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Thermodynamics

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Spectrometer

A spectrometer consists mainly of three parts:-

1. Collimator

2. Prism table

3. Telescope

Collimator: It consists of an achromatic lens L₁, such that the slit S is at its focus. The slit is placed in front of a source of light and the width of the slit can be adjusted. The slit acts as a source of light and the rays coming out of the lens L₁ are parallel.

Table: The table can be adjusted and its position can be read with the help of the verniers V₁ and V₂. The table can be rotated about a vertical axis and its axis coincides with the axis of rotation of the telescope. A prism is placed on the table. A parallel beam of light is incident on the prism and the emergent beam is also parallel.

Telescope: It is an astronomical telescope fitted with a Ramsden’s eyepiece. If a photograph is to be taken, the eyepiece is replaced by a photographic plate.

Adjustments: The eye-piece of the telescope is adjusted such that the cross wires are clearly visible. The telescope is focused on a distant object. The parallax between the image and the crosswires is removed. Thus, the telescope is set for parallel rays. Then, the spectrometer is placed in front of a monochromatic light (sodium lamp) and the slit is adjusted. The position of the lens of the collimator is adjusted such that a sharp and well defined image is seen through the telescope.

Setting the table: The table can be leveled with the help of a spirit level. For accurate work the three leveling screws A, B and C provided with the prism table are adjusted. Parallel lines are marked on the table which are parallel to the line AB. The prism is placed with its edge a b perpendicular to the line AB. The table is adjusted so that both the faces ab and ac of the prism receive light coming out of the collimator. The telescope is brought into the position to receive light from the face ab. The screws A and B are adjusted till the image of the slit is in the centre of field.

Schuster’s method: The prism is placed on the table and the approximate position for minimum deviation is observed through the telescope. The telescope is rotated for a position making an angle more than the minimum deviation. In this position of the telescope, there will be two positions of the prism when the image of the slit can be viewed through the telescope. Suppose, in the first case, the prism is in the position A. The incident rays from the collimator are falling more obliquely on the prism. The telescope is focused so that the image is sharp and there is not parallax between the image and the cross wires. In this case the rays coming out of the prism become more parallel.

Keeping the position of the telescope fixed, the prism table is rotated such that the image of the slit is again viewed through the telescope. Suppose the prism is now in the position B. Here less oblique rays pass through the prism. The image in this case will be blurred. It is focused by adjusting the distances between the slit and the collimating lens. The telescope is not adjusted in this case.

The prism is again brought back to the position A and the process is repeated. When the image of the slit is sharp and there is no parallax between the image and the cross wires, the spectrometer is adjusted. The collimator produces parallel rays which are focused on the cross wires of the telescope after passing through the prism.