## APPARATUS REQUIRED

i) A spectrometer

ii) A Spirit level

iii) A sodium lamp

(iv) An eye-piece

(v) A diffraction grating with clamping arrangement

## THEORY

The bending of light round corners and spreading of light waves in the geometrical shadow of an object is called diffraction.

When a parallel beam of Monochromatic light is incident normally on a grating, the transmitted light gives rise to primary Maxima in certain directions given by the relation.

(a+b)sinθn = n-1

Where a is the width of a transparency, b that of an opacity, θn be the angle of diffraction for nth order maxims and λ be wavelength of light.

Therefore, λ = (a+b)sinθn /n

a = width of transparency

b = width of opacity

λ = wavelength of light

θn = angle of diffraction for n^{th} order maxima

## OBSERVATIONS

Vernier Constant = (1/60)

No. of lines per inch on the grating N = 15000

grating clement (a+b) = 2.54/N = 2.54/15000 cm = 1.69×10^{-4}

Direct reading of telescope= 118.43

## CALCULATION

Mean θ_{1} = 20.36^{0}

λ_{1} = (a+b)sinθ_{1} = 2.54/15000 x sin (20.36^{0})

= 5.89×10^{-5}cm

= 5.89×10^{-7}m

Mean θ_{2} = 176.45/4 = 44.11^{0}

λ_{2} = (a+b)sinθ_{2}/2 = 2.54x sin(44.11^{0})/ 15000×2 = 5.89×10^{-7}m

mean λ = λ_{1}+λ_{2}/2

= 5.81 x10^{-7}m +5.89 x10^{-7}m/2

Therefore, λ = 5.89 x10^{-7}m

## RESULT

Hence, from above calculation; the wavelength of Sodium light is found to be 5.89 x10^{-7}m.

## CONCLUSION

In this way, we can find the wavelength of sodium light using diffraction grating.

**Sources of Error**

i) The telescope May not be focused on the Middle or center of the brightest image of slit ii) Reading on the device might be slightly different than a dual.

iii) Carelessness of experiment on leveling the spectrometer correctly.

## PRECAUTION

i) level the spectrometer horizontally.

ii) Note the reading carefully.

iii) Use a good condition spectrometer which can easily rotate the telescope.

iv) Do the experiment in absence of external light.