Consider points $\mathrm{D}$ and $\mathrm{E}$ on the screen in Figure, where $\mathrm{BC}=\mathrm{CD}=\mathrm{DE}$. State and explain what you would expect to observe at $\mathrm{D}$ and $\mathrm{E}$.

The type of interference, and hence whether a bright or a dark fringe is seen on the screen, depends on the path difference between the rays of light arriving at the screen from the double-slit.

In a double-slit experiment using light from a helium– neon laser, a student obtained the following results:

Width of $10$ Fringes $10\mathrm{x} = 1.5 \mathrm{cm}$ Separation of slits $\mathrm{a} = 1.0 \mathrm{mm}$ Slit-to-screen distance $\mathrm{D} = 2.40 \mathrm{m}$ Determine the wavelength of the light. And 

If The student moved the screen to a distance of $4.8 \mathrm{m}$ From the slits. Determine the fringe separation $\mathrm{x}$ Now.

Use the equation $\lambda =\frac{ax}{D}$ to explain the following observations:

(a) With the slits closer together, the fringes are further apart.

Use the equation $\lambda =\frac{ax}{D}$ to explain the following observations:

(b) Interference fringes for blue light are closer together than for red light.

Use the equation $\lambda =\frac{ax}{D}$ to explain the following observations:

(c) In an experiment to measure the wavelength of light, it is desirable to have the screen as far from the slits as possible.

Monochromatic light is incident normally on a diffraction grating having $3000$lines per centimetre. The angular separation of the zeroth- and first-order maxima is found to be $10°$. If $\lambda =580 \mathrm{nm}$, calculate the angle $\theta$ for the second-order maximum.