Dispersive power

What is dispersive power?

• When a white is incident on a prism the emergent ray deviates from it’s original path and splits into seven colours. This phenomenon is known as Dispresion of light.

• The difference in the angle of deviation of extreme colours i.e red and voilet is known as angular dispersion.

• The ratio of the angular dispersion to the mean angle of dispersion is known as dispersive power.

Expression for dispersive power

• By finding the angular dispersion we have the expression for dispersive power, which is,

ω = δ/δm where δ= angular disperssion. δm=mean angle of deviation.

Set up for experiment

Experiment for finding dispersive power of a prism

Material required:

Spectrometer, mercury lamp, prism, reading lens spirit level.

Theory :Dispersive power of prism ω=(δv – δr)/δδv=angle of minimum deviation for voilet lightδr=angle of minimum deviation for red lightδ=angle of minimum deviation for yellow light

Procedure

• The axis of collimator and telescope must cut the axis of center of prism table for a clear pattern.

• With the help of rack and pinion method move the telescope such that the rays from the source fall parallel to the crosswire.

• The distance between slit and collimator is maintained such that the slit appears to be narrow.