PHYSICS LAB MANUALS

DIFFRACTION GRATING

Three physical quantity need to be measured with reference to the white light.

Table II : The details of the physical quantities to be measured.

S.No

Physical quantity

Independent/dependent

Measured

with

Measuring instrument’s

Minimum

(m)

Maximum (m)

Least count

1

\(\theta\)

Dependent

Circular scale with Vernier calipers

\(0.5^{\circ}\)

\(360^{\circ}\)

\(30^{''}\)


  1. Adjust the slit width to get the sufficient intensity through slit.
  2. Align the collimator, grating, telescope in such a way that we can see different wavelengths (colors) through the telescope lens.
  3. Record \(\theta_{w}\) of white light for right side, then record \(\theta_{1}\) (right side of white light) for different colors.
  4. Repeat the above step for the left side of white light and take it as \(\theta_{2}\) (left side of white light), including \(\theta_{w}\) for left.
  5. Subtract the \(\theta_{1}\) angles from \(\theta_{w}\) of corresponding side and take it as \(\theta_{1}^{'}\), repeat the same thing for \(\theta_{2}\) angles and take it as \(\theta_{2}^{'}\) .
  6. Take the average of the \(\theta_{1}^{'}\) and \(\theta_{2}^{'}\). Denote this as \(\theta\) and then substitute in \(\sin\theta\)


Figure 2: Ray Diagram

TABLES FOR RECORDING THE DATA

S.No.

Color

\(\theta_{1}\)

\(\theta_{2}\)

\(\theta_{1}^{'}\)

\(\theta_{2}^{'}\)

\(\theta = \frac{\theta_{1}^{'}+\theta_{2}^{'}}{2}\)

\( \sin \theta \)

\( \lambda = \frac{a \sin \theta }{m}\)