Blue light from a tunable laser is shined on a pair of closely space slits. If the laser light is changed to red light on the same two slits, how will the spacing between the fringes on the distant diffraction screen change

The fringe spacing on a distant diffraction screen will increase.

Explanation:

Blue light has a shorter wavelength than red light, so, changing from blue to red light is basically increasing the wavelength of the light involved in the experiment.

In the double slit experiment, the fringe spacing on a diffraction screen is calculated from the equation below

ω = zλ/d

where ω is the fringe spacing

z is the distance of the slit to the screen

λ is the wavelength of the light used

d is separation or distance between the slits

From the equation, one can see that if other parameters are held constant, increasing the wavelength will lead to an increase in the spacing between the fringes, and hence, changing the light from blue to red light will increase the fringe spacing.