Determining wavelength using the Diffraction Grating

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Determining wavelength using the Diffraction Grating
1. CAUTION: The diffraction grating is a photographic reproduction and should NOT be touched. The deeper recess in the holder is intended to protect it from damage. Therefore, the glass is on the shallow side of the holder and the grating is on the deep side.
2. Place the grating on the center of the table with its scratches running vertically, and with the base material (glass) facing the light source. In this way, one can study diffraction without the complication of refraction (recall from the previous lab how light behaves when traveling through glass at other than normal incidence). Fix the grating in place using masking tape.
3. Rotate the table to make the grating perpendicular to the incident beam by eye. This is not critical since the average of R and L accommodates a minor misalignment.
4. Affirm maximum brightness for the straight through beam by adjusting the source-slit alignment. At this step, the slit should be narrow, perhaps a few times wider than the hairline. Search for the spectrum by moving the telescope to one side or the other. This spectrum should look much like that observed with the prism except that the order of the colors as you move away from zero degrees is reversed.
5. Search for the second- and third-order spectra. Do not measure the higher-order angles, but record the order of colors away from zero degrees.
6. For each of the colour of white light source measure the angles R and L to the nearest tenth of a degree by placing the hairline on the stationary side of the slit.

Refractive index, Cauchy's constant by spectrometer


(A) Measurement of the angle of the prism.

(i) Determine the least count of the spectrometer.

(ii) Place the prism on the prism table with its refracting angle A towards the collimator

and with its refracting edge A at the centre. In this case some of the light falling on each face will be reflected and can be received with the help of the telescope.

(iii) The telescope is moved to one side to receive the light reflected from the face AB and the cross wires are focused on the image of the slit.The reading of the two verniers are taken.

(iv) The telescope is moved in other side to receive the light reflected from the face AC and again the cross wires are focused on the image of the slit.The readings of the two verniers are taken.

(v) The angle through which the telescope is moved or the difference in the two positions gives twice the refracting angle A of the prism. Therefore half of this angle gives the refracting angle of the prism.

(B) Measurement of the angle of minimum deviations:

(i) Place the prism so that its centre coincides with the centre of of the prism table and light falls on one of the polished faces and emerges out of the other polished face, after refraction. In this position the spectrum of light is obtained.

(ii) The spectrum is seen through the telescope and the telescope is adjusted for minimum

deviation position for a particular color (wavelength) in the following way:

Set up telescope at a particular color and rotate the prism table in one direction, of course thetelescope should be moved in such a way to keep the spectral line in view. By doing so a position will come where a spectral line recede in opposite direction although the rotation of the table is continued in the same direction. The particular position where the spectral line begins to recede in opposite direction is the minimum deviation position for that color. Note the readings of two verniers.

(iii) Remove the prism table and bring the telescope in the line of the collimator. See the slit directly through telescope and coincide the image of slit with vertical crosswire. Note the readings of the two verniers.


The difference in minimum deviation position and direct position gives the angle

of minimum deviation for that color.

(v)The same procedure is repeated to obtain the angles of minimum deviation for that color.

(i) Put the interferometer on a rigid table and level it with the help of three leveling screws. (ii) Adjust the position of movable mirror M1 by using the coarse adjustment so that M1 and M2 are approximately equidistant from the beam splitter. (iii) Place the sodium lamp at left hand side of the interferometer at a distance of about 50 cm. Lamp should be approximately in line with the centre of the beam splitter G1 and fixed mirror M2. (iv) Bring the pin hole in front of the sodium lamp. Now put your eye close to the beam splitter. Two virtual images of the pin hole will be seen. Adjust the tilt screws of the fixed mirror so that the two virtual images coincide. For this, you may have to tilt also the three screw of the movable mirror M1. (v) Use the fine tilt adjustment of the fixed mirror to make the two virtual images of the pin hole coincide exactly. (vi) Now remove the pin hole and put the ground glass plate between the sodium lamp and beam splitter. You will see fringes but may not be circular. Again use the fine tilt adjustment of the fixed mirror till the circular fringes centred in the field of view are seen. (vii) Adjust the position of the movable mirror M1 to get about 20 good contrast fringes, centered in the field of view. (viii) Place the telescope focused to infinity for about 30cm away from the beam splitter. (ix) The telescope points towards the movable mirror. The fringes will be seen through the telescope. The centre of the fringes should not shift laterally, if the movable mirror is displaced by means of drum. If this condition is not achieved, repeat the steps vi, vii, viii till this condition is achieved.

Determination of the wavelength of monochromatic light

(i) The mirrors M1 and M2 are adjusted so that circular fringes are visible in the field of view.

(ii) If M1 and M2 are equidistant from the beam splitter G1. the field of view will be perfectly dark.


The mirror M2 is kept fixed and the mirror M1 is moved with the help of the fine

movement screw and the number of fringes that cross the field of view is counted.

Suppose for the monochromatic light of wavelength λ, the distance through which the

mirror is moved =d and the number of fringes that cross the centre of the field of view =


Then d=nλ/2 (1)

because for one fringe shift, the mirror moves through a distance equal to half the wavelength.

(iv) From the above relation λ can be determined.

Resolving Power of Telescope
(1) Mount the telescope and set the cross wire at vertical to the object (2) Glow the object by light and focus the telescope towards object (3) Open the slit of telescope in horizontal direction (4) Record the focal length of the eyepiece,written right on it.) (5) Clamp the telescope to one laboratory stand and the iris diaphragm to another laboratory
stand. (6) Place the resolution slide and the sodium lamp at the far end of the bench about four
metres from the iris diaphragm. Measure the distance D from the resolution slide to the iris diaphragm. (7) With the iris diaphragm wide open, focus the telescope on the resolution slide. (8) Use the least count of travelling microscope using following formula.
L.C. of travelling microscope = Value of smallest division on main scale/Total No. of division on vernier scale
(9)Find distance between two consecutive wires on given wire gauge using traveling microscope. Now place the wire gauge in front of the source of monochromatic light as shown.
(10)Adjust the telescope so as to observe clear image of wire gauge. Now attach given adjustable slit on objective of telescope in open mode.
(11) Continuously observing through the telescope, reduce the slit width gradually so that the image seen through the telescope just cease to be resolved.
(12) Measure the critical width at this position. Also measure the distance (D) of wire gauge from the objective of telescope as shown in the figure of Lab book.

Specific Rotation of Sugar Using Polarimeter
1. 1Find out the least count of analyser scale using following formula
L.C. of travelling microscope = Value of smallest division on main scale /Total No. of division on vernier scale
2. Look through the eyepiece so that the two halves of the half shade device are clearly visible .
3. Fill the polarimeter tube with distilled water taking care of that either there is no air bubble in the tube or if it is there, it remains at the centre part of the tube.
4. Place the tube in the polarimeter and observe through the eyepieceE . In general wefind two semi circles of different colours.
5. By rotating the analyser eyepiece system, the colour pair gets changed. Let us select a pair of different colour, say red and blue.
6. By rotating the analyser scale, the colour pair can be interchange. By rotating the analyser scale, the two colours can be mixed so that circular field of view appears gray instead of two semicircle of red and blue. Record the reading of analyser scale at this position.
7. Prepare a sugar solution by dissolving 10 gm., 8gm, 6gm, and 4gm of sugar in 100ml of distilled water separately. Fill the polarimeter tube with 10gm concentration of sugar solution.
8. Adjust the analyser scale until the field of view appears gray. Note down the analyser reading . Repeat the same procedure for other sugar concentrations i.e. 8gm,6gm,and 4gm. Adjust the analyser scale until the field of view appears gray every time.
9. Calculate the values of angle of rotation q for different sugar concentrations with respect to distilled water. Plot the graph of angle of rotation q against mass of sugar n. graph is straight line, find out its slope.
10. Use the formula for calculating specific rotation

Determining wavelength using the Fresnel's bi prism experiment
1 The sodium lamp was turned on and the apparatus was adjusted so the biprism, the eyepiece and the slit were lined up and at the same height, also so that the axis of the slit and bi-prism were parallel.
2 The biprism was placed 15cm to 20cm from the slit and with the naked eye the biprism .3 Rotary control was adjusted for the best visibility of the fringes, and using the eyepiece
the width of the slit was adjusted so that so that the fringes became as clear as possible. 4 By including the lens in the optical bench the two positions for the images of the slits
were found and thus d1 and d2 were determined. 5 Now by removing the lens the distance between 25 consecutive fringes was determined
and so a value for the fringe separation. This was repeated to determine a more accurate result.

Determining wavelength using the Newton’s rings



Set the equipments as shown in figure in lab manual and be careful not to touch the lens

surfaces and optical filters.

2 Turn power on the light source and try to get the interference fringes and not them on the screen in a clear way.

3 Try to put the optical center, of the interference fringes on the equal distance of the millimeter scale which occurs on the screen.


Measure the diameter of the bright rings starting from ring and arrange your


according to the table.


Draw the graph relation between(Dn)and(n)then find the slop.


The radius of the curvature can also be determined by the using a spherometer.

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Determining wavelength using the Diffraction Grating