What is the longest wavelength that can be observed in the third order for a transmission grating having 5200 slits/cm?

641 nm.

Explanation:

Given that,

A transmission grating has 5200 slits/cm.

We need to find the longest wavelength that can be observed in the third order. Using grating equation as follows :

...(1)

d is slit spacing

No fo slit per unit length :

We know that, N = 1/d

For longest wavelength, θ = 90°

From equation (1)

Hence, the longest wavelength in third order for a transmission grating is 641 nm.

heat the gas and measure the pressure