are broken.

Fractoluminescence is a subset of mechanoluminescence in which light is emitted as a result of forces acting on a solid. Other mechanoluminescent processes include triboluminescence in which the action of friction causes light to be emitted as chemical bonds in a substance are broken; piezoluminescence in which the action of pressure on a solid causes light to be emitted as electrons and holes recombine; and sonoluminescence is which bubbles in liquids excited by sound waves collapse, emitting light in the process. The exact process that causes sonoluminescence is unknown, though many suggestions including bremsstrahlung radiation, coronal discharge and proton tunnelling have been suggested.

Radioluminescence occurs when light is emitted as the result of bombardment by ionising radiation. It is radioluminescence that was previously used in glow-in-the-dark materials (in particular radium dials) and which is responsible for the glow produced by tritium illumination.

radioluminescenceA radioluminescent tritium light source.

Finally, thermoluminescence occurs when certain crystalline materials emit energy they had previously absorbed in the form of EM radiation or via bombardment of ionising radiation as a result of being heated.

Demonstrating refractive index

The refractive index of a material governs how much light bends as the light moves into it. You’ve probably seen this bending effect when looking at the surface of a swimming pool: the bottom of the pool looks closer to the surface than it actually is because light rays bend as they travel from water to air.

But if the refractive index of two materials is the same, as is the case for sunflower oil and Pyrex, then light doesn’t bend at all, and you end up with the nice effect demonstrated below.

To say that this demonstration impressed my pupils would be an understatement.

My favourite photograph from the 2012 Olympics

During the 2012 Olympics, the underwater cameras in the swimming pool have been tweeting regularly. On Sunday, the PoolCam sent out my favourite image of the whole London 2012 Olympic Games: a magnificent demonstration of total internal reflection.

Light refracts as it travels from one medium to another. Total internal reflection occurs when light travels from a medium with a high refractive index to one with a low refractive index at an angle above the critical angle for those two media.

In the photograph above you can see out of the pool at the top of the image because the angle of incidence is less than the critical angle. Beyond the critical angle, light is totally internally reflected and the bottom of the pool is reflected back towards the camera. Because the angle is the same in all directions this creates a semicircle, which is visible at the top of the image.

If the camera is on the bottom of the pool looking up you therefore see a perfect “see-through” circle looking at the roof, surrounded by water reflecting the bottom of the pool. This effect can be seen in a previous @L2012PoolCam photograph:

Cosmic Latte

Cosmic Latte is the jokey name given to the overall colour of the Universe. If all the different wavelengths and intensities of light in the Universe were added together, the result is the light beige colour seen above.

The raw data from the study* is shown below.

Click to enlarge

A number of peaks are visible in the spectrum, in particular the H-alpha line emitted by ionised hydrogen gas.

* Ivan Baldry et al, “The 2dF Galaxy Redshift Survey: Constraints on Cosmic Star Formation History from the Cosmic Spectrum”, The Astrophysical Journal 569:582-594 (2004). DOI: 10.1086/339477.