Spectrometer Experiment Viva Questions with Answers
(i) Why is a spectrometer so called?
Ans: It is an instrument to study the spectra of different sources of light.
(ii) What is meant by monochromatic source of light?
Ans: It is a source that emits waves of the same colour or wave length.
(iii) Why do we use sodium vapour lamp in the laboratory as source of light?
Ans: It emits nearly monochromatic light (yellow) of wavelength 5890 Å.
(iv) What is the function of a colimator?
Ans: It is to produce parallel rays
(v) What are the parts of a telescope?
Ans: It has an objective and an eye-piece and a cross wire at the focal plane of the eye piece.
(vi) What is meant by minimum deviation?
Ans: As the angle of incidence at one face of a prism increases the angle of deviation decreases reaches a minimum value and then increases. The minimum angle of deviation is called angle of minimum deviation
(vii) Why are the two readings of the spectrometer taken?
Ans: They are taken in order to eliminate any error arising due to the non coincidence of the centre of the graduated scale and the axis of rotation.
(viii). What is refraction of light?
The phenomenon of the bending of light from its straight path at the surface of two different optical media is called refraction of light.
(ix). What are the laws of refraction?
(a) The incident ray, the normal to the point of incidence and the refracted ray are in the same plane.
(b) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for any two given media. It is also called Snell’s law.
(x). What is the unit of refractive index?
It has no unit since it is the ratio of two similar quantities.
(xi). What happens, when a prism is set in minimum deviation position?
(a) The angle of emergence becomes equal to the angle of incidence.
(b) The refracted ray travels parallel to the base of the prism.
(xii). What are the uses of a spectrometer?
To find the refractive index and dispersive power, to study spectrum, to find unknown wavelength etc.
(xiii) Aim of Spectrometer Experiment?
To find the refractive index of a solid prism by measuring the angle of the prism A and the angle of minimum deviation D.
(xiv) Apparatus of Spectrometer Experiment?
Spectrometer, source of light, prism, reading lens etc.
(xv) Explain Adjustments of the spectrometer?
1. Adjustment of the eye piece: The telescope is turned towards a white wall and the eyepiece is pushed in or pulled out till the crosswires are clearly seen.
2. Adjustment of telescope for parallel rays: The telescope is turned towards a distant object and it is adjusted to obtain a well defined image of the object without parallax with the crosswires.
3. Adjustment of collimator for parallel rays: The slit of the collimator is illuminated with the source of light. The telescope is brought in line with the collimator. Looking through the telescope the collimator is adjusted to obtain a clear image of the slit without parallax with the crosswires.
4. Leveling of the prism table: The prism table is leveled with the help of a spirit level by working the leveling screws.
(xvi) Explain experiment to determine the angle of the prism?
After making the preliminary adjustments, the prism is mounted on the prism table with its refracting edge vertical. The slit is illuminated with a monochromatic source of light. The prism table is rotated so that rays of light coming out of the collimator fall symmetrically on the two faces of the prism.
The telescope is turned and the image of the slit formed by the reflection of light at the face AB is obtained in the field of view of the telescope. The telescope is clamped in this position and the tangential screw is adjusted to make the vertical cross-wire coincide with the image of the slit. The readings of both the verniers are taken. The telescope is turned to obtain the image of slit formed by the reflection of light at the face AC in the field of view. The telescope is adjusted till the vertical crosswire coincides with the image of the slit. The readings of the verniers are again noted. The difference between the readings of the same verniers gives twice the angle of the prism. From this the angle of the prism 'A' is determined.
(xvii) Explain experiment to determine the angle of minimum deviation (D)?
The prism table is turned so that light from the collimator falls on one of the faces of the pnsm and emerges through the other face.
The telescope is turned to view the refracted image. Looking through the telescope the prism table is slowly rotated in the direction of decreasing angle of deviation, i.e., the refracted ray should move towards the direct ray. The telescope is also rotated so that the image of the slit is in its field of view. It will be found that at a certain position, the image remains stationary for a moment and turns back. The prism table is fixed in this position. The telescope is also fixed so that the image of the slit coincides with the vertical crosswire. The readings of the verniers are taken.
The prism is removed. The telescope is brought in line with the collimator. The vertical cross-wire is made to coincide with the direct image of the slit. The readings of the verniers are again noted.
The difference in the readings of the same vernier gives the angle of minimum deviation. The average value of minimum deviation D is calculated.
(xviii) Determine the refractive index of the material of the prism?
The angle of the prism A and its minimum deviation D are determined as described above. The refractive index of the material of the prism is calculated using the formula.
n = [sin(A + D)/2] / [sin(A/2)]