Holographic Diffraction Grating Film

Introduction

Use this holographic diffraction grating film for the simple observation of spectral emission lines. It separates, or diffracts, light just as a traditional ruled diffraction grating would, but it has the advantage that it can be held at any angle relative to the light source.

Concepts

• Emission spectra
• Diffraction

Materials

Safety Precautions

Do not touch the spectrum tube and/or power supply while they are on because the voltage running through them is quite high. When the power supply is turned on, the spectrum tube will become very hot in a short period of time. Turn off the spectrum tube power supply and allow the spectrum tube to cool completely before removing it from the power supply. Do not look directly into the sun through the diffraction grating film. Wear chemical splash goggles and always follow safe laboratory procedures.

Procedure

1. Insert the spectrum tube into the spectrum tube power supply. Plug in the spectrum tube power supply and turn it on. Turn off the lights.
2. Observe the light being given off by the gas in the tube through the holographic diffraction grating film. Look to either side of the spectrum tube to see the emission line spectrum for the gas.

Teacher Tips

• Tube life is extended if operation is cyclic for no more than 30 seconds “on” and 30 seconds “off,” etc.
• It is easier to see the emission lines of the gas in the spectrum tube in a darkened room.
• It is easiest to observe the emission spectrum by holding the film right next to your eye (not at arm’s length).
• Have students view and compare the emission spectra for several different gases in spectrum tubes. They should observe that as the number of electrons in the element increases, so does the complexity of the emission spectrum.
• When using several different spectrum tubes, space them at least one meter apart to reduce interference from adjacent tubes.
• Holographic diffraction gratings can also be used to observe flame test of metal ions such as sodium, potassium, lithium, calcium, strontium, and copper. The spectra of these metal ions will be observed as diffracted, diffuse flames, not as discrete lines. If nichrome wire is used for the flame test, the diffraction of the light emitted from it can also be observed. Better results are obtained using a slit in front of the burner to produce a narrow beam of light.

Discussion

A traditional ruled diffraction grating is a glass or metal plate that contains many equally spaced parallel grooves. The grooves are generally etched using a diamond cutter. Diffraction gratings prepared in this manner are called master gratings (see Figure 1). These master gratings are then used to manufacture replica gratings, which are formed by pouring a liquid plastic on the master grating, allowing it to harden, and stripping it off. The stripped plastic is then fastened to a flat piece of glass or other backing and becomes the diffraction grating film.

Holographic diffraction gratings are different from traditional ruled diffraction gratings in that they are formed by an interference fringe field of two laser beams whose standing wave pattern is exposed to a polished substrate coated with photo resist (see Figure 2). Processing of the exposed medium results in a pattern of straight lines with a sinusoidal cross section as opposed to a sawtooth-like cross section of a ruled grating. Holographic master gratings are replicated by a process identical to that used for ruled gratings. When light strikes the grooves on the diffraction grating film, it is separated, or diffracted, into its component wavelengths. This characteristic makes diffraction gratings ideal for studying the emission spectra of light sources, such as gases in spectrum tubes. When the light from a gas in a spectrum tube is viewed through the diffraction grating film, the individual emission lines can be seen to either side of the spectrum tube. Holographic gratings are an ideal choice for spectroscopy experiments because they produce less stray light than ruled gratings. In fact, they can reduce stray light by a factor of 10 to 100 below that of ruled gratings.

Several sets of repeating emission lines may be observed through a diffraction grating. Each of these sets of lines represent an order of the spectrum, with the intensity of the lines further away from the light source being less intense than those directly adjacent to the light source. Blazing is a term that refers to the angle of the grooves on a diffraction grating. By controlling the blaze angle (see Figure 1) of a ruled master grating, a large fraction of the energy can be concentrated in a single order. Due to their sinusoidal cross section, holographic gratings cannot be easily blazed and their efficiency is usually considerably less than a comparable ruled grating. One exception occurs when the groove-spacing-to-wavelength ratio is nearly one. In this case, a holographic grating has virtually the same efficiency as a ruled grating. For example, a holographic grating with 1800 grooves per millimeter (556 nanometer spacing) has essentially the same efficiency at 500 nanometers as a ruled grating. The holographic diffraction grating film in 2" x 2" cardboard mounts available through Flinn Scientific (Catalog No. AP1047) contains 1000 grooves per millimeter (1000 nanometer spacing). In general, the angular dispersion of diffraction orders is increased as the number of grooves per millimeter is increased. Therefore, if more diffraction orders need to be viewed, a holographic diffraction grating with fewer grooves per unit length should be chosen.